Biology:SCARB2

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A representation of the 3D structure of the protein myoglobin showing turquoise α-helices.
Generic protein structure example

Lysosomal integral membrane protein 2 (LIMP-2) is a protein that in humans is encoded by the SCARB2 gene.[1] LIMP-2 is expressed in brain, heart, liver, lung and kidney, mainly in the membrane of lysosome organelles; however, in cardiac muscle, LIMP-2 is also expressed at intercalated discs. LIMP-2 in a membrane protein in lysosomes that functions to regulate lysosomal/endosomal transport. Mutations in LIMP-2 have been shown to cause Gaucher disease, myoclonic epilepsy, and action myoclonus renal failure syndrome. Abnormal levels of LIMP-2 have also been found in patients with hypertrophic cardiomyopathy.

Structure

Human LIMP-2 has a theoretical molecular weight of 54.3 kDa and is 478 amino acids in length.[2]

Though LIMP-2 was initially discovered in 1985 by Lewis et al. from rat liver lysosomes,[3] LIMP-2 was cloned in 1992 by two groups, one isolated LIMP-2 from human metastatic pancreatic islet tumor cells, and one from rat liver lysosomal membranes.[4][5] LIMP-2 was isolated as a protein of approximate molecular weight 85 kDa, synthesized from a precursor oform of approximately 77 kDa. The weight discrepancy between its theoretical (54.3 kDa) and observed (85 kDa) is due to the presence of 10 high mannose-type N-linked oligosaccharide chains in the human form of this protein, compared to 11 in mouse and rat.[6] LIMP-2 has two hydrophobic regions, one near the N-terminus and one near the C-terminus, as well as a short isoleucine/leucine-rich cytoplasmic tail consisting of 20 amino acids that serves as the lysosomal targeting sequence.[7][8] LIMP-2 has been shown to be expressed in brain, heart, liver, lung and kidney.[6]

Function

The protein encoded by this gene is a type III glycoprotein that is located primarily in limiting membranes of lysosomes and endosomes. Studies of the similar protein in mice and rat suggested that this protein may participate in membrane transportation and the reorganization of endosomal/lysosomal compartment.[9] In rat hepatic cells, LIMP-2 exhibited a half-life for internalization and lysosomal transport of 32 min and 2.0 h, respectively, which resembled that of well-known lysosomal proteins, lamp-1 and lamp-2, albeit different amino acid sequences in their cytoplasmic tails.[10]

LIMP2 has recently been identified as a novel component of intercalated discs in cardiac muscle. Intercalated discs are composed of gap junctions, adherens junctions and desmosomes, and are critical for the mechanical and electrical coupling of adjacent cardiomyocytes. The discovery of LIMP-2 as a component of this complex came about from a genetic screen of a homozygous, hypertensive transgenic rat model of renin overexpression, in which a population of these rats rapidly develop heart failure and another remains compensated.[11] Out of 143 differentially-regulated genes, LIMP-2 was identified to be significantly upregulated in heart failure-prone rat cardiac muscle biopsies, which also proved true in human heart failure. Further analysis employing a LIMP-2 knockout mouse demonstrated that animals lacking LIMP-2 failed to flight a normal hypertrophic response following Angiotensin II treatment, however they developed interstitial fibrosis and dilated cardiomyopathy coordinate with disrupted intercalated disc structure. Biochemical and immunohistochemical analyses discovered that LIMP-2 interacts with N-cadherin at intercalated discs, a function outside of lysosomal membranes. Knockdown of LIMP-2 with RNA interference decreased the binding of N-cadherin to the phosphorylated form of beta-catenin, and LIMP-2 overexpression had the reverse effect.[12]

LIMP-2 plays other roles in other organs. Characteristic tubular proteinuria observed in LIMP-2 knockout mice has been shown to be due to a failure of in lysosomal/endosomal fusion, thus proteins reabsorbed in the proximal tubule of the kidney are not properly proteolyzed, causing the proteinuria.[13] Deficiency of LIMP-2 in mice was also reported to impair cell membrane transport processes and cause pelvic junction obstruction, deafness, and peripheral neuropathy.[14]

Clinical significance

In patients with hypertrophic cardiomyopathy due to aortic stenosis, SCARB2 mRNA is significantly upregulated, suggesting that LIMP-2 may act as a hypertrophic marker.[12]

Mutations in SCARB2 have been shown to cause action myoclonus renal failure syndrome, a rare syndrome characterized by progressive neurological disease and associated with proteinuria, kidney failure, and Focal segmental glomerulosclerosis.[15][16][17]

Mutations in SCARB2 have also been shown to cause Gaucher disease and myoclonic epilepsy,[18] as LIMP-2 is critical for the proper sorting and targeting of glucocerebrosidase enzyme (the enzyme deficient in Gaucher disease) to lysosomes.

SCARB2 is a receptor for two viruses that cause hand, foot, and mouth disease in children, Enterovirus 71 and Coxsackievirus A16.[19]

Interactions

LIMP-2 has been shown to interact with:

References

  1. ↑ "Entrez Gene: SCARB2 scavenger receptor class B, member 2". https://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=950. 
  2. ↑ "Protein sequence of human SCARB2 (Uniprot ID: Q14108)". http://www.heartproteome.org/copa/ProteinInfo.aspx?QType=Protein%20ID&QValue=Q14108. 
  3. ↑ "Glycoproteins of the lysosomal membrane". The Journal of Cell Biology 100 (6): 1839–47. Jun 1985. doi:10.1083/jcb.100.6.1839. PMID 3922993. 
  4. ↑ "Isolation and sequencing of a cDNA clone encoding the 85 kDa human lysosomal sialoglycoprotein (hLGP85) in human metastatic pancreas islet tumor cells". Biochemical and Biophysical Research Communications 184 (2): 604–11. Apr 1992. doi:10.1016/0006-291X(92)90632-U. PMID 1374238. 
  5. ↑ "Isolation and characterization of a novel membrane glycoprotein of 85,000 molecular weight from rat liver lysosomes". Chemical & Pharmaceutical Bulletin 40 (1): 170–3. Jan 1992. doi:10.1248/cpb.40.170. PMID 1576668. 
  6. ↑ 6.0 6.1 "Identification and characterization of a major lysosomal membrane glycoprotein, LGP85/LIMP II in mouse liver". Journal of Biochemistry 122 (4): 756–63. Oct 1997. doi:10.1093/oxfordjournals.jbchem.a021820. PMID 9399579. 
  7. ↑ "Lysosomal targeting of Limp II membrane glycoprotein requires a novel Leu-Ile motif at a particular position in its cytoplasmic tail". The Journal of Biological Chemistry 269 (7): 5210–7. Feb 1994. doi:10.1016/S0021-9258(17)37676-7. PMID 8106503. 
  8. ↑ "The residues Leu(Ile)475-Ile(Leu, Val, Ala)476, contained in the extended carboxyl cytoplasmic tail, are critical for targeting of the resident lysosomal membrane protein LIMP II to lysosomes". The Journal of Biological Chemistry 269 (9): 6622–31. Mar 1994. doi:10.1016/S0021-9258(17)37418-5. PMID 7509809. 
  9. ↑ "Lysosomal integral membrane protein-2: a new player in lysosome-related pathology". Molecular Genetics and Metabolism 111 (2): 84–91. Feb 2014. doi:10.1016/j.ymgme.2013.12.005. PMID 24389070. 
  10. ↑ "Cycling of an 85-kDa lysosomal membrane glycoprotein between the cell surface and lysosomes in cultured rat hepatocytes". Journal of Biochemistry 116 (3): 670–6. Sep 1994. PMID 7852289. 
  11. ↑ "Thrombospondin-2 is essential for myocardial matrix integrity: increased expression identifies failure-prone cardiac hypertrophy". Circulation Research 95 (5): 515–22. Sep 2004. doi:10.1161/01.RES.0000141019.20332.3e. PMID 15284191. 
  12. ↑ 12.0 12.1 12.2 "Lysosomal integral membrane protein 2 is a novel component of the cardiac intercalated disc and vital for load-induced cardiac myocyte hypertrophy". The Journal of Experimental Medicine 204 (5): 1227–35. May 2007. doi:10.1084/jem.20070145. PMID 17485520. 
  13. ↑ "Tubular proteinuria in mice and humans lacking the intrinsic lysosomal protein SCARB2/Limp-2". American Journal of Physiology. Renal Physiology 300 (6): F1437–47. Jun 2011. doi:10.1152/ajprenal.00015.2011. PMID 21429972. https://semanticscholar.org/paper/043036feba45697fe4277754ffb24b0d50469433. 
  14. ↑ "LIMP-2/LGP85 deficiency causes ureteric pelvic junction obstruction, deafness and peripheral neuropathy in mice". Human Molecular Genetics 12 (6): 631–46. Mar 2003. doi:10.1093/hmg/ddg062. PMID 12620969. 
  15. ↑ "A nonsense mutation in the LIMP-2 gene associated with progressive myoclonic epilepsy and nephrotic syndrome". Human Molecular Genetics 17 (14): 2238–43. Jul 2008. doi:10.1093/hmg/ddn124. PMID 18424452. 
  16. ↑ "Array-based gene discovery with three unrelated subjects shows SCARB2/LIMP-2 deficiency causes myoclonus epilepsy and glomerulosclerosis". American Journal of Human Genetics 82 (3): 673–84. Mar 2008. doi:10.1016/j.ajhg.2007.12.019. PMID 18308289. 
  17. ↑ "Novel SCARB2 mutation in action myoclonus-renal failure syndrome and evaluation of SCARB2 mutations in isolated AMRF features". BMC Neurology 11: 134. 27 October 2011. doi:10.1186/1471-2377-11-134. PMID 22032306. 
  18. ↑ "A mutation in SCARB2 is a modifier in Gaucher disease". Human Mutation 32 (11): 1232–8. Nov 2011. doi:10.1002/humu.21566. PMID 21796727. 
  19. ↑ "Scavenger receptor B2 is a cellular receptor for enterovirus 71". Nature Medicine 15 (7): 798–801. Jul 2009. doi:10.1038/nm.1992. PMID 19543282. 

Further reading




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