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Ganglioside GM2 Activator (Cerebroside sulfate activator protein, GM2-AP, Shingolipid activator protein 3, SAP-3, GM2AP)

Ganglioside GM2 activator protein (GM2AP) is product of the GM2A gene and consists of 170 amino acids. It is a small (20 kDa), heat stable, long-lived, protease-resistant glycosylated protein that normally resides in the lysozome. GM2AP is an essential cofactor to β-hexosaminidase A (Hex A) in the degradation of GM2 ganglioside to GM3 form. The protein mediates the interaction between the water-soluble exohydrolase and its membrane-embedded glycolipid substrate at the lipid–water interface. It also acts as a lipid transfer protein. Because of the presence of an N-terminal signal peptide, N-linked glycosylation, and disulfide bonds, it is clear that the activator is synthesized in the endoplasmic reticulum. The protein is first synthesized as a precursor which is then glycosylated, modified, and cleaved at 32Ser to the mature form. GM2AP contains at least three functional elements: a hydrophobic β-cup structure forming a spacious hydrophobic cavity, an oligosaccharide binding site, and an area that interacts with alpha subunit of HexA. Butkus and Coghlan revealed by northern analysis that GM2AP mRNA has a widespread distribution in ovine tissue. In the kidney, GM2 was expressed in all major renal arteries and arterioles. In the liver, the expression of the gene was prominent in the hepatic vein and duct. Antibodies raised against the GM2AP confirmed that the protein is present at the same sites as the mRNA. A deficiency of the GM2AP due to a mutation in respective gene can lead to the accumulation of GM2 in the lysosomes of primarily neuronal cells, where the synthesis of the more complex gangliosides is the greatest. This storage leads to neuronal cell death and one of three similar neurodegenerative diseases collectively known as GM2 gangliosidosis. On the other hand, up-regulation of GM2AP expression by oxidized LDL enhances ganglioside accumulation in the aorta, which may be strongly associated with atherosclerosis. Highashi et al observed that administration of GM2AP to mice significantly increased serum insulin levels and blockage of GM2AP function by specific antibodies inhibited insulin secretion. They also observed higher concentration of GM2AP in human serum of obese subjects by ELISA. Areas of investigation: - Diabetology - Metabolic syndrome - CNS disorders

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