Signaling via growth factor receptors frequently results in the concomitant activation of phospholipase Cγ (PLCγ) and phosphatidylinositol (PI) 3-kinase. While it is well established that tyrosine phosphorylation of PLCγ is necessary for its activation, we show here that PLCγ is regulated additionally by the lipid products of PI 3-kinase. We demonstrate that the pleckstrin homology (PH) domain of PLCγ binds to phosphatidylinositol 3,4,5-trisphosphate [PdtIns(3,4,5)P 3 ], and is targeted to the membrane in response to growth factor stimulation, while a mutated version of this PH domain that does not bind PdtIns(3,4,5)P 3 is not membrane targeted. Consistent with these observations, activation of PI 3-kinase causes PLCγ PH domain-mediated membrane targeting and PLCγ activation. By contrast, either the inhibition of PI 3-kinase by overexpression of a dominant-negative mutant or the prevention of PLCγ membrane targeting by overexpression of the PLCγ PH domain prevents growth factor-induced PLCγ activation. These experiments reveal a novel mechanism for cross-talk and mutual regulation of activity between two enzymes that participate in the control of phosphoinositide metabolism.

The GGAs are a multidomain protein family implicated in protein trafficking between the Golgi and endosomes. Here, the VHS domain of GGA2 was shown to bind to the acidic cluster-dileucine motif in the cytoplasmic tail of the cation-independent mannose 6-phosphate receptor (CI-MPR). Receptors with mutations in this motif were defective in lysosomal enzyme sorting. The hinge domain of GGA2 bound clathrin, suggesting that GGA2 could be a link between cargo molecules and clathrin-coated vesicle assembly. Thus, GGA2 binding to the CI-MPR is important for lysosomal enzyme targeting.

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Src-homology 3 is a small protein domain of about GO amino acid residues. It is probably made of p-sheets. SI-13 is prcscn~ in a large number of eukaryotic proteins which arc involved in sigma' A transduction, cell polnrizntion and membranc-cytoskcleton interactions. Here WC review its occurrence and discuss possible functions of this domain.