SCREENING FOR MOLECULES INTERACTING WITH PROTEASOMES IN THERMOPLASMA-ACIDOPHILUM

Thermoplasma acidophilum cell extracts were fractionated by gel filtra tion. Proteasomes were eluted as two major peaks. The first one (molec ular mass(r) about 2 MDa) contained proteasomes associated with DNA/pr otein protein complexes. Proteasomes eluted in the other peak were par tially resolved into three subpeaks and based on their preferential hy drolysis of casein, Z-GGL-MCA, and suc-LLVY-MCA, were designated C, L, and Y, respectively. Further purification of proteasomes from peak Y resulted in a homogenous enzyme preparation, whereas proteasomes purif ied from peak C contained a homomultimeric protein composed of 20 kDa subunits. Thus, association of proteasomes with this protein seems to be responsible for the observed increase in molecular mass and for inh ibition of caseinolytic activity by Ca2+-ions.