REGULATION OF SIALYLTRANSFERASE ACTIVITIES BY PHOSPHORYLATION AND DEPHOSPHORYLATION

The composition of tissue gangliosides is thought to result mainly fro m the active regulation and selective expression of specific enzymes r esponsible for their metabolism. In the last few years, we have purifi ed several rat brain sialyltransferases to homogeneity; the availabili ty of these highly purified enzymes enabled us to investigate their re gulation and expression at the molecular level. Thus, we studied the r egulation of sialyltransferase activities, in particular, CMP-NeuAc:GM 1 and CMP-NeuAc:LacCer sialyltransferases by a phosphorylation/dephosp horylation mechanism. Protein kinase C was added to a standard enzyme assay mixture containing [gamma-P-32]ATP, and the activity of the enzy me was measured after various incubation times, We found that treatmen t of several sialyltransferases by protein kinase C decreased their ac tivities in a time-dependent manner. Analyses of P-32-labeled amino ac ids revealed that the major phosphorylation site of CMP-NeuAc:GM1 alph a 2-->3 sialyrtransferase (ST-IV) was serine and that for GMPNeuAc:LaC cer alpha 2-->3 sialyltransferase (ST-I) was primarily threonine. Part ial recovery of the enzyme activity could be achieved by treatment of the phosphorylated sialyltransferases with rat brain protein phosphata se. We conclude that the activities of sialyltransferases can be modul ated by protein kinase C and protein phosphatase and this may represen t a potential regulatory mechanism for ganglioside biosynthesis.