Diversity in molecular recognition of phospholipids by PH domains: An examination of three unusual PH domain -phospholipid interactions

Membrane targeting of PH domains in vivo is generally a function of delocalized electrostatic attraction between the positively-charged PH surface and acidic, membrane-associated phospholipids, supplemented with high affinity interactions between specific phosphoinositides and conserved residues in the lipid-binding site of the PH domain. In instances where membrane targeting of PH domains is observed in vivo in the absence of high affinity phosphoinositide binding in vitro, an adjacent domain, oligomerization, or a membrane-associated protein ligand have been shown to be involved. Several members of the OSBP/FAPP1/Osh PH family were identified as PtdIns4P-specific domains in vitro and Golgi membrane-specific targeting modules in vivo. By performing in vitro phosphoinositide binding studies on the PH domains of OSBP, FAPP1, and Osh1p and solving the structure of PtdIns 4P-liganded and unliganded Osh1p PH, it is demonstrated that the PH domains of Osh1p and OSBP are promiscuous for phosphoinositides in vitro, and not PtdIns4P-specific as previously suggested. Furthermore, it is shown that delocalized electrostatic attraction alone can solely account for membrane surface binding in vitro. This Osh1p PH domain structure presented is the first of a PH domain complexed to a monophosphoinositide. Temporal and spatial regulation of 3-phosphoinositide production has an important impact in the recruitment of signaling molecules to specific cellular compartments. All 3-phosphoinositide-binding PH domains characterized to date are either PtdIns(3,4,5)P3-specific or dual-specific, PtdIns(3,4.5)P3/PtdIns(3,4) P2 binders, with the possible exception of TAPP1 PHn. We demonstrate that the PH domain of SH3BP2 is possibly the most PtdIns(3,4) P2-specific domain identified to date, including TAPP1 PHn. Finally, relatively modest differences in the structural features of PH domains that contribute to its unique properties can be exploited for therapeutic intervention. We reveal that the alkylphospholipid drug perifosine is a low affinity, Akt1/PKBα PH domain-specific competitive inhibitor in vitro