The GM2 activator protein does not play a critical role in endosome and lysosome membrane fusion

A recent report (Kuwana, T., Mullock, B.M., and Luzio, J.P. (1993) Biochem Soc Trans 21, 299-300) presented data suggesting that the rat homologue of the human GM2 activator protein may stimulate the fusion of late endosomes with lysosomes. To evaluate this possibility we monitored the bulk endocytosis and lysosomal incorporation of horseradish peroxidase by normal human fibroblasts and by mutant fibroblasts which lack functional lysosomal GM2 activator protein. A comparison of the peroxidase as well as the endogenous hexosaminidase distributions between endosomal and lysosomal fractions from these two cell lines failed to reveal any quantitative differences. We conclude that the GM2 activator protein is not needed for efficient fusion of lysosomal and endosomal membranes in vivo.