Morphology and molecular composition of isolated postsynaptic junctional structures

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Abstract
The solubilization of isolated pig or rat brain synaptosomal plasma membranes by various detergents was studied and in each case found to depend upon detergent concentration. By using conditions sufficient to extract maximally protein and phospholipid from the membranes, postsynaptic junctional particles were isolated with each of 4 detergents and their ultrastructural appearances and protein contents compared. Two basic structural forms were identified. One, isolated with Triton X-100, consisted of a planar array of dense-staining particles approximately 20 nm in diameter. It resembled the postsynaptic density seen in undigested synaptosomal plasma membranes. The other, isolated with sodium deoxycholate, contained less protein. It had the same overall shape and dimensions as the postsynaptic density, but consists of a branching network of short 5 nm fibers (the postsynaptic junctional lattice) making up an array of contiguous polygons, each approximately 20 nm across. The interior of these polygonal elements seemed to be hydrophobic since it could not be penetrated by metallic salts used for negative staining. The dense-staining 20 nm subunits observed at the postsynaptic junctional site may be composed of hydrophobic proteins inserted into the hollow cores of the lattice polygons. Electrophoretic analysis of the proteins present in the various postsynaptic junctional preparations identified 2 major common components with molecular masses of 275,000 and 47,500. The latter is tentatively identified as actin. Components comigrating, respectively, with .alpha.- and .beta.-tubulin are present, and the relation of the lattice structure to subjacent microtubules is discussed.