PROTEASOME (PROSOME) SUBUNIT VARIATIONS DURING THE DIFFERENTIATION OFMYELOID U937 CELLS

20S proteasomes (prosomes/multicatalytic proteinase) are protein parti cles built of 28 subunits in variable composition. We studied the chan ges in proteasome subunit composition during the differentiation of U9 37 cells induced by phorbol-myristate-acetate or retinoic acid plus I, 25-dihydroxy-cholecalciferol by western blot, flow cytometry and immun o-fluorescence. p25K (C3), p27K (IOTA) and p30/33K (C2) subunits were detected in both the nucleus and cytoplasm of undifferentiated cells. Flow cytometry demonstrated a biphasic decrease in proteasome subunits detection during differentiation induced by RA+VD. PMA caused an earl y transient decrease in these subunits followed by a return to their c ontrol level, except for p30/33K, which remained low. Immune-fluoresce nce also showed differences in the cytolocalization of the subunits, w ith a particular decrease in antigen labeling in the nucleus of RA+VD- induced cells, and a scattering in the cytoplasm and a reorganization in the nucleus of PMA-induced cells. Small amounts of proteasomal prot eins were seen on the outer membrane of non-induced cells; these membr ane proteins disappeared when treated with RA+VD, whereas some increas ed on PMA-induced cells. The differential changes in the distribution and type of proteasomes in RA+VD and PMA-induced cells indicate that, possibly, 20S proteasomes may play a role in relation to the mechanism s of differentiation and the inducer used.