Tubulin in postsynaptic junctional lattice

Abstract

TUBULIN, the primary protein of microtubules^1,2, has been identified as a major soluble protein in isolated synaptosomes^3,4. Indirect evidence for the presence of tubulin in the synaptosomal plasma membranes themselves has been accumulating for some time. Colchicine-binding activity, used as an assay for the presence of tubulin^1,2, has been found in synaptosomal plasma membranes^5–7. These membranes, prepared by several different techniques, all show a prominent protein band of apparent molecular weight 52,000–53,000 when solubilised and electrophoresed in sodium dodecyl sulphate–polyacrylamide gel systems^8–12; such gel techniques applied to well characterised microtubule preparations yield molecular weights for tubulin which have been reported to range from 52,000 to 56,000 (refs 1 and 2), the precise value depending on the particular electrophoretic system used. Sequential solubilisation of synaptosomal plasma membranes with Triton X-1000 and N-lauroyl sarcosinate yields isolated postsynaptic densities¹³ whose major protein has a molecular weight of 53,000 (ref. 14), and similarly digestion of the synaptosomal lipid unit membrane by sodium deoxycholate results in isolated postsynaptic junctional lattices¹⁵ with a major protein of approximate molecular weight 53,000 (ref. 12). We report here the identification of tubulin in the postsynaptic junctional lattices of rat forebrain both by two-dimensional electrophoresis of ¹²⁵I-labelled tryptic peptides and by electron microscope immunohistochemistry.