Protein Domains New Tricks for the Old PH Domain

L. Bryan Ray

Science, Science Signaling, AAAS, Washington, DC 20005, USA

The pleckstrin homology (PH) domain binds to phosphoinositide products formed by activity of PI3K (phosphatidylinositol 3-kinase) and thus has been described as a modular domain that allows controlled membrane localization of various signaling molecules. Swanson et al. now show that the PH domain in Skap-hom (Src-kinase–associated phosphoprotein of 55 kD homolog), an adaptor protein that functions in linking integrins to cytoskeletal rearrangement, has a critical role in controlling targeting of Skap-hom to membrane ruffles, but by a different mechanism. Their structural studies of Skap-hom showed that the protein forms dimers through a dimerization domain that interacts with the PH domain in a way that alters the structure of the phosphoinositide-binding portion of the PH domain and prevents phosphoinositide binding. In mouse bone marrow macrophages, mutant Skap-hom proteins lacking the PH domain still bound to actin-rich ruffles, but treatment of the cells with a pharmacological inhibitor of PI3K caused dissociation of Skap-hom from the cytoskeleton. The authors reasoned that the PH domain might mediate an autoinhibitory domain interaction within Skap-hom that prevents association of Skap-hom with ruffles and that binding of 3'-phosphoinositides would relieve this inhibition. Indeed, mutations that disrupted both phosphoinositide binding by the PH domain and the intramolecular interaction with the dimerization domain yielded a protein whose association of Skap-hom to ruffles was similar to that of the wild-type protein. Thus, the authors propose that signaling through activity of PI3 kinases and phosphatases is likely to regulate association of Skap-hom with membrane ruffles, although it remains to be determined whether this PH domain–mediated molecular switch controls actual generation of the ruffles, targeting of Skap-hom to the ruffles, or both.

K. D. Swanson, Y. Tang, D. F. Ceccarelli, F. Poy, J. P. Sliwa, B. G. Neel, M. J. Eck, The Skap-hom dimerization and PH domains comprise a 3'-phosphoinositide-gated molecular switch. Mol. Cell 32, 564–575 (2008). [PubMed]