Molecular Mechanisms Involved in the Actions 
of Calcium-mediated Hormones 
GTP analogue. These findings demonstrate the 
complete reconstruction of a signaling pathway 
for a calcium-mediated agonist from purified 
components. 
The second major research area arose from the 
observation that many hormones, neurotransmit- 
ters, and growth factors break down another cell 
membrane phospholipid, phosphatidylcholine 
(PC), in a wide variety of cells. This breakdown 
yields DAG and phosphatidic acid (PA) and in- 
volves two other phospholipases (C and D), 
which are being purified and characterized. Fur- 
ther work has shown that PC breakdown is regu- 
lated by mechanisms involving G proteins, cal- 
cium ions, protein kinase C, and tyrosine kinases. 
The latter two mechanisms are currently being 
explored in several cell types. 
In a related area, the regulation of different iso- 
zymes of protein kinase C by certain molecular 
species of DAG and PA is being studied. This in- 
volves the purification of a protein kinase C iso- 
zyme that is stimulated by PA but not DAG. These 
studies should elucidate the functions and mecha- 
nisms of this novel signaling system. 
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