The precursor of sulfatide activator protein is processed to three different proteins

The enzymic degradation of a number of sphingolipids in the lysosomes is stimulated by small acid glycoproteins named activator proteins. We purified and sequenced a new protein, called component C, which seems to be related to sulfatide activator and to a recently described activator of glucosylceramidase (A1 activator) (Kleinschmidt, T., Christomanou, H. & Braunitzer, G. (1987) Biol. Chem. Hoppe-Seyler 368, 1571-1578). It consists of 78 amino acids and carries one carbohydrate chain at aparagine 20. Component C shows 21.5% sequence homology to sulfatide activator and 34.2% homology to A1 activator. Structural similarities between these three proteins have also been detected. Recently the cDNA sequence of the sulfatide activator precursor has been published (Dewji, N.N., Wenger, D.A. & O'Brien, J.S. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 8652-8656). We could align the protein sequences of sulfatide activator, A1 activator and component C with that of this large precursor protein. After minor corrections of the DNA sequence we obtained total fit. Thus it seems that three different proteins are derived from the sulfatide activator precursor by proteolytic processing. Possible processing sites were found on the precursor at sites adjacent to the N-termini and C-termini of the mature proteins. The processing of sulfatide activator was studied by Fujibayashi and Wenger (Fujibayashi, S. & Wenger, D.A. (1986) Biochim. Biophys. Acta 875, 554-562). Their data support our assumption that processing occurs by simultaneous cleavage at all possible sites.