Membrane protein biosynthesis at the endoplasmic reticulum
- The molecular mechanisms of protein integration and maturation.
- The quality control of misfolded membrane proteins.
The research in my laboratory is aimed at understanding how proteins are inserted into lipid bilayers to form biologically functional membranes. Our studies concentrate on the endoplasmic reticulum (ER) where a large number of integral membrane proteins are synthesized, and we have been able to identify several protein components of the ER that mediate the integration of membrane proteins. At the core of the integration process is the Sec61 complex which plays a central role in membrane protein insertion at the ER. We are presently studying the detailed molecular mechanisms by which polytopic, or multi-spanning, membrane proteins are integrated into the lipid bilayer via the Sec61 complex. The majority of membrane proteins are integrated into the ER membrane "co-translationally", that is whilst they are still being synthesized by the ribosome, and the growing polypeptide chain can be modified in several different ways including the addition of N-linked glycans. These N-linked glycans serve a number of functions, including the recruitment of specific molecular chaperones to bind to the newly made protein and we are particularly interested in the role of specific subunits of the enzyme complex (the oligosaccharyltransferase) that mediates N-glycosylation, and their functional relationship with the Sec61 translocon of the ER membrane. My group is also investigating the synthesis of a second class of integral membrane proteins, the so called tail-anchored proteins. Tail-anchored proteins are characterised by their ability to be integrated "post-translationally", and both the pathway for their delivery to the ER membrane, and the mechanisms by which they become membrane integrated are controversial. We are investigating both of these stages, and have identified distinct pathways by which newly synthesised tail-anchored proteins are delivered to the ER membrane. The quality control function of the endoplasmic reticulum is now well defined and is based on the ability of a variety of chaperones to bind to and retain misfolded proteins that are eventually retro-translocated from the ER back in to the cytosol where they are degraded at the proteasome. Whilst there has been much work on the quality control, of soluble, secretory, proteins in the ER lumen, much less is known about the quality control of polytopic integral membrane proteins. We are currently using a variety of techniques, inclusing small molecule inhibitors, to investigate the ER components that mediate the recognition of aberrant membrane proteins and the pathways by which these polypeptides are removed from the bilayer for degradation at the proteasome.
Hypothetical routes for the biogenesis of tail-anchored proteins:
Multiple routes have been described for the biogenesis of tail-anchored membranbe proteins at the endoplasmic reticulum and different precurosrs most likely exploit one or more different pathways. See also Rabu et al (2009), Journal of Cell Science 122: 3605-3612.