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Single-molecule denaturation and degradation of proteins by the AAA+ ClpXP protease.

ABSTRACT: ClpXP is an ATP-fueled molecular machine that unfolds and degrades target proteins. ClpX, an AAA+ enzyme, recognizes specific proteins, and then uses cycles of ATP hydrolysis to denature any native structure and to translocate the unfolded polypeptide into ClpP for degradation. Here, we develop and apply single-molecule fluorescence assays to probe the kinetics of protein denaturation and degradation by ClpXP. These assays employ a single-chain variant of the ClpX hexamer, linked via a single biotin to a streptavidin-coated surface, and fusion substrates with an N-terminal fluorophore and a C-terminal GFP-titin-ssrA module. In the presence of adenosine 5'-[gamma-thio]triphosphate (ATPgammaS), ClpXP degrades the titin-ssrA portion of these substrates but stalls when it encounters GFP. Exchange into ATP then allows synchronous resumption of denaturation and degradation of GFP and any downstream domains. GFP unfolding can be monitored directly, because intrinsic fluorescence is quenched by denaturation. The time required for complete degradation coincides with loss of the substrate fluorophore from the protease complex. Fitting single-molecule data for a set of related substrates provides time constants for ClpX unfolding, translocation, and a terminal step that may involve product release. Comparison of these single-molecule results with kinetics measured in bulk solution indicates similar levels of microscopic and macroscopic ClpXP activity. These results support a stochastic engagement/unfolding mechanism that ultimately results in highly processive degradation and set the stage for more detailed single-molecule studies of machine function.

SUBMITTER: Shin Y

PROVIDER: S-EPMC2773733 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:In Escherichia coli, SigmaS (?S) is the master regulator of the general stress response. The cellular levels of ?S are controlled by transcription, translation and protein stability. The turnover of ?S, by the AAA+ protease (ClpXP), is tightly regulated by a dedicated adaptor protein, termed RssB (Regulator of Sigma S protein B)-which is an atypical member of the response regulator (RR) family. Currently however, the molecular mechanism of ?S recognition and delivery by RssB is only poorly understood. Here we describe the crystal structures of both RssB domains (RssBN and RssBC) and the SAXS analysis of full-length RssB (both free and in complex with ?S). Together with our biochemical analysis we propose a model for the recognition and delivery of ?S by this essential adaptor protein. Similar to most bacterial RRs, the N-terminal domain of RssB (RssBN) comprises a typical mixed (??)5-fold. Although phosphorylation of RssBN (at Asp58) is essential for high affinity binding of ?S, much of the direct binding to ?S occurs via the C-terminal effector domain of RssB (RssBC). In contrast to most RRs the effector domain of RssB forms a ?-sandwich fold composed of two sheets surrounded by ?-helical protrusions and as such, shares structural homology with serine/threonine phosphatases that exhibit a PPM/PP2C fold. Our biochemical data demonstrate that this domain plays a key role in both substrate interaction and docking to the zinc binding domain (ZBD) of ClpX. We propose that RssB docking to the ZBD of ClpX overlaps with the docking site of another regulator of RssB, the anti-adaptor IraD. Hence, we speculate that docking to ClpX may trigger release of its substrate through activation of a "closed" state (as seen in the RssB-IraD complex), thereby coupling adaptor docking (to ClpX) with substrate release. This competitive docking to RssB would prevent futile interaction of ClpX with the IraD-RssB complex (which lacks a substrate). Finally, substrate recognition by RssB appears to be regulated by a key residue (Arg117) within the ?5 helix of the N-terminal domain. Importantly, this residue is not directly involved in ?S interaction, as ?S binding to the R117A mutant can be restored by phosphorylation. Likewise, R117A retains the ability to interact with and activate ClpX for degradation of ?S, both in the presence and absence of acetyl phosphate. Therefore, we propose that this region of RssB (the ?5 helix) plays a critical role in driving interaction with ?S at a distal site.

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Single-molecule denaturation and degradation of proteins by the AAA+ ClpXP protease.

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