Local translation of yeast mRNA at the endoplasmic reticulum requires the brefeldin A resistance protein Bfr1.

Brefeldin A resistance factor 1 (Bfr1p) is a nonessential RNA-binding protein and multicopy suppressor of brefeldin A sensitivity in Deletion of leads to multiple defects, including altered cell shape and size, change in ploidy, induction of P-bodies and chromosomal missegregation. Bfr1p has been shown to associate with polysomes, binds to several hundred mRNAs, and can target some of them to P-bodies. Although this implies a role of Bfr1p in translational control of mRNAs, its molecular function remains elusive. In the present study, we show that mutations in RNA-binding residues of Bfr1p impede its RNA-dependent colocalization with ER, yet do not mimic the known cellular defects seen upon deletion. However, a Bfr1 RNA-binding mutant is impaired in binding to mRNA, which encodes an enzyme required for the final step of ergosterol biosynthesis. Consistently, Δ strains show a strong reduction in Erg4p protein levels, most likely because of degradation of misfolded Erg4p. Polysome profiling of or mutant strains reveals a strong shift of mRNA to polysomes, consistent with a function of Bfr1p in elongation or increased ribosome loading. Collectively, our data reveal that Bfr1 has at least two separable functions: one in RNA binding and cotranslational protein translocation into the ER and one in ploidy control or chromosome segregation.