Phospholipase C

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PIP2 cleavage to IP3 and DAG initiates intracellular calcium release and PKC activation.

Phospholipase C (PDB 1AH7, EC 3.1.4.3) is a key enzyme in phosphatidylinositol (PIP₂) metabolism and lipid signaling pathways. It is activated by either G_αq protein or Gβγ subunits (making it part of a G protein-coupled receptor signal transduction pathway) or by transmembrane receptors with intrinsic or associated tyrosine kinase activity.

It hydrolyzes PIP₂, a phosphatidylinositol, into two second messagers, inositol triphosphate (IP₃) and diacylglycerol (DAG), which then go on to modulate the activity of downstream proteins during cellular signalling, e.g. calcium channels in the endoplasmic reticulum of various cell types and protein kinase C, respectively.

The Phospholipase C family consists of 13 isozymes split between six subfamilies, PLC-δ, -β, -γ, -ε, -ζ, and the recently discovered -η isoform. The molecular weights of each being 85kDa for the δ form, 120-155kDa for both the β and γ forms, and 230-260kDa for the ε form. They all require calcium for catalytic activity, although the only isoform which is known to be inactive at basal intracellular calcium levels is the δ subfamily of enzymes.

PLC-δ is activated by high calcium levels and binding to PIP₂ through its PH domain. It is also thought to be the archetypical PLC.
PLC-β is activated by G protein subunits.
PLC-γ is activated by receptor tyrosine kinases.
PLC-ε is activated by Ras.
PLC-ζ is thought to play an important role in vertebrate fertilization, although, its mode of activation is currently unclear.
PLC-η has been found to be important for proper neuronal functioning.

All members of the family contain X and Y catalytic domains, SH2 (phosphotyrosine binding) domains are only found in the γ form, and only the ε form contains the RA (Ras Associating) domain. The -β, -δ and -γ forms all contain PH domains at their N-termini, however, the -β subfamily is distinguished from the others by the presence of a long C-terminal extension which is required for activation by G_αq subunits.