Regulation of sialyltransferase activities by phosphorylation and dephosphorylation

The composition of tissue gangliosides is thought to result mainly from the active regulation and selective expression of specific enzymes responsible for their metabolism. In the last few years, we have purified several rat brain sialyltransferases to homogeneity; the availability of these highly purified enzymes enabled us to investigate their regulation and expression at the molecular level. Thus, we studied the regulation of sialyltransferase activities, in particular, CMP-NeuAc:GM1 and CMP-NeuAc:LacCer sialyltransferases by a phosphorylation/dephosphorylation mechanism. Protein kinase C was added to a standard enzyme assay mixture containing [gamma-32P]ATP, and the activity of the enzyme was measured after various incubation times. We found that treatment of several sialyltransferases by protein kinase C decreased their activities in a time-dependent manner. Analyses of 32P-labeled amino acids revealed that the major phosphorylation site of CMP-NeuAc:GM1 alpha 2-->3 sialyltransferase (ST-IV) was serine and that for CMP-NeuAc:LacCer alpha 2-->3 sialyltransferase (ST-I) was primarily threonine. Partial recovery of the enzyme activity could be achieved by treatment of the phosphorylated sialyltransferases with rat brain protein phosphatase. We conclude that the activities of sialyltransferases can be modulated by protein kinase C and protein phosphatase and this may represent a potential regulatory mechanism for ganglioside biosynthesis.