The heterotrimeric Asi ubiquitin ligase (encoded by ASI1, ASI2, and ASI3) mediates protein degradation in the inner nuclear membrane in Saccharomyces cerevisiae. Asi1p and Asi3p possess catalytic domains, while Asi2p functions as an adaptor for a subset of Asi substrates. We hypothesized the Asi complex is an important mediator of protein quality control, and we predicted that Asi would be required for optimal growth in conditions associated with elevated abundance of aberrant proteins. Loss of Asi1p or Asi3p, but not Asi2p, sensitized yeast to hygromycin B, which promotes translational infidelity by distorting the ribosome A site. Surprisingly, loss of quality control ubiquitin ligase Hul5p did not sensitize yeast to hygromycin B. Our results are consistent with a prominent role for an Asi subcomplex that includes Asi1p and Asi3p (but not Asi2p) in protein quality control.
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