Molecular Mechanisms Involved in the Actions of Calcium-medtated Hormones 
phospholipase C. Possible functions of the larger 
protein are being examined. 
The second major research area arose from the 
observation that hormones and neurotransmitters 
that break down PIP2 in their target cells also 
break down another cell membrane phospho- 
lipid, phosphatidylcholine (PC). This break- 
down yields DAG and phosphatidic acid (PA) and 
involves two other phospholipases (C and D), 
which are being purified and characterized. Fur- 
ther work has shown that PC breakdown is regu- 
lated in several ways: by mechanisms involving G 
proteins, Ca^^, protein kinase C, and tyrosine ki- 
nases. In vivo and in vitro approaches are being 
used to explore these mechanisms. 
The third major research area involves studies 
of the actions and metabolism of PA in cells. This 
work has shown that high concentrations of PA 
accumulate rapidly in cells stimulated with many 
hormones, neurotransmitters, and growth fac- 
tors. Although it is widely believed that PA acts as 
a second messenger, its cellular targets and func- 
tions remain unknown. We have recently identi- 
fied a protein kinase that is activated by PA and 
may therefore mediate its cellular effects. This 
en2yme is being purified and characterized. 
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