Six-fold rotational symmetry of ClpQ, the E. coli homolog of the 20S proteasome, and its ATP-dependent activator, ClpY.

ClpQ (HslV) is a homolog of the beta-subunits of the 20S proteasome. In E. coli, it is expressed from an operon that also encodes ClpY (HslU), an ATPase homologous to the protease chaperone, ClpX. ClpQ (subunit Mr 19,000) and ClpY (subunit Mr 49,000) were purified separately as oligomeric proteins with molecular weights of approximately 220,000 and approximately 350,000, respectively, estimated by gel filtration. Mixtures of ClpY and ClpQ displayed ATP-dependent proteolytic activity against casein, and a complex of the two proteins was isolated by gel filtration in the presence of ATP. Image processing of negatively stained electron micrographs revealed strong six-fold rotational symmetry for both ClpY and ClpQ, suggesting that the subunits of both proteins are arranged in hexagonal rings. The molecular weight of ClpQ combined with its symmetry is consistent with a double hexameric ring, whereas the data on ClpY suggest only one such ring. The symmetry mismatch previously observed between hexameric ClpA and heptameric ClpP in the related ClpAP protease is apparently not reproduced in the symmetry-matched ClpYQ system.