[Mass-spectrometric analysis of proteasomal subunits possessing endoribonuclease activity]

Proteasomes act as the main apparatus of non-lysosomal intracellular proteolysis and participate in the regulation of most important cellular processes. Despite considerable progress in the understanding of proteasome's functioning, some issues, in particular, RNase activity of these ribonucleoprotein complexes and its regulation remain scarcely explored. In this paper we found several proteins corresponding by electrophoretic mobility to subunits of the complex 20S proteasome to possess endoribonuclease activity with respect to both sense and antisense sequences of the c-myc mRNA 3'-UTR. Mass-spectrometric analysis of tryptic hydrolysates of these proteins revealed in the samples the presence of 20S proteasome subunits--αl (PSMA6), α5 (PSMA5), α6 (PSMA1) and α7 (PSMA3). A number of novel phosphorylation sites in subunits αl (PSMA6) and α7 (PSMA3), and the form of subunit α5 (PSMA5) with a deletion of N-terminal 20 amino acid residues detected. The observed differences of individual subunits in the possession endonuclease activity could be apparently explained by postranslational modifications of these proteins, in particular--by phosphorylation. It is shown that the specificity of the proteasomal RNase activity varies after dephosphorylation and also influenced by Ca and Mg cations. The conclusions made about the impact of the PTM status of proteasome subunits on the specificity of their RNase activity.