514 publications from this institution
In eukaryotic cells, polypeptides are N glycosylated after passing through the membrane of the ER into the ER lumen. This modification is effected cotranslationally by the multimeric oligosaccharyltransferase (OST) enzyme. Here, we report the first cross-linking of an OST subunit to a nascent chain that is undergoing translocation through, or integration into, the ER membrane. A photoreactive probe was incorporated into a nascent chain using a modified Lys-tRNA and was positioned in a cryptic glycosylation site (-Q-K-T- instead of -N-K-T-) in the nascent chain. When translocation intermediates with nascent chains of increasing length were irradiated, nascent chain photocross-linking to translocon components, Sec61α and TRAM, was replaced by efficient photocross-linking solely to a protein identified by immunoprecipitation as the STT3 subunit of the OST. No cross-linking was observed in the absence of a cryptic sequence or in the presence of a competitive peptide substrate of the OST. As no significant nascent chain photocross-linking to other OST subunits was detected in these fully assembled translocation and integration intermediates, our results strongly indicate that the nascent chain portion of the OST active site is located in STT3.
Membrane-protein integration, folding and assembly processes in vivo depend on complex targeting, translocation, chaperoning, and sorting machineries that somehow read the 'molecular code' built into the nascent polypeptide, ultimately producing a properly folded protein integrated into the correct target membrane. Although the main molecular constituents and the basic mechanistic principles of many of these machines are known in outline, the codes remain poorly defined and there is little quantitative information on how protein sequence affects the final structure of membrane proteins. By carefully designing model protein constructs, we have derived the first true biological hydrophobicity scale and have been able to get a first impression of how the position of a given type of residue within a transmembrane segment affects its ability to promote membrane insertion.
