ARE PROSOME PARTICLES FORMED BY THE DISINTEGRATION OF PROTEASOMES

Quail reticulocyte 19S ''prosome'' fractions isolated on sucrose densi ty gradients contain two kinds of particles: cylindrical proteasomes a nd ferritin. When samples of this fraction are prepared for electron m icroscopy using the one-step stain protocol described in this paper, m ost of the particles have a rectangular image resembling the proteasom e. However, when samples are prepared for electron microscopy using th e two-step stain protocol described here, there are few rectangular im ages. Their place is taken by round particles that resemble the prosom e. Thus it appears that the round, raspberry-shaped particles called p rosomes and the ring-like proteasome particles may be artifacts of spe cimen preparation for electron microscopy. We propose that proteasome particles may disintegrate when prepared for electron microscopy by me thods such as the two-step stain protocol and that prosome particles r epresent the component parts of the proteasome. Furthermore, based on the enhancement of proteasome images obtained using the one-step main protocol we propose that, instead of consisting of a stack of four rin gs, the proteasome is constructed of three components, i.e., a spheric al central particle flanked by two flat hexagonal end caps.