Structural Studies on the PH Domains of Dbl, Sos1, IRS-1, and .beta.ARK1 and Their Differential Binding to G.beta..gamma. Subunits

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Abstract
Pleckstrin homology (PH) domains are approximately 110 amino acid residues in length and are structurally conserved in a number of intracellular signaling proteins. A role for these domains has been postulated for beta ARK, which binds to G beta gamma subunits. We have quantified the binding of individual (His)6-tag PH domains of human Db1, human Sos1, rat IRS-1, human beta ARK, and human beta ARK with an extra 33-residue C-terminal extension (beta ARK + C) to G beta gamma subunits. Our in vitro binding studies show that all of the PH domains (apart from Sos1), bind G beta gamma subunits in a dose-dependent manner, but beta ARK + C binds 4 times as much G beta gamma at saturation as the others. The IRS-1 PH domain has a similar half-maximal concentration of G beta gamma binding (18 nM) to beta ARK + C (30 nM), suggesting that the IRS-1 PH domain has sufficient determinants for G beta gamma binding. The beta ARK PH domain alone has a half-maximal value of 45 nM but a drastically reduced extent of G beta gamma binding, suggesting that both the PH domain and the C-terminal 33 residues are necessary for maximal binding. Db1 has a half-maximum concentration of G beta gamma binding of 45 nM and a maximal extent of binding similar to that of beta ARK, but it is difficult to demonstrate saturable binding of G beta gamma to Sos1. Since it was previously predicted that the C-terminal PH domain of Pleckstrin [Tyers, M., et al. (1988) Nature 333, 470-473] contains a potential calcium binding site, we have tested the different PH domains for calcium binding. Only the PH domain of Db1 bound 45Ca2+ with a Kd of 10 microM. CD spectroscopy of the purified recombinant PH domains indicated that they are predominantly beta-sheet structures.(ABSTRACT TRUNCATED AT 250 WORDS)

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