MOLECULAR-CLONING OF THE MOUSE PROTEASOME SUBUNITS MC14 AND MECL-1 - RECIPROCALLY REGULATED TISSUE EXPRESSION OF INTERFERON-GAMMA-MODULATEDPROTEASOME SUBUNITS

primary structures of the interferon-gamma-inducible mouse 20S proteas ome subunit MECL-1 and its alternate homolog MC14 were determined. Nor thern analysis of mouse tissues revealed that MECL-1 mRNA predominantl y occurred in thymus, lymph nodes, and spleen, whereas small amounts w ere detected in nonlymphoid tissues such as kidney, muscle, and testis . Unexpectedly, probing RNA blots with MC14 showed that tissues with h igh MECL-1 expression contained little MC14 and vice versa. A very sim ilar reciprocal tissue expression was subsequently found for the homol ogous subunit pairs LMP2 and delta as well as LMP7 and MB1. The subuni t protein composition of 20S proteasomes purified from liver, thymus, and lung reflected RNA expression. The impact of a regulated reciproca l tissue expression is discussed with respect to thymic selection and the induction of tolerance in potentially autoreactive T cells.