The hydrolysis of a minor membrane phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP2) by a specific phospholipase C (PLC) is one of the earliest key events in the regulation of various cell functions by more than 100 extracellular signaling molecules. This reaction produces two intracellular messengers, diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3), which mediate the activation of protein kinase C and intracellular calcium release, respectively. Furthermore, a decrease in the amount of PIP2 itself is likely an important signal because PIP2 is an activator for phospholipase D and phospholipase A2, modulates actin polymerization by interacting with various actin-binding proteins, and serves as a membrane-attachment site for many signaling proteins that contain pleckstrin homology (PH) domains. Consequently, the activity of PLC is strictly regulated in cells through several distinct mechanisms that link multiple PLC isoforms to various receptors.
The 12 mammalian PLC isozymes identified to date (excluding alternatively spliced forms) are all single polypeptides and can be divided into five types: β, γ, δ, ε and ζ, of which four PLC-β, two PLC-γ, four PLC-δ, one PLC-ε and one PLC-ζ proteins are known. Two regions of high sequence homology, designated X and Y, constitute the PLC catalytic domain. The β-, γ-, and δ-type isozymes all contain an NH2-terminal PH domain, an EF-hand domain located between the PH and X domains, and a C2 domain, which is sometimes represented as part of an extended Y domain. Whereas PLC-β and PLC-δ isozymes contain a short sequence of 50 to 70 amino acids that separates the X and Y regions, PLC-γ isozymes have a long sequence of ~400 amino acids that contains Src homology (two SH2 and one SH3) domains. PLC-ε, differs from the other three types of isozymes in that it possesses an NH2-terminal Ras guanine nucleotide exchange factor (RasGEF)–like domain and one or two COOH-terminal Ras binding (RA) domains. PLC-ζ has domain features similar to PLC-δ, but lacks the PH domain.
The receptor-mediated activation of PLC-β isozymes is achieved mainly via the α subunits of the Gq/11 subfamily of heterotrimeric G proteins or the Gβγ dimers. The region of PLC-β that interacts with Gaq/11 differs from that responsible for interaction with Gβγ. Binding of polypeptide growth factors (platelet-derived growth factor, epidermal growth factor, fibroblast growth factor) to their receptors results in activation of the intrinsic protein tyrosine kinase (PTK) activity that causes the phosphorylation of PLC-γ1 at tyrosines 771, 783 and 1254. Phosphorylation of tyrosine 783 was shown to be essential for the growth factor-dependent activation of PLC-γ1. Nonreceptor PTKs also phosphorylate and activate PLC-γ isozymes in response to the ligation of certain cell surface receptors listed in the table. These receptors, most of which comprise multiple polypeptide chains, do not themselves possess PTK activity, but activate a wide variety of nonreceptor PTKs such as the members of Src, Syk and Btk families.
The mechanism by which PLC-δ is coupled to membrane receptors remains unclear. PLC-ε can be activated by growth factors, Gα12/13 via the small G proteins Ras, Rap or Rho, and by Gβγ. All PLC isozymes are activated by calcium in vitro, but PLC-δ isozymes are more sensitive to calcium compared with the other isozymes. Furthermore, PLC-δ can be tethered to PIP2-containing membranes via its PH domain in the absence of other signals.
The Table below contains accepted modulators and additional information.