Phospholipid independent phosphorylation of protamine by protein kinase C: effects of polyanions

The ability of purified protein kinase C (PKC) to phosphorylate protamine sulphate was found to be totally independent of phospholipid cofactors, whereas the phosphorylation of protamine free base was markedly increased by the presence of phosphatidylserine (PS). The hypothesis of an activation of PKC by the sulphate groups of protamine sulphate was confirmed by the high phosphorylation of protamine free base in the presence of non-peptide polyanionic compounds, such as glycoaminoglycans or polynucleotides. The catalytic fragment of PKC supported protamine base phosphorylation with the same polyanionic dependency. Light scattering intensity measurements showed that this phosphorylation correlated to the substrate/cofactor aggregation. These data support the view that apparent phospholipid-independent activation of PKC results from the formation of aggregates in the assay and this could result in the non-specific activation of this enzyme through its catalytic domain.