Stimulation of phospholipase Cβ by membrane interactions, interdomain movement, and G protein binding — How many ways can you activate an enzyme?

Abstract

Signaling proteins are usually composed of one or more conserved structural domains. These domains are usually regulatory in nature by binding to specific activators or effectors, or species that regulate cellular location, etc. Inositol-specific mammalian phospholipase C (PLC) enzymes are multidomain proteins whose activities are controlled by regulators, such as G proteins, as well as membrane interactions. One of these domains has been found to bind membranes, regulators, and activate the catalytic region. The recently solved structure of a major region of PLC-β2 together with the structure of PLC-δ1 and a wealth of biochemical studies poises the system towards an understanding of the mechanism through which their regulations occurs.

Differential Regulation of Phospholipase C-β2 Activity and Membrane Interaction by Gαq, Gβ1γ2, and Rac2

“…PLC␦ 1 and the Dictyostelium discoideum CRAC protein) were found to undergo dynamic membrane-cytoplasm exchange along with lateral diffusion over short distances (35,36). Interestingly, we found a similar behavior for PLC␤ 2 (7), although this protein is likely to interact with the membrane through several distinct sites; these include the catalytic triosephosphate isomerase barrel, the C-terminal region, and possibly the EF hands motif and the C2 domain, as well as its PH domain, which, unlike that of PLC␦, appears to be unable to bind phosphoinositides (16,18).…”

“…In contrast, direct interaction of the PH domain with activated Rho GTPases is both necessary and sufficient for their stimulatory function (6,17,18). Functional evidence for a connection between PLC␤ 2 and Rho GTPases in cells is provided by the chemoattractant receptor system, which activates Rac/Cdc42 and PLC␤ 2 (19 -22).The mechanisms by which heterotrimeric G proteins and Rho GTPases regulate PLC␤ isozymes are only partially understood (16,23,24), especially in live cells. Because both PLC␤ substrate(s) and stimulators are membrane-bound (16,23,24), recruitment of PLC␤ from the cytoplasm to the membrane is clearly required for effective enzyme activation.…”

“…Recent results show that ␤␥ and Rac/Cdc42 activate PLC␤ 2 by interacting, at least in part, with different regions of the effector enzyme. Thus, although the pleckstrin homology (PH) domain of PLC␤ 2 is dispensable for activation of the enzyme by ␤␥ dimers (6), this domain also interacts with ␤␥, suggesting that the latter binds to at least two sites on the enzyme (16). In contrast, direct interaction of the PH domain with activated Rho GTPases is both necessary and sufficient for their stimulatory function (6,17,18).…”

“…The mechanisms by which heterotrimeric G proteins and Rho GTPases regulate PLC␤ isozymes are only partially understood (16,23,24), especially in live cells. Because both PLC␤ substrate(s) and stimulators are membrane-bound (16,23,24), recruitment of PLC␤ from the cytoplasm to the membrane is clearly required for effective enzyme activation.…”

“…Because both PLC␤ substrate(s) and stimulators are membrane-bound (16,23,24), recruitment of PLC␤ from the cytoplasm to the membrane is clearly required for effective enzyme activation. This view is consistent with the loss of PLC␤ activation following mutations that interfere with the membrane association of ␣ q or ␤␥ (25,26).…”