Docking of components in a bacterial complex

Proteases are enzymes that cut up other proteins for the purposes of tailoring or degradation. Some depend on ATP as an energy source for unfolding protein substrates, and these are often organized into rings of ATPase subunits stacked coaxially onto rings of protease subunits1. Bochtler et al .2 have reported a crystal structure for the ATP-dependent protease complex HslVU (also known as ClpYQ) from Escherichia coli. They claim this consists of a double hexamer of the protease HslV flanked by hexamers of an ATPase, HslU, which mainly lie in a ring of ATPase domains whose I-domains protrude to form a smaller ring that binds HslV. Based on cryo-electron microscopy of HslVU in buffer conditions that support enzymatic activity, we find that the HslU rings bind in the opposite orientation — that is, their I-domains protrude distally instead of making contact with HslV. Redefinition of this interaction has implications for the functional architecture of the complex.