PH Domains

PH Domains
Pleckstrin homology (PH) domains are about 120 amino acid-long protein modules that were first described in pleckstrin, the major protein kinase C substrate in platelets. PH domains have since been identified in several key regulatory proteins with characteristic structural features that include two orthogonal beta sheets that form alpha sandwich with an a helix at the COOH terminus, and variable loops that create a highly charged surface. It has been generally accepted that PH domains provide a structural basis for the interaction of certain regulatory proteins with membranes. Some PH domains bind with high affinity (low µM or nM K d ) to specific phosphoinositides such as phosphatidylinositol- 4,5-bisphosphate, PI-3,4-P2 or PI-3,4,5-P3. Binding to phosphoinositides may allow PH proteins to respond to lipid messengers for example by relocation to membranes. The C-termini of some PH domains have also been reported to bind the beta/gamma subunits of heterotrimeric G-proteins. For example, the PH domain of phospholipase C (PLC) delta binds inositol 1,4,5-tris-phosphate (Ins[1,4,5]-P3 ) and associates with lipid vesicles containing phosphatidylinositol 4,5-bisphosphate (PtdIns-[4,5] P2 ), but only weakly binds other inositol phosphates and PtdIns(4)P. Similarly, the PH domain of the Akt protein kinase appears to bind PtdIns(3,4)-P2 but not Ptd-Ins(4,5)-P2. Because different PH domains specifically bind specific phosphoinositides, PH domains (usually with a fluorescent tag like GFP) have widely used to label the metabolism and distribution of these phosphoinositides.
 

An example showing membrane translocation of GFP tagged PH domain (Akt) after application of PDGF