▪ Abstract Eleven distinct isoforms of phosphoinositide-specific phospholipase C (PLC), which are grouped into four subfamilies (β, γ, δ, and ∍), have been identified in mammals. These isozymes catalyze the hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] to inositol 1,4,5-trisphosphate and diacylglycerol in response to the activation of more than 100 different cell surface receptors. All PLC isoforms contain X and Y domains, which form the catalytic core, as well as various combinations of regulatory domains that are common to many other signaling proteins. These regulatory domains serve to target PLC isozymes to the vicinity of their substrate or activators through protein-protein or protein-lipid interactions. These domains (with their binding partners in parentheses or brackets) include the pleckstrin homology (PH) domain [PtdIns(3)P, βγ subunits of G proteins] and the COOH-terminal region including the C2 domain (GTP-bound α subunit of Gq) of PLC-β; the PH domain [PtdIns(3,4,5)P3] and Src homology 2 domain [tyrosine-phosphorylated proteins, PtdIns(3,4,5)P3] of PLC-γ; the PH domain [PtdIns(4,5)P2] and C2 domain (Ca2+) of PLC-δ; and the Ras binding domain (GTP-bound Ras) of PLC-∍. The presence of distinct regulatory domains in PLC isoforms renders them susceptible to different modes of activation. Given that the partners that interact with these regulatory domains of PLC isozymes are generated or eliminated in specific regions of the cell in response to changes in receptor status, the activation and deactivation of each PLC isoform are likely highly regulated processes.
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