Phylogenic relationships of the amino acid sequences of prosome (proteasome, MCP) subunits.

Prosomes [or proteasomes, Multi-Catalytic Proteinase (MCP) are multisubunit protein complexes, found from archaebacteria to man, the structure of which (a 4-layer cylinder) is remarkable conserved. They were first observed as subcomplexes of untranslated mRNP, and then as a multicatalytic proteinase with several proteolytic activities. A number of sequences from subunits of these complexes are now available.

The prosomal RNA-binding protein p27K is a member of the alpha-type human prosomal gene family.

Monoclonal antibodies demonstrated high conservation during evolution of a prosomal protein of M(r) 27,000 and differentiation--specific expression of the epitope. More than 90% of the reacting antigen was found as a p27K protein in the free messenger ribonucleoprotein (mRNP) fraction but another protein of M(r) 38,000, which shared protease fingerprint patterns with the p27K polypeptide, was also labelled in the nuclear and polyribosomal fractions. Sequencing of cDNA recombinant clones encoding the p27/38K protein and comparison with another prosomal protein, p30-33K, demonstrated the existence of a common characteristic sequence pattern containing three highly conserved segments.

Disruption of prosomes by some bivalent metal ions results in the loss of their multicatalytic proteinase activity and cancels the nuclease resistance of prosomal RNA.

Prosomes are ribonucleoprotein particles constituted by a variable set of about 20 proteins found associated with untranslated mRNA. In addition, they contain a small RNA, the presence of which has been an issue of controversy for a long time. The intact particles have a multicatalytic proteinase (MCP) activity and are very stable; we have never observed autodigestion of the particle by its intrinsic proteinase activity.

Two mRNAs exist for the Hs PROS-30 gene encoding a component of human prosomes.

Screening of a lambda gt11 cDNA expression library of the HeLa cell genome with a monoclonal antibody that specifically recognizes prosomal 30-33-kDa proteins, allowed isolation of a 1264-nucleotide (nt) recombinant cDNA containing a 327-nt untranslated 5'-end. The amino acid (aa) sequence deduced from this cDNA revealed a protein of 269 aa (M(r) of 30,227) that includes a consensus box characteristic for Tyr phosphorylation, also observed in other prosomal proteins. Comparison with another prosomal 27-kDa protein, cloned in our laboratory, indicated the presence of three prosome-specific homology boxes observed in these proteins from archaebacteria to man.