GM2 - activator is a protein that occurs in vertebrates and is essential in the breakdown of the gangliosides, special lipids in the body. This degradation takes place in lysosomes within the cells where the GM2 activator is found. GM2 activator allows the extraction of single GM2 molecules from the membrane, binds GM2 and presented it to the GM2 -degrading enzyme hexosaminidase A. In humans, mutations in the gene Gm2a the cause of GM2 gangliosidosis type AB.

GM2 and other lipids are not soluble in water. To be able to edit it with the ( water-soluble ) enzymes, they or their hydrophobic parts must be embedded in a hydrophobic bubble that is part of the GM2 - activator. GM2 - activator recognizes the sialic acid residues in gangliosides, such that only these particular lipids are taken up from the membrane.

Due to its properties of the GM2 - activator as a whole could act as a transport protein for glycosphingolipids. Increased expression of GM2 activator caused by oxidized LDL, could be the cause of local accumulation of glycosphingolipids in arteriosclerosis.