From owner-cellbiol@net.bio.net Thu Dec 05 22:00:00 1996 Path: biosci!agate!howland.erols.net!panix!news.panix.com!not-for-mail From: iayork@panix.com (Ian A. York) Newsgroups: bionet.cellbiol Subject: Re: protein translocation Date: 6 Dec 1996 10:30:09 -0500 Organization: Panix Lines: 39 Message-ID: <589e61$nkd@panix.com> References: NNTP-Posting-Host: panix.com In article , Mazen Saleh wrote: > >does anyone know of an example where the ER protein translocation >complex works in reverse. i.e translocate a protein from inside >the ER to the cytoplasmic side?. I need an example, preferably, >of an endogenous cellular protein. I know of no examples that exactly fit your criteria, but I can think of a couple that almost fit. First, there's a well-known but poorly defined pathway whereby peptides and/or proteins can leave the ER and become degraded. This has been shown in antigen processing assays in a number of ways. The route by which this proteins/ peptides leave the ER is not known, except that it isn't the TAP transporter complex. The ER protein translocation machinery is still one candidate, though only because nothing else has been excluded. The other example was first (I think) clearly described by Hidde Ploegh's lab. In humans cells infected with HCMV the class I MHC heavy chain is very rapidly shunted back out of the ER and is rapidly degraded. Based on the speed of this, it's plausible that the protein never has a chance to leave the translocation machinery, so it may be 'simply' falling back out. More recently a very similar phenomenon was described by some other group in JBC - sorry, I can't remember the authors; it was within the last couple of months. In this case the cells were uninfected and involved several proteins, and the authors suggessted that this was a general pathway that is important in degradation of ER proteins. It seems to me that ER proteins in the cytoplasm are likely to be degraded very rapidly - the environment in the cytoplasm must be fairly hostile to an ER protein, and it's likely to become misfolded and hence degraded. For that reason it may be difficult to track those proteins which go from the ER to the cytoplasm. Ian -- Ian York (iayork@panix.com) "-but as he was a York, I am rather inclined to suppose him a very respectable Man." -Jane Austen, The History of England .