Pleckstrin homology (PH) domains in signal transduction

An intact N terminus of the gamma subunit is required for the Gbetagamma stimulation of rhodopsin phosphorylation by human beta-adrenergic receptor kinase-1 but not for kinase binding

Cleavage after lysine 32 in the Ggamma2 subtype and after lysine 36 in the Ggamma3 subtype of purified mixed brain Gbetagamma by endoproteinase Lys-C blocks Gbetagamma-mediated stimulation of phosphorylation of rhodopsin in urea-extracted rod outer segments by recombinant human beta-adrenergic receptor kinase (hbetaARK1) holoenzyme while hbetaARK1 binding to rod outer segments is partially affected. This treatment does not attenuate the binding of the treated Gbetagamma to C-terminal fragments of hbetaARK1 containing the pleckstrin homology domain. Lys-C proteolysis also does not alter the association of the Gbetagamma with phospholipids, its ability to support pertussis toxin-catalyzed Galphao/Galphai ADP-ribosylation, or its ability to inhibit forskolin-stimulated platelet adenylate cyclase. The Gbeta subunit remains noncovalently associated with the cleaved Ggamma fragments. Thus, in addition to recruiting hbetaARK1 to its receptor substrate, Ggamma contributes secondary and/or tertiary structural features to activate the kinase.

The pleckstrin homology domain: an intriguing multifunctional protein module

Pleckstrin homology (PH) domains are a family of compact protein modules defined by sequences of roughly 100 amino acids. These domains are common in vertebrate, Drosophila, C. elegans and yeast proteins, suggesting an early origin and fundamental importance to eukaryotic biology. Many enzymes which have important regulatory functions contain PH domains, and mutant forms of several such proteins are implicated in oncogenesis and developmental disorders. Numerous recent studies show that PH domains bind various proteins and inositolphosphates. Here I discuss PH domains in detail and conclude that they form a versatile family of membrane binding and protein localization modules.

Pleckstrin homology domains: a fact file

Structures of three different pleckstrin homology domains have been determined within the past year. They have a common core consisting of a seven-stranded and strongly bent beta-sheet and a C-terminal alpha-helix that packs against the beta-sheet. Phosphatidylinositol 4,5-bisphosphate and related compounds specifically bind to pleckstrin homology domains, suggesting that the domain may be involved in reversible anchorage to membranes or in recognition of a second messenger, such as inositol 1,4,5-trisphosphate. Pleckstrin homology domains have also been suggested to bind to the G beta gamma complex, but direct evidence for this is missing.

Developmental regulation of expression and activity of multiple forms of the Drosophila RAC protein kinase

We have characterized the Drosophila homologue of the proto-oncogenic RAC protein kinase (DRAC-PK). The DRAC-PK gene gives rise to two transcripts with the same coding potential, generated by the use of two different polyadenylation signals. Each transcript encodes two polypeptides because of the presence of a weaker initiator ACG codon, upstream from the major AUG, such that the larger protein contains an N-terminal extension. Like the human isoforms, DRAC-PKs possess a novel signaling region, the pleckstrin homology domain. DRAC-PK proteins have a similar expression pattern, being regulated both maternally and zygotically, and are expressed throughout Drosophila development. Antisera specific for recombinant DRAC-PK and for its C terminus detected two polypeptides of 66 and 85 kDa in Drosophila extracts. The antirecombinant antisera also recognized a polypeptide of 120 kDa from Drosophila, which apparently shared an epitope related to DRAC-PK sequences. The role of p120 appears to be restricted compared with that of DRAC-PK, since it was not detected in larvae or adult flies. There was no spatial restriction of DRAC-PK expression during embryogenesis, suggesting that localized activation might be a regulatory mechanism for its function. DRAC-PK possesses an intrinsic kinase activity that is approximately 8-fold higher in adult flies than in 0-3-h embryos undergoing rapid mitotic cycles.