UBIQUITIN-PROTEIN CONJUGATES IN DIFFERENT STRUCTURES OF THE CENTRAL-NERVOUS-SYSTEM OF THE RAT

The capacity to form ubiquitin (Ub)-protein conjugates was investigate d in the cytosol of different structures of the rat central nervous sy stem (CNS) in order to confirm the presence of this extralysosomal, ad enosine triphosphate (ATP)-dependent, protein degradation system as we ll as its structural localization. Using I-125-Ub, We found that in th e presence of ATP, the cytosol obtained from whole brains was able to form high molecular weight Ub-protein conjugates. These conjugates cou ld be detected after sodium dodecyl sulfate-polyacrylamide gel electro phoresis (SDS-PAGE) and radioautography. The formation of these conjug ates was much higher in the cerebral cortex than in the brain stem, wh ich is mainly constituted by white matter, being intermediate in the c ytosol isolated from whole brain total homogenates. These results sugg ested to us that under normal conditions the capacity to form Ub-prote in conjugates was mainly located in structures containing neuronal cel l bodies. Strong support for this contention was obtained when the cyt osol isolated from rat optic nerves or from oligodendroglial cells iso lated from whole brain was found to be totally unable to form Ub-prote in conjugates. The inability of certain CNS structures to form conjuga tes with Ub could be attributed, among other reasons, to the lack of e nzymes catalyzing the various steps of the Ub degradation system, to t he absence of short half-life (target) proteins in those structures, o r to the lack of activity of the enzymes catalyzing the reaction due t o regulatory control mechanisms operating under normal conditions. (C) 1994 Wiley-Liss, Inc.