Biology:Integrin

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Short description: Instance of a defined set in Homo sapiens with Reactome ID (R-HSA-374573)
Integrin alphaVbeta3 extracellular domains
PDB 1jv2 EBI.jpg
Structure of the extracellular segment of integrin alpha Vbeta3.[1]
Identifiers
SymbolIntegrin_alphaVbeta3
PfamPF08441
Pfam clanCL0159
InterProIPR013649
SCOP21jv2 / SCOPe / SUPFAM
OPM superfamily176
OPM protein2knc
Membranome13
Integrin alpha cytoplasmic region
PDB 1dpk EBI.jpg
Structure of chaperone protein PAPD.[2]
Identifiers
SymbolIntegrin_alpha
PfamPF00357
InterProIPR000413
PROSITEPDOC00215
SCOP21dpk / SCOPe / SUPFAM
Integrin, beta chain (vWA)
Integrinalpha.png
Identifiers
SymbolIntegrin_beta
PfamPF00362
InterProIPR002369
SMARTSM00187
PROSITEPDOC00216
SCOP21jv2 / SCOPe / SUPFAM
Integrin beta 7 cytoplasmic domain: complex with filamin
PDB 2brq EBI.jpg
crystal structure of the filamin a repeat 21 complexed with the integrin beta7 cytoplasmic tail peptide
Identifiers
SymbolIntegrin_b_cyt
PfamPF08725
InterProIPR014836
SCOP21m8O / SCOPe / SUPFAM

Integrins are transmembrane receptors that help cell-cell and cell-extracellular matrix (ECM) adhesion.[3] Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle, organization of the intracellular cytoskeleton, and movement of new receptors to the cell membrane.[4] The presence of integrins allows rapid and flexible responses to events at the cell surface (e.g. signal platelets to initiate an interaction with coagulation factors).

Several types of integrins exist, and one cell generally has multiple different types on its surface. Integrins are found in all animals while integrin-like receptors are found in plant cells.[3]

Integrins work alongside other proteins such as cadherins, the immunoglobulin superfamily cell adhesion molecules, selectins and syndecans, to mediate cell–cell and cell–matrix interaction. Ligands for integrins include fibronectin, vitronectin, collagen and laminin.

Structure

Integrins are obligate heterodimers composed of α and β subunits. Several genes code for multiple isoforms of these subunits, which gives rise to an array of unique integrins with varied activity. In mammals, integrins are assembled from eighteen α and eight β subunits,[5] in Drosophila five α and two β subunits, and in Caenorhabditis nematodes two α subunits and one β subunit.[6] The α and β subunits are both class I transmembrane proteins, so each penetrates the plasma membrane once, and can possess several cytoplasmic domains.[7]

alpha (mammal)
gene protein synonyms
ITGA1 CD49a VLA1
ITGA2 CD49b VLA2
ITGA3 CD49c VLA3
ITGA4 CD49d VLA4
ITGA5 CD49e VLA5
ITGA6 CD49f VLA6
ITGA7 ITGA7 FLJ25220
ITGA8 ITGA8
ITGA9 ITGA9 RLC
ITGA10 ITGA10 PRO827
ITGA11 ITGA11 HsT18964
ITGAD CD11D FLJ39841
ITGAE CD103 HUMINAE
ITGAL CD11a LFA1A
ITGAM CD11b MAC-1
ITGAV CD51 VNRA, MSK8
ITGA2B CD41 GPIIb
ITGAX CD11c
beta (mammal)
gene protein synonyms
ITGB1 CD29 FNRB, MSK12, MDF2
ITGB2 CD18 LFA-1, MAC-1, MFI7
ITGB3 CD61 GP3A, GPIIIa
ITGB4 CD104
ITGB5 ITGB5 FLJ26658
ITGB6 ITGB6
ITGB7 ITGB7
ITGB8 ITGB8

Variants of some subunits are formed by differential RNA splicing; for example, four variants of the beta-1 subunit exist. Through different combinations of the α and β subunits, 24 unique mammalian integrins are generated, excluding splice- and glycosylation variants.[8]

Integrin subunits span the cell membrane and have short cytoplasmic domains of 40–70 amino acids. The exception is the beta-4 subunit, which has a cytoplasmic domain of 1,088 amino acids, one of the largest of any membrane protein. Outside the cell membrane, the α and β chains lie close together along a length of about 23 nm; the final 5 nm N-termini of each chain forms a ligand-binding region for the ECM. They have been compared to lobster claws, although they don't actually "pinch" their ligand, they chemically interact with it at the insides of the "tips" of their "pinchers".

The molecular mass of the integrin subunits can vary from 90 kDa to 160 kDa. Beta subunits have four cysteine-rich repeated sequences. Both α and β subunits bind several divalent cations. The role of divalent cations in the α subunit is unknown, but may stabilize the folds of the protein. The cations in the β subunits are more interesting: they are directly involved in coordinating at least some of the ligands that integrins bind.

Integrins can be categorized in multiple ways. For example, some α chains have an additional structural element (or "domain") inserted toward the N-terminal, the alpha-A domain (so called because it has a similar structure to the A-domains found in the protein von Willebrand factor; it is also termed the α-I domain). Integrins carrying this domain either bind to collagens (e.g. integrins α1 β1, and α2 β1), or act as cell-cell adhesion molecules (integrins of the β2 family). This α-I domain is the binding site for ligands of such integrins. Those integrins that don't carry this inserted domain also have an A-domain in their ligand binding site, but this A-domain is found on the β subunit.

In both cases, the A-domains carry up to three divalent cation binding sites. One is permanently occupied in physiological concentrations of divalent cations, and carries either a calcium or magnesium ion, the principal divalent cations in blood at median concentrations of 1.4 mM (calcium) and 0.8 mM (magnesium). The other two sites become occupied by cations when ligands bind—at least for those ligands involving an acidic amino acid in their interaction sites. An acidic amino acid features in the integrin-interaction site of many ECM proteins, for example as part of the amino acid sequence Arginine-Glycine-Aspartic acid ("RGD" in the one-letter amino acid code).

Structure

Despite many years of effort, discovering the high-resolution structure of integrins proved to be challenging, as membrane proteins are classically difficult to purify, and as integrins are large, complex and highly glycosylated with many sugar 'trees' attached to them. Low-resolution images of detergent extracts of intact integrin GPIIbIIIa, obtained using electron microscopy, and even data from indirect techniques that investigate the solution properties of integrins using ultracentrifugation and light scattering, were combined with fragmentary high-resolution crystallographic or NMR data from single or paired domains of single integrin chains, and molecular models postulated for the rest of the chains.

The X-ray crystal structure obtained for the complete extracellular region of one integrin, αvβ3,[1] shows the molecule to be folded into an inverted V-shape that potentially brings the ligand-binding sites close to the cell membrane. Perhaps more importantly, the crystal structure was also obtained for the same integrin bound to a small ligand containing the RGD-sequence, the drug cilengitide.[9] As detailed above, this finally revealed why divalent cations (in the A-domains) are critical for RGD-ligand binding to integrins. The interaction of such sequences with integrins is believed to be a primary switch by which ECM exerts its effects on cell behaviour.

The structure poses many questions, especially regarding ligand binding and signal transduction. The ligand binding site is directed towards the C-terminal of the integrin, the region where the molecule emerges from the cell membrane. If it emerges orthogonally from the membrane, the ligand binding site would apparently be obstructed, especially as integrin ligands are typically massive and well cross-linked components of the ECM. In fact, little is known about the angle that membrane proteins subtend to the plane of the membrane; this is a problem difficult to address with available technologies. The default assumption is that they emerge rather like little lollipops, but there is little evidence for this. The integrin structure has drawn attention to this problem, which may have general implications for how membrane proteins work. It appears that the integrin transmembrane helices are tilted (see "Activation" below), which hints that the extracellular chains may also not be orthogonal with respect to the membrane surface.

Although the crystal structure changed surprisingly little after binding to cilengitide, the current hypothesis is that integrin function involves changes in shape to move the ligand-binding site into a more accessible position, away from the cell surface, and this shape change also triggers intracellular signaling. There is a wide body of cell-biological and biochemical literature that supports this view. Perhaps the most convincing evidence involves the use of antibodies that only recognize integrins when they have bound to their ligands, or are activated. As the "footprint" that an antibody makes on its binding target is roughly a circle about 3 nm in diameter, the resolution of this technique is low. Nevertheless, these so-called LIBS (Ligand-Induced-Binding-Sites) antibodies unequivocally show that dramatic changes in integrin shape routinely occur. However, how the changes detected with antibodies look on the structure is still unknown.

Activation

When released into the cell membrane, newly synthesized integrin dimers are speculated to be found in the same "bent" conformation revealed by the structural studies described above. One school of thought claims that this bent form prevents them from interacting with their ligands, although bent forms can predominate in high-resolution EM structures of integrin bound to an ECM ligand. Therefore, at least in biochemical experiments, integrin dimers must apparently not be 'unbent' in order to prime them and allow their binding to the ECM. In cells, the priming is accomplished by a protein talin, which binds to the β tail of the integrin dimer and changes its conformation.[10][11] The α and β integrin chains are both class-I transmembrane proteins: they pass the plasma membrane as single transmembrane alpha-helices. Unfortunately, the helices are too long, and recent studies suggest that, for integrin gpIIbIIIa, they are tilted with respect both to one another and to the plane of the membrane. Talin binding alters the angle of tilt of the β3 chain transmembrane helix in model systems and this may reflect a stage in the process of inside-out signalling which primes integrins.[12] Moreover, talin proteins are able to dimerize[13] and thus are thought to intervene in the clustering of integrin dimers which leads to the formation of a focal adhesion. Recently, the Kindlin-1 and Kindlin-2 proteins have also been found to interact with integrin and activate it.[14]

Function

Integrins have two main functions, attachment of the cells to the ECM and signal transduction from the ECM to the cells.[15] They are also involved in a wide range of other biological activities, including extravasation, cell-to-cell adhesion, cell migration, and as receptors for certain viruses, such as adenovirus, echovirus, hantavirus, and foot-and-mouth disease, polio virus and other viruses. Recently, the importance of integrins in the progress of autoimmune disorders is also gaining attention of the scientists. These mechanoreceptors seem to regulate autoimmunity by dictating various intracellular pathways to control immune cell adhesion to endothelial cell layers followed by their trans-migration. This process might or might not be dependent on the sheer force faced by the extracellular parts of different integrins.[16]

A prominent function of the integrins is seen in the molecule GpIIb/IIIa, an integrin on the surface of blood platelets (thrombocytes) responsible for attachment to fibrin within a developing blood clot. This molecule dramatically increases its binding affinity for fibrin/fibrinogen through association of platelets with exposed collagens in the wound site. Upon association of platelets with collagen, GPIIb/IIIa changes shape, allowing it to bind to fibrin and other blood components to form the clot matrix and stop blood loss.

Attachment of cell to the ECM

Integrins couple the cell-extracellular matrix (ECM) outside a cell to the cytoskeleton (in particular, the microfilaments) inside the cell. Which ligand in the ECM the integrin can bind to is defined by which α and β subunits the integrin is made of. Among the ligands of integrins are fibronectin, vitronectin, collagen, and laminin. The connection between the cell and the ECM may help the cell to endure pulling forces without being ripped out of the ECM. The ability of a cell to create this kind of bond is also of vital importance in ontogeny.

Cell attachment to the ECM is a basic requirement to build a multicellular organism. Integrins are not simply hooks, but give the cell critical signals about the nature of its surroundings. Together with signals arising from receptors for soluble growth factors like VEGF, EGF, and many others, they enforce a cellular decision on what biological action to take, be it attachment, movement, death, or differentiation. Thus integrins lie at the heart of many cellular biological processes. The attachment of the cell takes place through formation of cell adhesion complexes, which consist of integrins and many cytoplasmic proteins, such as talin, vinculin, paxillin, and alpha-actinin. These act by regulating kinases such as FAK (focal adhesion kinase) and Src kinase family members to phosphorylate substrates such as p130CAS thereby recruiting signaling adaptors such as CRK. These adhesion complexes attach to the actin cytoskeleton. The integrins thus serve to link two networks across the plasma membrane: the extracellular ECM and the intracellular actin filamentous system. Integrin α6β4 is an exception: it links to the keratin intermediate filament system in epithelial cells.[17]

Focal adhesions are large molecular complexes, which are generated following interaction of integrins with ECM, then their clustering. The clusters likely provide sufficient intracellular binding sites to permit the formation of stable signaling complexes on the cytoplasmic side of the cell membrane. So the focal adhesions contain integrin ligand, integrin molecule, and associate plaque proteins. Binding is propelled by changes in free energy.[18] As previously stated, these complexes connect the extracellular matrix to actin bundles. Cryo-electron tomography reveals that the adhesion contains particles on the cell membrane with diameter of 25 +/- 5 nm and spaced at approximately 45 nm.[19] Treatment with Rho-kinase inhibitor Y-27632 reduces the size of the particle, and it is extremely mechanosensitive.[20]

One important function of integrins on cells in tissue culture is their role in cell migration. Cells adhere to a substrate through their integrins. During movement, the cell makes new attachments to the substrate at its front and concurrently releases those at its rear. When released from the substrate, integrin molecules are taken back into the cell by endocytosis; they are transported through the cell to its front by the endocytic cycle, where they are added back to the surface. In this way they are cycled for reuse, enabling the cell to make fresh attachments at its leading front.[21] The cycle of integrin endocytosis and recycling back to the cell surface is important also for not migrating cells and during animal development.[22]

Signal transduction

Integrins play an important role in cell signaling by modulating the cell signaling pathways of transmembrane protein kinases such as receptor tyrosine kinases (RTK). While the interaction between integrin and receptor tyrosine kinases originally was thought of as uni-directional and supportive, recent studies indicate that integrins have additional, multi-faceted roles in cell signaling. Integrins can regulate the receptor tyrosine kinase signaling by recruiting specific adaptors to the plasma membrane. For example, β1c integrin recruits Gab1/Shp2 and presents Shp2 to IGF1R, resulting in dephosphorylation of the receptor.[23] In a reverse direction, when a receptor tyrosine kinase is activated, integrins co-localise at focal adhesion with the receptor tyrosine kinases and their associated signaling molecules.

The repertoire of integrins expressed on a particular cell can specify the signaling pathway due to the differential binding affinity of ECM ligands for the integrins. The tissue stiffness and matrix composition can initiate specific signaling pathways regulating cell behavior. Clustering and activation of the integrins/actin complexes strengthen the focal adhesion interaction and initiate the framework for cell signaling through assembly of adhesomes.[24]

Depending on the integrin's regulatory impact on specific receptor tyrosine kinases, the cell can experience:

Knowledge of the relationship between integrins and receptor tyrosine kinase has laid a foundation for new approaches to cancer therapy. Specifically, targeting integrins associated with RTKs is an emerging approach for inhibiting angiogenesis.[26]

Integrins are localised at the growth cone of regenerating neurons.[27]

Integrins and nerve repair

Integrins have an important function in neuroregeneration after injury of the peripheral nervous system (PNS).[27] Integrins are present at the growth cone of damaged PNS neurons and attach to ligands in the ECM to promote axon regeneration. It is unclear whether integrins can promote axon regeneration in the adult central nervous system (CNS). There are two obstacles that prevent integrin-mediated regeneration in the CNS: 1) integrins are not localised in the axon of most adult CNS neurons and 2) integrins become inactivated by molecules in the scar tissue after injury.[27]

Vertebrate integrins

The following are 16 of the ~24 integrins found in vertebrates:

Name Synonyms Distribution Ligands
α1β1 VLA-1 Many Collagens, laminins[28]
α2β1 VLA-2 Many Collagens, laminins[28]
α3β1 VLA-3 Many Laminin-5
α4β1 VLA-4[28] Hematopoietic cells Fibronectin, VCAM-1[28]
α4β7 LPAM-1 T cells MAD-CAM1[29][30]
α5β1 VLA-5; fibronectin receptor widespread fibronectin[28] and proteinases
α6β1 VLA-6; laminin receptor widespread laminins
α7β1 muscle, glioma laminins
αLβ2 LFA-1[28] T-lymphocytes ICAM-1, ICAM-2[28]
αMβ2 Mac-1, CR3[28] Neutrophils and monocytes Serum proteins, ICAM-1[28]
αIIbβ3 Fibrinogen receptor; gpIIbIIIa[31] Platelets[28] fibrinogen, fibronectin[28]
αVβ1 neurological tumors vitronectin, osteopontin,[32] fibrinogen
αVβ5 widespread, esp. fibroblasts, epithelial cells vitronectin, osteopontin,[32] and adenovirus
αVβ6 proliferating epithelia, esp. lung and mammary gland fibronectin; TGFβ1+3
αVβ8 neural tissue; peripheral nerve fibronectin; TGFβ1+3
α6β4 Epithelial cells[28] Laminin[28]

Beta-1 integrins interact with many alpha integrin chains. Gene knockouts of integrins in mice are not always lethal, which suggests that during embryonal development, one integrin may substitute its function for another in order to allow survival. Some integrins are on the cell surface in an inactive state, and can be rapidly primed, or put into a state capable of binding their ligands, by cytokines. Integrins can assume several different well-defined shapes or "conformational states". Once primed, the conformational state changes to stimulate ligand binding, which then activates the receptors — also by inducing a shape change — to trigger outside-in signal transduction.

See also

References

  1. ↑ 1.0 1.1 "Crystal structure of the extracellular segment of integrin alpha Vbeta3". Science 294 (5541): 339–45. October 2001. doi:10.1126/science.1064535. PMID 11546839. Bibcode: 2001Sci...294..339X. 
  2. ↑ "Structural basis of chaperone function and pilus biogenesis". Science 285 (5430): 1058–61. August 1999. doi:10.1126/science.285.5430.1058. PMID 10446050. 
  3. ↑ 3.0 3.1 "Integrins: bidirectional, allosteric signaling machines". Cell 110 (6): 673–87. September 2002. doi:10.1016/s0092-8674(02)00971-6. PMID 12297042. 
  4. ↑ "Integrin signaling". Science 285 (5430): 1028–32. August 1999. doi:10.1126/science.285.5430.1028. PMID 10446041. 
  5. ↑ "Integrins". Molecular Biology of the Cell (4th ed.). New York: Garland Science. 2002. https://www.ncbi.nlm.nih.gov/books/NBK26867/. 
  6. ↑ "Integrin structure". Biochemical Society Transactions 28 (4): 311–39. 2000. doi:10.1042/0300-5127:0280311. PMID 10961914. 
  7. ↑ "Electron microscopy and structural model of human fibronectin receptor". The EMBO Journal 7 (13): 4093–9. December 1988. doi:10.1002/j.1460-2075.1988.tb03303.x. PMID 2977331. 
  8. ↑ "Integrins: bidirectional, allosteric signaling machines". Cell 110 (6): 673–87. September 2002. doi:10.1016/S0092-8674(02)00971-6. PMID 12297042. 
  9. ↑ "Cilengitide Merck". Current Opinion in Investigational Drugs 4 (6): 741–5. June 2003. PMID 12901235. 
  10. ↑ "Talin controls integrin activation". Biochemical Society Transactions 32 (Pt3): 434–7. June 2004. doi:10.1042/BST0320434. PMID 15157154. 
  11. ↑ "The Talin head domain binds to integrin beta subunit cytoplasmic tails and regulates integrin activation". The Journal of Biological Chemistry 274 (40): 28071–4. October 1999. doi:10.1074/jbc.274.40.28071. PMID 10497155. 
  12. ↑ "The final steps of integrin activation: the end game". Nature Reviews. Molecular Cell Biology 11 (4): 288–300. April 2010. doi:10.1038/nrm2871. PMID 20308986. 
  13. ↑ "Native talin is a dumbbell-shaped homodimer when it interacts with actin". Journal of Structural Biology 112 (1): 3–10. 1994. doi:10.1006/jsbi.1994.1002. PMID 8031639. 
  14. ↑ "Kindlin-1 and -2 directly bind the C-terminal region of beta integrin cytoplasmic tails and exert integrin-specific activation effects". The Journal of Biological Chemistry 284 (17): 11485–97. April 2009. doi:10.1074/jbc.M809233200. PMID 19240021. 
  15. ↑ "Integrin transmembrane signaling and cytoskeletal control". Current Opinion in Cell Biology 7 (5): 681–9. October 1995. doi:10.1016/0955-0674(95)80110-3. PMID 8573343. 
  16. ↑ Banerjee, S; Nara, R; Chakraborty, S; Chowdhury, D; Haldar, S (2022). "Integrin Regulated Autoimmune Disorders: Understanding the Role of Mechanical Force in Autoimmunity.". Frontiers in Cell and Developmental Biology 10: 852878. doi:10.3389/fcell.2022.852878. PMID 35372360. 
  17. ↑ "Multiple functions of the integrin alpha6beta4 in epidermal homeostasis and tumorigenesis". Molecular and Cellular Biology 26 (8): 2877–86. April 2006. doi:10.1128/MCB.26.8.2877-2886.2006. PMID 16581764. 
  18. ↑ "The non-equilibrium thermodynamics and kinetics of focal adhesion dynamics". PLOS ONE 5 (8): e12043. August 2010. doi:10.1371/journal.pone.0012043. PMID 20805876. Bibcode: 2010PLoSO...512043O. 
  19. ↑ "Dissecting the molecular architecture of integrin adhesion sites by cryo-electron tomography". Nature Cell Biology 12 (9): 909–15. September 2010. doi:10.1038/ncb2095. PMID 20694000. 
  20. ↑ "Mechanosensitive channels". Theoretical and Computational Biophysics Group, Beckman Institute for Advanced Science and Technology: University of Illinois at Urbana-Champaign. http://www.ks.uiuc.edu/Research/MscLchannel/. 
  21. ↑ "Endocytic Trafficking of Integrins in Cell Migration" (in en). Current Biology 25 (22): R1092-105. November 2015. doi:10.1016/j.cub.2015.09.049. PMID 26583903. 
  22. ↑ "Integrin trafficking in cells and tissues". Nature Cell Biology 21 (2): 122–132. February 2019. doi:10.1038/s41556-018-0223-z. PMID 30602723. 
  23. ↑ "beta1A integrin expression is required for type 1 insulin-like growth factor receptor mitogenic and transforming activities and localization to focal contacts". Cancer Research 65 (15): 6692–700. August 2005. doi:10.1158/0008-5472.CAN-04-4315. PMID 16061650. 
  24. ↑ "Extracellular matrix and cell signalling: the dynamic cooperation of integrin, proteoglycan and growth factor receptor". The Journal of Endocrinology 209 (2): 139–51. May 2011. doi:10.1530/JOE-10-0377. PMID 21307119. 
  25. ↑ 25.0 25.1 25.2 "9 - Neoplasms of the Prostate" (in en). Urologic Surgical Pathology (Fourth ed.). Philadelphia: Content Repository Only!. 2020-01-01. pp. 415–525.e42. ISBN 978-0-323-54941-7. 
  26. ↑ "β1 integrin targeting potentiates antiangiogenic therapy and inhibits the growth of bevacizumab-resistant glioblastoma". Cancer Research 73 (10): 3145–54. May 2013. doi:10.1158/0008-5472.CAN-13-0011. PMID 23644530. 
  27. ↑ 27.0 27.1 27.2 "Integrins promote axonal regeneration after injury of the nervous system". Biological Reviews of the Cambridge Philosophical Society 93 (3): 1339–1362. February 2018. doi:10.1111/brv.12398. PMID 29446228. 
  28. ↑ 28.00 28.01 28.02 28.03 28.04 28.05 28.06 28.07 28.08 28.09 28.10 28.11 28.12 Molecular cell biology (fifth ed.). New York: W.H. Freeman and CO. 2004. ISBN 978-0-7167-4366-8. https://archive.org/details/molecularcellbio00harv. 
  29. ↑ Wang, Caihong; McDonough, Jacquelyn S.; McDonald, Keely G.; Huang, Conway; Newberry, Rodney D. (2008-09-15). "Alpha4beta7/MAdCAM-1 interactions play an essential role in transitioning cryptopatches into isolated lymphoid follicles and a nonessential role in cryptopatch formation". Journal of Immunology 181 (6): 4052–4061. doi:10.4049/jimmunol.181.6.4052. ISSN 1550-6606. PMID 18768861. 
  30. ↑ Wagner, N.; Löhler, J.; Kunkel, E. J.; Ley, K.; Leung, E.; Krissansen, G.; Rajewsky, K.; Müller, W. (1996-07-25). "Critical role for beta7 integrins in formation of the gut-associated lymphoid tissue". Nature 382 (6589): 366–370. doi:10.1038/382366a0. ISSN 0028-0836. PMID 8684468. https://pubmed.ncbi.nlm.nih.gov/8684468. 
  31. ↑ "Cell adhesion molecules--update". Veterinary Pathology 34 (1): 61–73. January 1997. doi:10.1177/030098589703400113. PMID 9150551. 
  32. ↑ 32.0 32.1 Kazanecki, CC; Uzwiak, DJ; Denhxxardt, DT (1 November 2007). "Control of osteopontin signaling and function by post-translational phosphorylation and protein folding.". Journal of Cellular Biochemistry 102 (4): 912–24. doi:10.1002/jcb.21558. PMID 17910028. 

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