The 3'-UTR mediates the cellular localization of an mRNA encoding a short plasma membrane protein.

Cotranslational synthesis of proteins into the endoplasmic reticulum is preceded by targeting of the translating mRNA once a signal peptide emerges from the ribosome exit tunnel. Many mRNAs, however, are unlikely to be targeted by this process because they encode proteins that do not contain a signal peptide or because they are too short to be recognized by the signal recognition particle. Herein we tested the possible involvement of the 3'-UTR in the localization of an mRNA that encodes a very short Saccharomyces cerevisiae protein (Pmp1). We found by ribosome density mapping, sedimentation analysis, differential centrifugation, and fluorescent in situ hybridization that the 3'-UTR is essential for the association of the transcript with membrane compartments. Fusion of the 3'-UTR to heterologous open reading frames conferred on them a sedimentation and cellular localization pattern resembling that of PMP1. Mutation analysis revealed that a repeating UG-rich sequence within the 3'-UTR is important for membrane association. Taken together, our results reveal an essential role for elements within the 3'-UTR in the localization of an mRNA that is likely to be ignored by the standard signal-dependant mechanism.

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