Novel propeptide function in 20 S proteasome assembly influences beta subunit composition

The assembly of eukaryotic 20 S proteasomes involves the formation of half- proteasomes where precursor beta-type subunits gather in position on an alp ha-subunit ring, followed by the association of two half-proteasomes and be ta-subunit processing. In vertebrates three additional beta-subunits (beta 1i/LMP2, beta 2iIMECL1, and beta 5i/LMP7) can be synthesized and substitute d for constitutive homologues (beta 1/delta, beta 2/Z, and beta 5/X) to yie ld immunoproteasomes, which are important for generating certain antigenic peptides. We have shown previously that when all six beta-subunits are pres ent, cooperative assembly mechanisms limit the diversity of proteasome popu lations. Specifically, LMP7 is incorporated preferentially over X into prep roteasomes containing LMP2 and MECL1. We show here that the LMP7 propeptide is responsible for this preferential incorporation, and it also enables LM P7 to incorporate into proteasomes containing delta and Z. In contrast, the X propeptide restricts incorporation to proteasomes with delta and Z, Furt hermore, we demonstrate that the LMP7 propeptide can function in trans when expressed on LMP2, and that its NH2-terminal and mid-regions are particula rly critical for function, In addition to identifying a novel propeptide fu nction, our results raise the possibility that one consequence of LMP7 inco rporation into both immunoproteasomes and delta/Z proteasomes may be to inc rease the diversity of antigenic peptides that can be generated.