The signal recognition particle receptor alpha subunit assembles co-translationally on the endoplasmic reticulum membrane during an mRNA-encoded translation pause in vitro

Many proteins, including the alpha subunit of the signal recognition particle receptor (SR alpha), are targeted within the cell by poorly defined mechanisms. A 140 residue N-terminal domain of SR alpha targets and anchors the polypeptide to the endoplasmic reticulum membrane by a mechanism independent of the pathway involving the signal recognition particle. To investigate the mechanism of membrane anchoring, translation pause sites on the SR alpha mRNA were used to examine the targeting of translation intermediates. A strong pause site at nucleotide 507 of the mRNA open reading frame corresponded with the shortest nascent SR alpha polypeptide able to assemble on membranes, An mRNA sequence at this pause site that resembles a class of viral -1 frameshift sequences caused translation pausing when transferred into another mRNA context, Site-directed mutagenesis of the mRNA greatly reduced translation pausing without altering the polypeptide sequence, demonstrating unambiguously a role for this mRNA sequence in translation pausing. SR alpha polypeptides synthesized from the non-pausing mRNA were impaired in co-translational membrane anchoring, Furthermore, co-translational membrane assembly of SR alpha appears to anchor polysomes translating SR alpha to membranes.