β2 subunit propeptides influence cooperative proteasome assembly

Vertebrate proteasomes are structurally heterogeneous, consisting of both "constitutive" (or "standard") proteasomes and "immunoproteasomes." Constitutive proteasomes contain three ubiquitously expressed catalytic subunits, Delta (beta1), Z (beta2), and X (beta5), whereas immunoproteasomes contain three interferon-gamma-inducible catalytic subunits, LMP2 (beta1i), MECL (beta2i), and LMP7 (beta5i). We recently have demonstrated that proteasome assembly is biased to promote immunoproteasome homogeneity when both types of catalytic subunits are expressed in the same cell. This cooperative assembly is due in part to differences between the LMP7 (beta5i) and X (beta5) propeptides. In the current study we demonstrate that differences between the MECL (beta2i) and Z (beta2) propeptides also influence cooperative assembly. Specifically, replacing the MECL propeptide with that of Z enables MECL incorporation into otherwise constitutive (Delta(+)/X+) proteasomes and facilitates X incorporation into otherwise immunoproteasomes (MECL+/LMP2(+)). We also show, using MECL-/- mice, that LMP2 incorporation does not require MECL, in contrast with previous suggestions that their incorporation is mutually codependent. These results enable us to refine our model for cooperative proteasome assembly by determining which combinations of inducible and constitutive subunits are favored over others, and we propose a mechanism for how propeptides mediate cooperative assembly.