SOLUTION STRUCTURE OF A SYNDECAN-4 CYTOPLASMIC DOMAIN AND ITS INTERACTION WITH PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE

Syndecan-4, a transmembrane heparan sulfate proteoglycan, is a corecep tor with integrins in cell adhesion. It has been suggested to form a t ernary signaling complex with protein kinase C alpha and phosphatidyli nositol 4,5-bisphosphate (PLP2). Syndecans each have a unique, central , and variable (V) region in their cytoplasmic domains, and that of sy ndecan-4 is critical to its interaction with protein kinase C and PIP2 . Two oligopeptides corresponding to the variable region (4V) and whol e domain (4L) of syndecan-4 cytoplasmic domain were synthesized for nu clear magnetic resonance (NMR) studies. Data from NMR and circular dic hroism indicate that the cytoplasmic domain undergoes a conformational transition and forms a symmetric dimer in the presence of phospholipi d activator PIP2. The solution conformations of both free and PIP2-com plexed 4V have been determined by two-dimensional NMR spectroscopy and dynamical simulated annealing calculations. The 4V peptide in the pre sence of PIP2 formed a compact dimer with two twisted strands packed p arallel to each other and the exposed surface of the dimer consisted o f highly charged and polar residues. The overall three-dimensional str ucture in solution exhibits a twisted clamp shape having a cavity in t he center of dimeric interface. In addition, it has been observed that the syndecan-4V strongly interacts not only with fatty acyl groups bu t also the anionic head group of PIP2. These findings reveal that PIP2 promotes oligomerization of syndecan-4 cytoplasmic domain for transme mbrane signaling and cell-matrix adhesion.