CHARACTERIZATION OF GRANULAR PARTICLES ISOLATED FROM POSTSYNAPTIC DENSITIES

We describe here the isolation and biochemical characterization of a p opulation of protein aggregates from the postsynaptic density (PSD) pr epared from pig cerebral cortex. The protein constituents of these agg regates are linked together primarily by disulfide bonds. Negative sta ining electron microscopy revealed that the isolated protein aggregate s were granular objects with an average outside diameter of similar to 21 nm and with small protrusions on their surface. The major constitu ents of the isolated granular aggregates consist of tubulin and an uni dentified protein of 70 kDa in size. Small amounts of the alpha subuni t of calcium/calmodulin-dependent protein kinase II and subunits of lp ha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and NMDA subtype s of glutamate receptors were also detected by immunoblotting. Actin, however, was not found in these granular aggregates. We propose that t hese granular protein aggregates correspond to the similar to 20-nm-di ameter granular particles of the PSD on the basis of their biochemical and morphological characteristics. The spatial arrangement of these g ranular aggregates relative to other components of the postsynaptic te rminal is also postulated here.