Actin-like and tubulin-like proteins in synaptic junctional complexes.

Synaptic junctional complexes (SJCs), isolated by a procedure which preserves presynaptic dense projections (PDPs) contain as their major component a polypeptide (P55) which comigrates with tubulin on sodium dodecyl sulfate - polyacrylamide gels and another major polypeptide (P45) which comigrates with muscle actin. We report here the characterization of P55 and P45 by two-dimensional polyacrylamide gel electrophoresis and by peptide mapping of the radioiodinated polypeptides. Our results indicate that P55 and P45 are homologous with tubulin and actin respectively, in terms of molecular weight, protein charge, and primary structure. Comparison of the sodium dodecyl sulfate - polyacrylamide gel electrophoresis patterns of SJCs and postsynaptic densities (PSDs) indicates that P55 and P45 are associated mainly with PDPs but are also found in the PSD.