SOLUTION STRUCTURE OF THE PLECKSTRIN HOMOLOGY DOMAIN OF DROSOPHILA BETA-SPECTRIN

Background: The pleckstrin homology (PH) domain, which is approximatel y 100 amino acids long, has been found in about 70 proteins involved i n signal transduction and cytoskeletal function, a frequency comparabl e to SH2 (src homology 2) and SH3 domains. PH domains have been shown to bind the beta gamma-subunits of G-proteins and phosphatidylinositol 4,5-bisphosphate (PIP2). It is conceivable that the PH domain of beta -spectrin plays a part in the association of spectrin with the plasma membrane of cells. Results: We have solved the solution structure of t he 122-residue PH domain of Drosophila beta-spectrin. The overall fold consists of two antiparallel beta-sheets packing against each other a t an angle of approximately 60 degrees to form a beta beta-sandwich, a two-turn alpha-helix unique to spectrin PH domains, and a four-turn C -terminal alpha-helix. One of the major insertions in beta-spectrin PH domains forms a long, basic surface loop and appears to undergo slow conformational exchange in solution. This loop shows big spectral chan ges upon addition of D-myo-inositol 1,4,5-trisphosphate (IP3). Conclus ions: We propose that the groove at the outer surface of the second be ta-sheet is an important site of association with other proteins. This site and the possible lipid-binding site can serve to localize the sp ectrin network under the plasma membrane. More generally, it has to be considered that the common fold observed for the PH domain structures solved so far does not necessarily mean that all PH domains have simi lar functions. In fact, the residues constituting potential binding si tes for ligands or other proteins are only slightly conserved between different PH domains.