NAC acts as a modulator of SRP function. It can bind to signal sequences directly. SRP initially displaces NAC from RNCs; however, when the signal sequence emerges, trimeric NAC·RNC·SRP complexes form. Upon docking NAC·RNC·SRP complexes to the ER, NAC remains bound, allowing NAC to shield cytosolically exposed nascent chain domains.

bRibosome stalling is an important incident enabling the cellular quality control machinery to detect aberrant mRNA. Saccharomyces cerevisiae Hbs1-Dom34 and Ski7 are homologs of the canonical release factor eRF3-eRF1, which recognize stalled ribosomes, promote ribosome release, and induce the decay of aberrant mRNA. Polyadenylated nonstop mRNA encodes aberrant proteins containing C-terminal polylysine segments which cause ribosome stalling due to electrostatic interaction with the ribosomal exit tunnel. Here we describe a novel mechanism, termed premature translation termination, which releases C-terminally truncated translation products from ribosomes stalled on polylysine segments. Premature termination during polylysine synthesis was abolished when ribosome stalling was prevented due to the absence of the ribosomal protein Asc1. In contrast, premature termination was enhanced, when the general rate of translation elongation was lowered. The unconventional termination event was independent of Hbs1-Dom34 and Ski7, but it was dependent on eRF3. Moreover, premature termination during polylysine synthesis was strongly increased in the absence of the ribosome-bound chaperones ribosome-associated complex (RAC) and Ssb (Ssb1 and Ssb2). On the basis of the data, we suggest a model in which eRF3-eRF1 can catalyze the release of nascent polypeptides even though the ribosomal A-site contains a sense codon when the rate of translation is abnormally low.

Background: Legionella pneumophila glucosyltransferase Lgt1 modifies elongation factor 1A and Hbs1 (Hsp70 subfamily B suppressor 1). Results: In Saccharomyces cerevisiae deleted of endogenous eEF1A and Hbs1, Lgt1 inhibits growth by glucosylation of ectopically expressed eEF1A at serine 53. Conclusion: Glucosylation of eEF1A but not of Hbs1 is essential for Lgt1-induced toxicity. Significance: Yeast as a model to study the action of Legionella glucosyltransferases.

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