Fused Ig-PH domain of plant-specific actin-binding protein
This family is a fused Ig and PH domain found on plant-specific actin-binding proteins or SCABs. SCAB proteins bind, bundle and stabilize actin filaments and regulate stomatal movement. The Ig-PH fusion domain is at the C-terminus. This domain has the N-terminal Ig beta-sandwich fold consisting of two antiparallel beta-sheets built from strands beta1 and beta2 and strands beta3-beta6, respectively. The C-terminus of the fused domains adopts the PH fold, of seven beta-strands, beta7-beta13 and two alpha-helices, alpha1 and alpha2 arranged into a beta-barrel. The Ig and PH domains appear to be truly fused together into an integral structure which displays a few conserved patches on the surface, particularly of the PH part. The canonical phosphoinositide-binding pocket of the classic PH domain is degenerate in this fused one, and the charge on the pocket suggest that the Ig-PH domain contains a non-canonical binding site for inositol phosphates. There are a handful of bacterial members at low threshold but they are missing the PH part of the fused domain, and appear to match little else.