The localization of mRNAs encoding secreted/membrane proteins (mSMPs) to the endoplasmic reticulum (ER) likely facilitates the co-translational translocation of secreted proteins. However, studies have shown that mSMP recruitment to the ER in eukaryotes can occur in a manner that is independent of the ribosome, translational control, and the signal recognition particle, although the mechanism remains largely unknown. Here, we identify a cis- acting RNA sequence motif that enhances mSMP localization to the ER and appears to increase mRNA stability, and both the synthesis and secretion of secretome proteins. Termed SECReTE, for s ecretion- e nhancing c is re gulatory t argeting e lement, this motif is enriched in mRNAs encoding secretome proteins translated on the ER in eukaryotes and on the inner membrane of prokaryotes. SECReTE consists of ≥10 nucleotide triplet repeats enriched with pyrimidine (C/U) every third base ( i . e . NNY , where N = any nucleotide, Y = pyrimidine) and can be present in the untranslated as well as the coding regions of the mRNA. Synonymous mutations that elevate the SECReTE count in a given mRNA ( e . g . SUC2 , HSP150 , and CCW12 ) lead to an increase in protein secretion in yeast, while a reduction in count led to less secretion and physiological defects. Moreover, the addition of SECReTE to the 3’UTR of an mRNA for an exogenously expressed protein ( e . g . GFP) led to its increased secretion from yeast cells. Thus, SECReTE constitutes a novel RNA motif that facilitates ER-localized mRNA translation and protein secretion. Author summary Proteins destined for secretion from the cell, including soluble secreted and membrane proteins (SMPs), are translocated into the endoplasmic reticulum (ER) either directly upon translation on the ER surface or post-translationally, as in the case of type II membrane proteins. Interestingly, several studies have demonstrated that mRNAs encoding SMPs (mSMPs) are also enriched on the ER, yet how they target this organelle is less clear. The signal recognition particle (SRP), which recognizes N-terminal hydrophobic signals of nascent polypeptides and targets them to an ER-localized receptor, was proposed to mediate the co-translational ER targeting of mRNA. However, more recent studies show that SRP inactivation, as well as the inhibition of translation, do not prevent targeting. Thus, how mSMPs reach the ER and whether the process is translation-independent remain open. Here we identify a cis -acting sequence element in mSMPs that appears to facilitate mRNA stability and localization to the ER and, more importantly, enhances protein secretion. This motif, entitled “SECReTE” ( s ecretion- e nhancing c is re gulatory t argeting e lement) is enriched in nearly all mSMPs in eukaryotes and its addition or removal from mRNAs results in either enhanced or reduced protein secretion, respectively. Thus, SECReTE is a RNA sequence motif that regulates protein translation and secretion.
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