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Specialized ribosome-associated molecular chaperones

During their synthesis on ribosomes, proteins are particularly susceptible to aggregation, which prohibits their proper folding. We are studying ribosome-associated molecular chaperones that are tethered near the site from which the nascent chain exits the ribosome tunnel. Our goal is to determine how this chaperone and associated factors aid in folding of newly synthesized proteins.

The Hsp70 Ssb, which is associated with ribosomes regardless of their translational state, is the major ribosome-associated chaperone. It can be crosslinked to short nascent chains that extend only a few amino acids beyond the ribosome exit site. Like all Hsp70s, Ssb has a J-type protein as a co-chaperone. Zuo1, Ssb's J-partner binds directly to the ribosome. Surprisingly, Zuo1 forms a stable complex with an Hsp70-related protein Ssz1, tethering it to the ribosome. Ssz1 does not appear to act as a classical Hsp70 however, as deletion of the putative peptide-binding domain has no obvious effect on its in vivo function. Intriguingly, when not bound to ribosomes, Ssz1 or Zuo1 activates a signal transduction pathway.

Questions we are addressing include:

  • Mechanism of action of the Ssb:Zuo1:Ssz1 triad. Zuo1 is the J-protein for Ssb. How does Ssz1 modulate Zuo1's function? Can Ssz1 function as a molecular chaperone in its own right? In short, how do the 3 components mechanistically function together as a chaperone machine?
  • in vivo roles of the ribosome associated chaperone triad. Cells lacking any component of the Ssb:Zuo1:Ssz1 triad are hypersensitive to aminoglycoside antibiotics and cold sensitive for growth. What is the functional basis of these phenotypes? Is this chaperone machine required for folding of a specific subset of proteins?
  • A signaling role for Zuo1 and Ssz1? Expression of domains of either Zuo1 or Ssz1 such that they are not ribosome-associated specifically activate a transcriptional response referred to as the pleiotropic drug resistance (PDR) pathway. What is the mechanism of activation of the Pdr1 transcription factor that is responsible for this activation? What role does this pathway play in the cell?

Selected Craig lab publications:

Meyer AE, Hung NJ, Yang P, Johnson AW, Craig EA. (2007) The specialized cytosolic J-protein, Jjj1, functions in 60S ribosomal subunit biogenesis. Proc Natl Acad Sci U S A. Jan 30;104(5):1558-63. [PDF]

Huang P, Gautschi M, Walter W, Rospert S, Craig EA. (2005) The Hsp70 Ssz1 modulates the function of the ribosome-associated J-protein Zuo1. Nat Struct Mol Biol. 12:497-504. [PDF]

Hundley HA, Walter W, Bairstow S, Craig EA. (2005) Human Mpp11 J protein: Ribosome-Tethered Molecular Chaperones Are Ubiquitous. Science. 308:1032-4. [PDF]

Kim SY, Craig EA (2005) Broad Sensitivity of Saccharomyces cerevisiae Lacking Ribosome-Associated Chaperone Ssb or Zuo1 to Cations, Including Aminoglycosides. Eukaryot Cell. Jan;4(1):82-9. [PDF]