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Abstract
One of the major phosphoproteins in synaptic plasma membranes (SPM) is the neuron-specific protein B-50 (Mr 48 kDa, IEP 4.5). Addition of purified protein kinase C (PKC) to native SPM increases B-50 phosphorylation. Exogenous PKC also phosphorylates B-50 in heat-inactivated SPM. Endogenous phosphorylation of B-50 in SPM is enhanced in a concentration-dependent manner by the tumor-promoting phorbol diesters 4β-phorbol 12-myristate,13-acetate, 4β-phorbol 12,13-dibutyrate (PDB) and 4β-phorbol 12,13-diacetate, with an EC50 of 7 × 10−8 M, 3 × 10−7 M and 10−6 M, respectively. This increase in the B-50 phosphorylation can be inhibited by ACTH1-24. PDB (10−6 M) also stimulates B-50 phosphorylation by exogenous PKC in native and heat-inactivated SPM (204 and 712%, respectively).
The increase in B-50 phosphorylation induced by the addition of PKC to SPM is accompanied by a decrease in the [32p]-incorporation into phosphatidylinositol 4,5-bisphosphate (PIP2). These data support the hypothesis that in neuronal membranes the degree of B-50 phosphorylation exerts a negative control on receptor-mediated hydrolysis of PIP2 in receptor systems coupled to phospholipase C.