The ribosome-associated complex RAC serves in a relay that directs nascent chains to Ssb

Ying Zhang,Genís Valentín Gesé,Karine Lapouge,Jürgen Kopp,Tina Wölfle,Charlotte Conz,Irmgard Sinning,Sabine Rospert

doi:10.1038/s41467-020-15313-w

Ying Zhang, Genís Valentín Gesé + Show 6 more

Open Access

https://doi.org/10.1038/s41467-020-15313-w

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Abstract

The conserved ribosome-associated complex (RAC) consisting of Zuo1 (Hsp40) and Ssz1 (non-canonical Hsp70) acts together with the ribosome-bound Hsp70 chaperone Ssb in de novo protein folding at the ribosomal tunnel exit. Current models suggest that the function of Ssz1 is confined to the support of Zuo1, however, it is not known whether RAC by itself serves as a chaperone for nascent chains. Here we show that, via its rudimentary substrate binding domain (SBD), Ssz1 directly binds to emerging nascent chains prior to Ssb. Structural and biochemical analyses identify a conserved LP-motif at the Zuo1 N-terminus forming a polyproline-II helix, which binds to the Ssz1-SBD as a pseudo-substrate. The LP-motif competes with nascent chain binding to the Ssz1-SBD and modulates nascent chain transfer. The combined data indicate that Ssz1 is an active chaperone optimized for transient, low-affinity substrate binding, which ensures the flux of nascent chains through RAC/Ssb.

Highlights

The conserved ribosome-associated complex (RAC) consisting of Zuo[1] (Hsp40) and Ssz[1] acts together with the ribosome-bound Hsp[70] chaperone Ssb in de novo protein folding at the ribosomal tunnel exit
Our results indicate that both RAC subunits contact nascent chains and form a relay that transfers them from Ssz[1] to Ssb
As the structure of the RAC core shows the presence of a complete SBDβ17, RAC should be able to bind substrates with low affinity besides its function as J-domain partner of Ssb

Summary

Introduction

The conserved ribosome-associated complex (RAC) consisting of Zuo[1] (Hsp40) and Ssz[1] (non-canonical Hsp70) acts together with the ribosome-bound Hsp[70] chaperone Ssb in de novo protein folding at the ribosomal tunnel exit. Based on these observations the current model is that Ssz[1] does not act as a typical Hsp[70] chaperone Instead, it may primarily play a structural role, supporting Zuo[1] function as the J-domain partner of Ssb[9,19]. Our results indicate that both RAC subunits contact nascent chains and form a relay that transfers them from Ssz[1] to Ssb. Structural analysis reveals that the Zuo[1] N-terminus contains a conserved motif (LP-motif) which binds to the Ssz1SBDβ in the same way as canonical Hsp70s bind their substrates. We show that Ssz[1] is an active chaperone with specific deviations from the canonical Hsp[70] mechanism

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