Species barrier in yeast [PSI^] prion transmission
Hideyuki Hara, Tom Nakayashiki and Yoshikazu Nakamura
Department of Basic Medical Science, Institute of Medical Science, Uni- versity of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639 Japan
<e-mail> hara@ims.u-tokyo.ac.jp
Abstract
The Sup35 protein of Saccharomyces cerevisiae is a eukaryotic polypep- tide release factor (eRF3), which is necessary for terminating protein syn- thesis at stop codons. The aggregated form of Sup35, which is referred to as the [PSr] element, self-propagates and is transmitted cytoplasmically in the manner of the "protein-only" transmission of mammalian prion dis- eases. In [PSr] cells, most of the Sup35 is converted from a soluble, ac- tive state into an insoluble, inactive state similar to the mammalian prion amyloid. As with mammalian prions, a species barrier prevents prion transmission between yeast species. The N-terminal of Sup35 of Sac- charomyces cerevisiae, necessary for [PSr], contains two spe- cies-signature elements - a Gln/Asn-rich region (residues 1-41; designated NQ) that is followed by oligopeptide repeats (designated NR).
In this study, we show that S. cerevisiae [PSr] is transmissible through plasmid shuffling and cytoplasmic transfer to heterotypic Sup35s whose NQ is replaced with the S. cerevisiae NQ. In addition to homology, the N-terminal location is essential for NQ mediated susceptibility to [PSr]
transmission amongst heterotypic Sup35s. Only heterotypic Sup35s with the NQ region swapped to the S. cerevisiae sequence could effectively cross-seed in vitro fiber formation reactions.
These findings suggest that NQ discriminates self from non-self, and is sufficient to initiate [PSI^] transmission irrespective of whether NR is het- erotypic. NR as well as NQ alone can coalesce into existing [PSr] ag- gregates, showing their independent potentials to interact with the identical sequence in the [PSI^] aggregate. The role of NQ and NR in [PSI^] ag- gregation is discussed.
217
