Molecular Mechanisms of Insulin and Polypeptide Hormone Action 
stimulate the rapid breakdown of membrane lip- 
ids, generating intracellular signals that activate 
an intracellular serine/threonine kinase known 
as protein kinase C, and increase concentrations 
of intracellular calcium. Activation of protein ki- 
nase C leads to the phosphorylation of many in- 
tracellular proteins and subsequent metabolic 
processes, including cell growth, hormone bio- 
synthesis, and neurotransmission. We have been 
studying one prominent substrate for protein ki- 
nase C, the myristoylated alanine-rich C kinase 
substrate (MARCKS) protein, which is phosphor- 
ylated within seconds of protein kinase C activa- 
tion in intact cells. Recent studies have suggested 
a role for this protein in regulation of the cellular 
availability of calmodulin, a calcium-binding 
protein that can activate a number of important 
enzymes that respond to changes in intracellular 
calcium. These studies may lead to the biochemi- 
cal characterization of a link between the protein 
kinase C and calcium pathways activated by sev- 
eral polypeptide growth factors. 
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