A functional chaperone triad on the yeast ribosome

Max-Planck Research Unit Enzymology of Protein Folding, Weinbergweg 22, D-06120 Halle (Saale), Germany

Communicated by Elizabeth Anne Craig, University of Wisconsin Medical School, Madison, WI (received for review November 23, 2001)

Abstract

The chaperones RAC (ribosome-associated complex), consisting of Ssz1p and zuotin, and Ssb1/2p are associated with ribosomes of yeast. Ssb1/2p was previously shown to form a crosslink product to polypeptides trapped in ribosome-nascent chain complexes (RNCs) in vitro. Here we show that an efficient crosslink of the nascent chain to Ssb1/2p depends on the presence of functional RAC. The crosslink to Ssb1/2p was significantly diminished if (i) RAC was removed from RNCs: a process reversed by addition of purified RAC; (ii) RAC carried a mutation in the J-domain of zuotin, leading to its inactivation in vivo; (iii) RAC's Ssz1p subunit was absent because RNCs were generated in a Δssz1-derived translation extract. In vivo the same specific set of growth defects caused by the absence of any of the three chaperones was also displayed by a Δssb1/2Δssz1Δzuo1 strain. The combination of in vitro and in vivo data supports a model in which Ssb1/2p, Ssz1p, and zuotin act in concert on nascent chains while they are being synthesized.

Footnotes

↵ * To whom reprint requests should be addressed: E-mail: rospert{at}enzyme-halle.mpg.de.
Abbreviations:

NAC,

nascent polypeptide-associated complex;

RAC,

ribosome-associated complex;

RNC,

ribosome-nascent chain complex;

BS3,

(bis-(sulfosuccinimidyl),-suberate)