Protein quality control systems associated with n o‐ g o and n onstop m RNA surveillance in yeast

Quality control systems eliminate aberrant proteins derived from aberrant m RNA s. Two E 3 ubiquitin ligases, L tn1 and N ot4, are involved in proteasomal protein degradation coupled to translation arrest. Here, we evaluated nonstop and translation arrest products degraded in a poly( A ) tail‐independent manner. L tn1 was found to degrade aberrant nonstop polypeptides derived from nonstop m RNA lacking a termination codon, but not peptidyl‐t RNA , even in the absence of the ribosome dissociation complex D om34: H bs1. The receptor for activated C kinase ( RACK 1/ ASC 1) was identified as a factor required for nascent peptide‐dependent translation arrest as well as L tn1‐dependent protein degradation. Both N ot4 and L tn1 were involved in the degradation of various arrest products in a poly( A ) tail‐independent manner. Furthermore, carboxyl terminus‐truncated degradation intermediates of arrest products were stabilized in a cdc48‐3 mutant defective in unfolding or the disassembly related to proteasomal degradation. Thus, we propose that stalled ribosomes may be dissociated into subunits and that peptidyl‐t RNA on the 60 S subunit is ubiquitinated by L tn1 and C dc48 is required for the degradation following release from t RNA .