Effector enzymes coupled to GTP binding proteins

A separate class of cell surface ADP-ribosylases targets the integrins, defensins, and the purinergic receptor P2X7.45

Phospholipase C

First hints of a signalling role for inositol phospholipids

The first hints of a role for the inositol-containing phospholipids in cell regulation emerged in 1953,46 but 20 years elapsed before a viable proposal concerning their role was forthcoming. Michell47 then pointed to the striking correlation between the activation of receptors that cause an increase in the metabolism

of phosphatidylinositol (PI) and activation of processes such as secretion and the contraction of smooth muscle that are dependent on Ca2 . Although a phospholipase C (PLC)-catalysed reaction was inferred, the precise identities of the substrate and the resulting second messenger(s) remained elusive. Then it was found that the rate of depletion of the polyphosphoinositides, in particular phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) (Figure 5.9) and not phosphatidylinositol itself, correlates with the onset of cellular activation.48 The

water-soluble product IP3, resulting from the hydrolysis of PI(4,5)P2, was found to release Ca2 from intracellular storage sites when introduced into permeabilized pancreatic cells.49 Within a very short time it was shown that this mechanism is ubiquitous and that the immediately accessible Ca2 stores are present in the endoplasmic reticulum which is endowed with receptors for IP3 (see Chapter 7). The hydrophobic product of PI(4,5)P2 breakdown is diacylglycerol (DAG) and this is retained in the membrane bilayer. It causes the activation of protein kinase C (see Figure 7.10, page 199 and Chapter 9).

The phospholipase family

Phospholipids possessing a glycerol backbone contain four ester bonds, all of which are potentially susceptible to enzyme-catalysed hydrolysis by specific phospholipases (Figure 5.10). Hydrolysis at three of these positions gives rise to products that are either second messengers or substrates for enzymes that yield further signalling molecules.

PLA2 and PLD

Phospholipase A2 (PLA2) hydrolyses mostly phosphatidylcholine yielding a lysophospholipid and releasing the fatty acid bound to the 2-position, generally arachidonate. Arachidonate is the substrate for the formation of prostaglandins and leukotrienes (potent signalling molecules with

autocrine and paracrine effects, involved in pathological processes such as inflammation). Phospholipase D (PLD) provides another, less transient source of DAG by cleaving phosphatidylcholine (the major membrane phospholipid) to form phosphatidate and water-soluble choline. Removal of the phosphate by phosphatidate phosphohydrolase produces DAG.

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