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Propagation of prions via action of molecular chaperones

Proteins can exist in more than one conformation. Particular proteins, called prions, can acquire conformations that are self-propagating. Yeast contains several prions whose conformation is profoundly affected by molecular chaperones. In each case, particular chaperones, a J-protein/Hsp70 pair and a AAA+ ATPase are absolutely required for maintenance of the prion form within a population of cells. Our goal is to understand the mechanism by which these chaperones affect self-replicating forms of proteins.

The function of the J-protein Sis1 and the Hsp70 Ssa is required for maintenance of the three major yeast prions,  [PSI+], [RNQ+] and [URE3]. The closely related J-protein Ydj1 will not suffice. These facts raise two fundamental questions: Why is a chaperone required for prion maintenance? What special features does the Sis1 J-protein possess that other J proteins do not?

Questions we are addressing include:

  • Requirement for Sis1 in propagation of [RNQ+]. Most yeast prions are at most modestly affected by the absence of a particular J protein. Recent in vivo data suggests that Sis1 (with its Hsp70 partner Ssa) works with the AAA+ ATPase Hsp104 to fragment prion fibrils, thus creating new seeds that are necessary for prion propagation. We are using genetic and biochemical tools to understand [RNQ+]'s exquisitively sensitive requirement for Sis1 function.
  • The basis of the specificity for the requirement of Sis1 in prion propagation.  Remarkably, the human ortholog of Sis1 is capable of [RNQ+] propagation, but the ortholog of Ydj1 is not. We have mapped the functional difference between Sis1 and Ydj1 to the glycine-rich region adjacent to the J domain. What is the function of the G/F region? We are using genetic and biochemical approaches to test the idea that the G/F region is critical for either targeting specific client proteins to Hsp70s or for modulating Hsp70s reaction cycle.
  • Essential cellular functions of Sis1. Sis1 is an essential protein. However, the prion protein Rnq1, which forms [RNQ+], is not an essential gene. Mammalian Hdj1 proteins are not only able to substitute for Sis1 in prion propagation, but also in it essential function(s). What is (are) the essential cellular function(s) of Sis1?

Selected Craig lab publications:

Higurashi, T., Hines, J.K., Sahi, C., Aron, R., and Craig E.A. (2008) Specificity of the J-Protein SIS1 in the Propagation of Three Yeast Prions. Proc Natl Acad Sci USA. 105:16596-16601. [PDF]

Aron R, Higurashi T, Sahi C, Craig EA. (2007) J-protein co-chaperone Sis1 required for generation of [RNQ+] seeds necessary for prion propagation. EMBO J. 26:3794-803. [PDF]

Lopez, N, Aron, R and Craig, EA (2003) Specificity of the Class II Hsp40 Sis1 in maintenance of the yeast prion [RNQ+] Mol. Biol. Cell. 14:1172-81. [PDF]

Johnson, J and Craig, E (2001) An essential role for the substrate-binding region of Hsp40s in Saccharomyces cerevisiae J. Cell Biol. 152:851-856. [PDF]