Structure doi:10.1016/j.str.2013.12.018

Membrane phosphoinositides (PIs) are important signaling molecules that can bind selective recognition modules within proteins whose activities are dependent on membrane localization. For instance, PI(4)P is found in the eukaryotic Golgi and plasma membrane and is recognized by PH, PX, ENTH and GAT domains within adaptor complexes and lipid-transfer proteins for localization to those membrane subcompartments. DrrA, the Legionella pneumophila guanine nucleotide exchange factor for the Rab1 GTPase, co-opts vesicular traffic within host cells by using its unique PI(4)-binding module (P4M). To understand the basis for binding and exploitation of host processes by DrrA, Del Campo et al. used biochemistry and also solved the crystal structure of a DrrA segment containing the P4M domain with dibutyl PI(4). They found that among seven PIs, DrrA preferentially incorporated into liposomes containing PI(4)P. KD measurements indicated that both the head group and the diacylglycerol moiety of PI(4)P and protein-lipid interactions contribute to the affinity of the P4M for membranes. The PI(4)-bound structure helped explain the PI(4)P specificity, revealing a constricted and deep binding pocket, in which the 4-phosphate is coordinated by two basic and three polar residues. The authors could also predict a DrrA membrane insertion motif (MIM). Mutational analysis validated residues important for binding affinity, particularly ones involved in contacting the PI(4) head group, as well as the location of the PI(4)P-specific MIM. Although some parallels exist to eukaryotic systems and other PI-binding modules—for instance, with similarities between the DrrA MIM and PX elements found in eukaryotic proteins—these results provide new insights into the mechanisms of PI(4)P binding.