PREFERENTIAL MODIFICATION OF NUCLEAR PROTEINS BY A NOVEL UBIQUITIN-LIKE MOLECULE

Sentrin is a novel ubiquitin-like protein that protects cells against both anti-Fas and tumor necrosis factor-induced cell death. Antiserum recognizing the N terminus of sentrin revealed the presence of a 18-kD a sentrin monomer, a 90-kDa band (p90), and multiple high molecular ma ss bands, Because sentrin possesses the conserved Gly-Gly residues nea r the C terminus, it is likely that these additional bands represent c onjugation of sentrin to other proteins in a manner that is similar to the ubiquitination pathway, Transient expression of hemagglutinin epi tope-tagged sentrin mutants in COS cells demonstrated that the sentrin C terminus is cleaved, which allows it to be conjugated to other prot eins via the conserved C-terminal Gly residue. Immunocytochemical stai ning and cell fractionation analysis demonstrated that sentrin monomer is localized pre dominantly to the cytosol, However, p90 and the majo rity of sentrinized proteins appeared to be localized to the nucleus, When the conserved Gly-Gly residues of sentrin were changed to Gly-Ala , only sentrin monomer and p90 but not the high molecular mass bands w ere observed, Thus, p90 generation appears to be required for the form ation of high molecular mass bands in the nucleus, Taken together, sen trinization represents a novel pathway for nuclear protein modificatio n, which is distinct from ubiquitination.