Characterization of an Alternatively Spliced GM2 Activator Protein, GM2A Protein: AN ACTIVATOR PROTEIN WHICH STIMULATES THE ENZYMATIC HYDROLYSIS OF N-ACETYLNEURAMINIC ACID, BUT NOT N-ACETYLGALACTOSAMINE, FROM GM2

Abstract

GM2 activator protein is a protein cofactor which stimulates the enzymatic hydrolysis of both GalNAc and NeuAc from GM2. We have previously isolated two cDNA clones, GM2 activator cDNA and GM2A cDNA, for human GM2 activator protein (Nagarajan, S., Chen, H.-C., Li, S.-C., Li, Y.-T., and Lockyer, J. M. (1992) Biochem. J. 282, 807-813). GM2A mRNA is an RNA alternative splicing product that contains exons 1, 2, 3, and intron 3 of the genomic DNA sequence of GM2 activator protein (Klima, H., Tanaka, A., Schnabel, D., Nakano, T., Schröder, M., Suzuki, K., and Sandhoff, K. (1991) FEBS Lett. 289, 260-264). GM2A cDNA encodes a protein (GM2A protein) containing 1-109 of the 160 amino acids of human GM2 activator protein, plus a tripeptide (VST) encoded by intron 3 at the COOH terminus. Thus, GM2A protein can be regarded as a form (truncated version) of GM2 activator protein. We have expressed GM2A cDNA in Escherichia coli using pT7-7 as the vector. The recombinant GM2A protein was purified to an electrophoretically homogeneous form and was found to stimulate the hydrolysis of NeuAc from GM2 by clostridial sialidase, but not the hydrolysis of GalNAc from GM2 by beta-hexosaminidase A. Like GM2 activator protein, GM2A protein also specifically recognized the terminal GM2 epitope in GalNAc-GD1a and stimulated the hydrolysis of only the external NeuAc from this ganglioside by clostridial sialidase. These results enabled us to discern the enzymatic hydrolyses of GalNAc and NeuAc from the GM2 epitope and established that the NeuAc recognition domain of GM2 activator protein is located within amino acids 1-109. The presence of GM2A mRNA in human tissues and the selective stimulation of NeuAc hydrolysis by GM2A protein indicate that this activator protein may be involved in the catabolism of GM2 through the asialo-GM2 pathway.