Specific postsynaptic density proteins bind tubulin and calmodulin-dependent protein kinase type II

Cytoskeletal interactions which contribute to the assembly of the postsynaptic density (PSD) were investigated. PSDs bound ¹²⁵I-tubulin specifically with an apparent K(m) of 2 X 10^-7 M and a B(max) of about 1 nmol/mg of protein. ¹²⁵I-Tubulin blots revealed that a group of polypeptides between M(r) 135,000 and 147,000 (P-140) was a major tubulin-binding PSD component. The P-140 polypeptides were highly enriched in the PSD fraction of purified synaptosomes and could not be detected in crude brain cytoplasm preparations. These polypeptides were subject to phosphorylation by endogenous calmodulin-dependent protein kinast type II, with a concomitant reduction in ¹²⁵I-tubulin binding. The tubulin-binding polypeptides could also associate with the radiolabeled α- and β-subunits of the calmodulin-dependent protein kinase. These observations are consistent with a role for the P-140 polypeptides in organizing the molecular structure of the PSD. The data also suggest that this structure may be modified by Ca²⁺-sensitive phosphorylation, thus permitting neuronal activity to modulate the cytoskeletal interactions of the PSD.