Biology:Vinculin

Short description: Mammalian protein found in Homo sapiens

Generic protein structure example

Vinculin is a globular protein approximately 115 x 85 x 65 angstroms in linear dimension.

In mammalian cells, vinculin is a membrane-cytoskeletal protein in focal adhesion plaques that is involved in linkage of integrin adhesion molecules to the actin cytoskeleton. Vinculin is a cytoskeletal protein associated with cell-cell and cell-matrix junctions, where it is thought to function as one of several interacting proteins involved in anchoring F-actin to the membrane.

Discovered independently by Benny Geiger and Keith Burridge, its sequence is 20%–30% similar to α-catenin, which serves a similar function.

Binding alternately to talin or α-actinin, vinculin's shape and, as a consequence, its binding properties are changed. The vinculin gene occurs as a single copy and what appears to be no close relative to take over functions in its absence. Its splice variant metavinculin (see below) also needs vinculin to heterodimerize and work in a dependent fashion.

Structure

Vinculin is a 117-kDa cytoskeletal protein with 1066 amino acids. The protein contains an acidic N-terminal domain and a basic C-terminal domain separated by a proline-rich middle segment. Vinculin consists of a globular head domain that contains binding sites for talin and α-actinin as well as a tyrosine phosphorylation site, while the tail region contains binding sites for F-actin, paxillin, and lipids.

Essentially, there is an 835 amino acid N-terminal head, which is split into four domains. This is linked to the C-terminal tail with a linker region.

The recent discovery of the 3D structure sheds light on how this protein tailors its shape to perform a variety of functions. For example, vinculin is able to control the cell's motility by simply altering its shape from active to inactive. When in its ‘inactive’ state, vinculin's conformation is characterized by the interaction between its head and tail domains. And, when transforming to the ‘active’ form, such as when talin triggers binding, the intramolecular interaction between the tail and head is severed. In other words, when talin's binding sites (VBS) of α-helices bind to a helical bundle structure in vinculin's head domain, the ‘helical bundle conversion’ is initiated, which leads to the reorganization of the α-helices (α1- α-4), resulting in an entirely new five-helical bundle structure. This function also extends to cancer cells, and regulating their movement and proliferation of cancer to other parts of the body.

Mechanism and function

Cell spreading and movement occur through the process of binding of cell surface integrin receptors to extracellular matrix adhesion molecules. Vinculin is associated with focal adhesion and adherens junctions, which are complexes that nucleate actin filaments and crosslinkers between the external medium, plasma membrane, and actin cytoskeleton. The complex at the focal adhesions consists of several proteins such as vinculin, α-actinin, paxillin, and talin, at the intracellular face of the plasma membrane.

In more specific terms, the amino-terminus of vinculin binds to talin, which, in turn, binds to β-integrins, and the carboxy-terminus binds to actin, phospholipids, and paxillin-forming homodimers. The binding of vinculin to talin and actin is regulated by polyphosphoinositides and inhibited by acidic phospholipids. The complex then serves to anchor actin filaments to the membrane and thus, helps to reinforce force on talin within the focal adhesions.

The loss of vinculin impacts a variety of cell functions; it disrupts the formation of the complex, and prevents cell adhesion and spreading. The absence of the protein demonstrates a decrease in spreading of cells, accompanied by reduced stress fiber formation, formation of fewer focal adhesions, and inhibition of lamellipodia extension. It was discovered that cells that are deficient in vinculin have growth cones that advance more slowly, as well as filopodia and lamellipodia that were less stable than the wild-type. Based on research, it has been postulated that the lack of vinculin may decrease cell adhesion by inhibiting focal adhesion assembly and preventing actin polymerization. On the other hand, overexpression of vinculin may restore adhesion and spreading by promoting recruitment of cytoskeletal proteins to the focal adhesion complex at the site of integrin binding. Vinculin's ability to interact with integrins to the cytoskeleton at the focal adhesion appears to be critical for control of cytoskeletal mechanics, cell spreading, and lamellipodia formation. Thus, vinculin appears to play a key role in shape control based on its ability to modulate focal adhesion structure and function.

Activation

Vinculin is present in equilibrium between an active and inactive state. The active state is triggered upon binding to its designated partner. These changes occur when vinculin interacts with focal adhesion points to which it is binding to. When vinculin resides in its inactive form, the protein is kept designated to the cytoplasm unlike the focal adhesion points bound from the active state. The molecule talin is thought to be the major initiator of vinculin activation due to its presence in focal complexes. The combinatorial model of vinculin states that either α-actinin or talin can activate vinculin either alone or with the assistance of PIP2 or actin. This activation takes place by separation of the head-tail connection within inactive vinculin.

Binding site

VBS
human vinculin head (1-258) in complex with talin's vinculin binding site 3 (residues 1944-1969)
Identifiers
Symbol	VBS
Pfam	PF08913
InterPro	IPR015009
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Vinculin binding sites are predominantly found in talin and talin-like molecules, enabling binding of vinculin to talin, stabilising integrin-mediated cell-matrix junctions. Talin, in turn, links integrins to the actin cytoskeleton. The consensus sequence for Vinculin binding sites is LxxAAxxVAxxVxxLIxxA, with a secondary structure prediction of four amphipathic helices. The hydrophobic residues that define the VBS are themselves 'masked' and are buried in the core of a series of helical bundles that make up the talin rod.

Splice variants

Smooth muscles and skeletal muscles (and probably to a lower extent in cardiac muscle) in their well-differentiated (contractile) state co-express (along with vinculin) a splice variant carrying an extra exon in the 3' coding region, thus encoding a longer isoform meta-vinculin (meta VCL) of ~150KD molecular weight — a protein whose existence has been known since the 1980s. Translation of the extra exon causes a 68- to 79-amino acid acid-rich insert between helices I and II within the C-terminal tail domain. Mutations within the insert region correlate with hereditary idiopathic dilated cardiomyopathy.

The length of the insert in metavinculin is 68 AA in mammals and 79 in frog. Compared metavinculin sequences from pig, man, chicken, and frog, and found the insert to be bipartite: the first part variable and the second highly conserved. Both vinculin isoforms co-localize in muscular adhesive structures, such as dense plaques in smooth muscles, intercalated discs in cardiomyocytes, and costameres in skeletal muscles. Metavinculin tail domain has a lower affinity for the head as compared with the vinculin tail. In case of metavinculin, unfurling of the C-terminal hydrophobic hairpin loop of tail domain is impaired by the negative charges of the 68-amino acid insert, thus requiring phospholipid-activated regular isoform of vinculin to fully activate the metavinculin molecule.

Interactions

Vinculin has been shown to interact with:

• CDH1,
• Paxillin, and
• SORBS1.

In cases of Small Intestinal Bacterial Overgrowth presented as IBS symptoms, anti-CdtB antibodies have been identified to affect vinculin function, which is required in gut motility.

Further reading

• "Cytoskeletal proteins talin and vinculin in integrin-mediated adhesion". Biochemical Society Transactions 32 (Pt 5): 831–6. November 2004. doi:10.1042/BST0320831. PMID 15494027. 
• "An additional exon in the human vinculin gene specifically encodes meta-vinculin-specific difference peptide. Cross-species comparison reveals variable and conserved motifs in the meta-vinculin insert". European Journal of Biochemistry 204 (2): 767–72. March 1992. doi:10.1111/j.1432-1033.1992.tb16692.x. PMID 1339348. 
• "Complementary physical and genetic techniques map the vinculin (VCL) gene on chromosome 10q". Genomics 13 (4): 1347–9. August 1992. doi:10.1016/0888-7543(92)90066-2. PMID 1505973. 
• "Complete sequence of human vinculin and assignment of the gene to chromosome 10". Proceedings of the National Academy of Sciences of the United States of America 87 (15): 5667–71. August 1990. doi:10.1073/pnas.87.15.5667. PMID 2116004. 
• "Detection of metavinculin in human platelets using a modified talin overlay assay". European Journal of Cell Biology 49 (1): 202–6. June 1989. PMID 2503380. 
• "Primary sequence of paxillin contains putative SH2 and SH3 domain binding motifs and multiple LIM domains: identification of a vinculin and pp125Fak-binding region". Journal of Cell Science 107 ( Pt 6) (6): 1583–91. June 1994. doi:10.1242/jcs.107.6.1583. PMID 7525621. 
• "Molecular cloning of human paxillin, a focal adhesion protein phosphorylated by P210BCR/ABL". The Journal of Biological Chemistry 270 (10): 5039–47. March 1995. doi:10.1074/jbc.270.10.5039. PMID 7534286. 
• "Initial assessment of human gene diversity and expression patterns based upon 83 million nucleotides of cDNA sequence". Nature 377 (6547 Suppl): 3–174. September 1995. PMID 7566098. http://www.columbia.edu/itc/biology/pollack/w4065/client_edit/readings/nature377_3.pdf. 
• "Pattern formation and handedness in the cytoskeleton of human platelets". Proceedings of the National Academy of Sciences of the United States of America 90 (8): 3280–3. April 1993. doi:10.1073/pnas.90.8.3280. PMID 7682697. 
• "F-actin binding site masked by the intramolecular association of vinculin head and tail domains". Nature 373 (6511): 261–4. January 1995. doi:10.1038/373261a0. PMID 7816144. 
• "The intracellular distribution of vinculin and alpha 2 integrin in epithelial cells and chondrocytes". Scanning 16 (5): 275–84. 1995. doi:10.1002/sca.4950160306. PMID 7994488. 
• "Interaction of vinculin with the clathrin heavy chain". Journal of Biochemistry 114 (4): 498–503. October 1993. doi:10.1093/oxfordjournals.jbchem.a124206. PMID 8276759. 
• "Organization of the human gene encoding the cytoskeletal protein vinculin and the sequence of the vinculin promoter". The Journal of Biological Chemistry 268 (6): 4318–25. February 1993. doi:10.1016/S0021-9258(18)53612-7. PMID 8440716. 
• "Leukocyte adhesion to vascular endothelium induces E-selectin linkage to the actin cytoskeleton". The Journal of Cell Biology 133 (2): 445–55. April 1996. doi:10.1083/jcb.133.2.445. PMID 8609175. 
• "Developmental regulation of focal contact protein expression in human melanocytes". Pigment Cell Research 8 (4): 221–8. August 1995. doi:10.1111/j.1600-0749.1995.tb00667.x. PMID 8610074. 
• "Modulation of expression and assembly of vinculin during in vitro fibrillar collagen-induced angiogenesis and its reversal". Experimental Cell Research 224 (2): 215–23. May 1996. doi:10.1006/excr.1996.0131. PMID 8612698. 
• "Dilated cardiomyopathy associated with deficiency of the cytoskeletal protein metavinculin". Circulation 95 (1): 17–20. January 1997. doi:10.1161/01.cir.95.1.17. PMID 8994410. 
• "Monocyte cells and cancer cells express novel paxillin isoforms with different binding properties to focal adhesion proteins". The Journal of Biological Chemistry 272 (11): 7437–44. March 1997. doi:10.1074/jbc.272.11.7437. PMID 9054445. 
• "Vinculin is associated with the E-cadherin adhesion complex". The Journal of Biological Chemistry 272 (51): 32448–53. December 1997. doi:10.1074/jbc.272.51.32448. PMID 9405455. 
• "The epidermal growth factor receptor modulates the interaction of E-cadherin with the actin cytoskeleton". The Journal of Biological Chemistry 273 (15): 9078–84. April 1998. doi:10.1074/jbc.273.15.9078. PMID 9535896. 

External links

• Overview of all the structural information available in the PDB for UniProt: P18206 (Human Vinculin) at the PDBe-KB.
• Overview of all the structural information available in the PDB for UniProt: Q64727 (Mouse Vinculin) at the PDBe-KB.