Biosynthesis, processing, and intracellular transport of G(M2) activator protein in human epidermal keratinocytes - The lysosomal targeting of the G(M2) activator is independent of a mannose-6-phosphate signal

The processing, intracellular transport, and endocytosis of the G(M2) activator protein (G(M2)AP), an essential cofactor of beta-hexosaminidase A for the degradation of ganglioside G(M2), was investigated in human epidermal keratinocytes. The G(M2)AP precursor is synthesized as an 18-kDa peptide, which is singly glycosylated, resulting in 22-kDa high mannose and 24-27-kDa complex glycoforms. A small portion of the 22-kDa form bears phosphomannosyl residues, About 30% of the G(M2)AP precursor is secreted during 12 h after synthesis, consisting almost exclusively of complex glycoforms. in a post-Golgi compartment, the intracellular remainder is converted to a 20-kDa mature form within 24 h, bearing a heavily trimmed N-glycan on a 17-kDa backbone, Interestingly, even nonglycosylated G(M2)AP is delivered to the lysosome, as shown by tunicamycin treatment and subcellular fractionation, Also, its endocytosis is independent of carbohydrate-linked signals and is even more effective for nonglycosylated G(M2)AP. We conclude that a mannose-6-phosphate-independent pathway for the lysosomal delivery of G(M2)AP exists in cultured human keratinocytes.