Studying the protein-protein interactions in the postsynaptic density by means of immunoabsorption and chemical crosslinking

Postsynaptic densities (PSDs), isolated from porcine cerebral cortices, are large disk-shaped aggregates consisting of hundreds of different proteins. To study the protein-protein interactions in such complex supramolecules, we developed a procedure to break up the PSD's overall structure, while preserving some interactions between individual proteins. Using the resulting PSD sample and an indirect immunoabsorption procedure, PSD-95 was isolated along with the alpha- and beta-subunits of calcium calmodulin-dependent protein kinase 11 (CaMKII alpha and CaMKII beta), alpha-tubulin, beta-tubulin, and Chapsyn110. Similarly, CaMKII alpha was isolated along with CaMKII beta, alpha-tubulin, beta-tubulin, and small amounts of PSD-95. The proteins isolated from PSDs treated with a cleavable bifunctional crosslinking reagent were further subjected to diagonal gel electrophoresis analysis, and the results indicated that CaMKII(x resides next to alpha-tubulin in the PSD. Overall, the results obtained here suggest that within the PSD, large aggregates of CaMKII alpha, CaMKII beta, alpha-tubulin, and beta-tubulin may occur that indirectly associate with PSD-95 and Chapsyn110. Such a protein organization would allow interactions with F-actin in the cytoplasm and with proteins, such as N-methyl-D-aspartate receptors, which reside on the postsynaptic membrane. Furthermore, it would facilitate binding to proteins such as the various microtubule-associated proteins that reside in the core region of the PSD.