Energy-dependent proteases often rely on adaptor proteins to modulate substrate recognition. The SspB adaptor binds peptide sequences in the stress-response regulator RseA and in ssrA-tagged proteins and delivers these molecules to the AAA+ ClpXP protease for degradation. The structure of SspB bound to an ssrA peptide is known. Here, we report the crystal structure of a complex between SspB and its recognition peptide in RseA. Notably, the RseA sequence is positioned in the peptide-binding groove of SspB in a direction opposite to the ssrA peptide, the two peptides share only one common interaction with the adaptor, and the RseA interaction site is substantially larger than the overlapping ssrA site. This marked diversity in SspB recognition of different target proteins indicates that it is capable of highly flexible and dynamic substrate delivery.
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T.A.B., I.L. and J.M.F. are employees of Howard Hughes Medical Institute (HHMI). We thank D. Bolon for the gift the SspB A74Q mutant, G. Hersch for the labeled ssrA peptide and other members of the Baker and Sauer labs for help and advice. This work was supported by HHMI and US National Institutes of Health (NIH) grant AI-16892. Studies conducted at the NE-CAT beamlines of the Advanced Photon Source were supported by award RR-15301 from the NIH National Center for Research Resources and by the US Department of Energy Office of Basic Energy Sciences under contract W-31-109-ENG-38.
The authors declare no competing financial interests.
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