GM2-activator Protein crystal structure
[SAP3_HUMAN] Defects in GM2A are the cause of GM2-gangliosidosis type AB (GM2GAB) [MIM:272750]; also known as Tay-Sachs disease AB variant. GM2-gangliosidosis is an autosomal recessive lysosomal storage disease marked by the accumulation of GM2 gangliosides in the neuronal cells. GM2GAB is characterized by GM2 gangliosides accumulation in the presence of both hexosaminidase A and B.  
[SAP3_HUMAN] The large binding pocket can accommodate several single chain phospholipids and fatty acids, GM2A also exhibits some calcium-independent phospholipase activity (By similarity). Binds gangliosides and stimulates ganglioside GM2 degradation. It stimulates only the breakdown of ganglioside GM2 and glycolipid GA2 by beta-hexosaminidase A. It extracts single GM2 molecules from membranes and presents them in soluble form to beta-hexosaminidase A for cleavage of N-acetyl-D-galactosamine and conversion to GM3.
Publication Abstract from PubMed
The GM2-activator protein (GM2-AP) is a small lysosomal lipid transfer protein essential for the hydrolytic conversion of ganglioside GM2 to GM3 by beta-hexosaminidase A. The crystal structure of human apo-GM2-AP is known to consist of a novel beta-cup fold with a spacious hydrophobic interior. Here, we present two new structures of GM2-AP with bound lipids, showing two different lipid-binding modes within the apolar pocket. The 1.9A structure with GM2 bound shows the position of the ceramide tail and significant conformational differences among the three molecular copies in the asymmetric unit. The tetrasaccharide head group is not visible and is presumed to be disordered. However, its general position could be established through modeling. The structure of a low-pH crystal, determined at 2.5A resolution, has a significantly enlarged hydrophobic channel that merges with the apolar pocket. Electron density inside the pocket and channel suggests the presence of a trapped phospholipid molecule. Structure alignments among the four crystallographically unique monomers provide information on the potential role for lipid binding of flexible chain segments at the rim of the cavity opening. Two discrete orientations of the S130-T133 loop define an open and a closed configuration of the hydrophobic channel that merges with the apolar pocket. We propose: (i) that the low-pH structure represents an active membrane-binding conformation; (ii) that the mobile S130-T133 loop serves as a gate for passage of ligand into the apolar pocket; and (iii) that this loop and the adjacent apolar V59-W63 loop form a surface patch with two exposed tryptophan residues that could interface with lipid bilayers.
Structural analysis of lipid complexes of GM2-activator protein.,Wright CS, Zhao Q, Rastinejad F J Mol Biol. 2003 Aug 22;331(4):951-64. PMID:12909021
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.