1. PLC-beta1 is easily cleaved by calpain and generates a 100-Kda enzyme that lacks the carboxyl-terminal 336 resiudes from Ser-881. The 100-Kda enzyme was catalytically as active as the intact PLC-beta1, but lost the capacity to be activated by alphaq. 2. Two new beta-type PLC isozymes, PLC-beta3 and PLC-beta4, were identified at both protein and DNA levels. 3. Studies of the activation of PLC-beta isozymse by alpha subunits of Gq class G proteins revealed that the extent of activation decreased in the order of PLC-beta1 > PLC-beta3 >> PLC-beta2, suggesting a certain degree of specificity in the interaction of Gqalpha subunits with different PLC-beta isozymes. 4. The beta gamma subunits of G proteins activated the beta-type, but not the gamma-type and delta-type PLC isozymes. The rank order for extent of activation was PLC-beta > PLC-beta2 > PLC-beta1 at all Ca2+ concentrations. The results suggest that beta gamma subunits might represent the active components of the long-sought pertussis-sensitive G proteins. 5. Activation of FcgammaRIII, the receptor responsible for the antibody- dependent cellular cytotoxicity in natural killer cells, is coupled to a nonreceptor protein tyrosine kinase, which phosphorylates and thereby activates PLC-gamma1 and PLC-gamma2. 6. Five members of the src-family tyrosine kinase (lck, lyn, hck, fyn, and src) phosphorylated PLC-gamma1 and PLC-gamma2 in vitro without any distinct specificity between PLC-gamma1 and PLC-gamma2, or between the five kinases. 7. The 47-Kda Nck was shown to be a target for a variety of protein kinases that might modulate the postulated role of Nck as an adaptor for the physical and functional coordination of signaling proteins.

National Institute of Health (NIH)
National Heart, Lung, and Blood Institute (NHLBI)
Intramural Research (Z01)
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National Heart, Lung, and Blood Institute
United States
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