The polybasic region that follows the plant homeodomain zinc finger 1 of Pf1 is necessary and sufficient for specific phosphoinositide binding.

The plant homeodomain (PHD) zinc finger is one of 14 known zinc-binding domains. PHD domains have been found in more than 400 eukaryotic proteins and are characterized by a Cys(4)-His-Cys(3) zinc-binding motif that spans 50-80 residues. The precise function of PHD domains is currently unknown; however, the PHD domains of the ING1 and ING2 tumor suppressors have been shown recently to bind phosphoinositides (PIs). We have recently identified a novel PHD-containing protein, Pf1, as a binding partner for the abundant and ubiquitous transcriptional corepressor mSin3A. Pf1 contains two PHD zinc fingers, PHD1 and PHD2, and functions to bridge mSin3A to the TLE1 corepressor. Here, we show that PHD1, but not PHD2, binds several monophosporylated PIs but most strongly to PI(3)P. Surprisingly, a polybasic region that follows the PHD1 is necessary for PI(3)P binding. Furthermore, this polybasic region binds specifically to PI(3)P when fused to maltose-binding protein, PHD2, or as an isolated peptide, demonstrating that it is sufficient for specific PI binding. By exchanging the polybasic regions between different PHD fingers we show that this region is a strong determinant of PI binding specificity. These findings establish the Pf1 polybasic region as a phosphoinositide-binding module and suggest that the PHD domains function down-stream of phosphoinositide signaling triggered by the interaction between polybasic regions and phosphoinositides.