Roles of double-loop (130~159 aa and 175~209 aa) in ClpY(HslU)-I domain for SulA substrate degradation by ClpYQ(HslUV) protease in Escherichia coli.

An Escherichia coli ATP-dependent two-component protease, ClpYQ(HslUV), targets the SulA molecule, an SOS induced protein. ClpY recognizes, unfolds and translocates the substrates into the proteolytic site of ClpQ for degradation. ClpY is divided into three domains N, I and C. The N domain is an ATPase; the C domain allows for oligomerization, while the I domain coordinates substrate binding. In the ClpYQ complex, two layer pore sites, pore I and II, are in the center of its hexameric rings. However, the actual roles of two outer-loop (130~159 aa, L1 and 175~209 aa, L2) of the ClpY-I domain for the degradation of SulA are unclear. In this study, with ATP, the MBP-SulA molecule was bound to ClpY oligomer(s). ClpYΔL1 (ClpY deleted of loop 1) oligomers revealed an excessive SulA-binding activity. With ClpQ, it showed increased proteolytic activity for SulA degradation. Yet, ClpYΔL2 formed fewer oligomers that retained less proteolytic activity, but still had increased SulA-binding activity. In contrast, ClpYΔpore I had a lower SulA-binding activity. ClpYΔ pore I ΔL2 showed the lowest SulA-binding activity. In addition, ClpY (Q198L, Q200L), with a double point mutation in loop 2, formed stable oligomers. It also had a subtle increase in SulA-binding activity, but displayed less proteolytic activity. As a result, loop 2 has an effect on ClpY oligomerization, substrate binding and delivery. Loop 1 has a role as a gate, to prevent excessive substrate binding. Thus, accordingly, ClpY permits the formation of SulA-ClpY(6x), with ATP(s), and this complex then docks through ClpQ(6x) for ultimate proteolytic degradation.