Discussion of all aspects of cellular structure, physiology and communication.
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I have a question about lipid trafficking, I don't understand why w e need to transfer lipid to a different place IN the cell. Can someone explain to me why move a lipid in the first place, and not have the cell make it where it's needed in the cell?
what kind of lipid are you talking about?
Your question is pretty ambiguous. In general, eukaryotic cells separate the place where they synthesize different stuff so that they can more strictly regulate the process without affecting other pathways. At least that's one thing I can think of right now on the spot.
Also, some cells, such as lymphocytes and probably all MHC-bearing cells, cluster these receptor complexes (namely T cell receptor + MHC I or II) on lipid rafts. I'm not sure if this counts as lipid trafficking, but at least it involves cells actively transporting certain types of lipids to one place in order to allow the molecules anchored to them do their job efficiently. Many other types of lipid rafts also exist.
I always thought that certain preferentially associate with lipid rafts because the rafts have much straighter chains, and thus are far less fluid. Can a cell actually target stuff to rafts? If yes, how?
I'm not sure about the exact mechanisms of MHC:TCR clustering, I read an article about it some time ago, but cannot find it anywhere at the moment. Gnnnh.
Anyway, I remember that for example when an antigen-presenting cell bearing MHC II molecules presents an MHC II:peptide complex to a T cell, it gathers a number MHC molecules on a single area on the cell surface, and the T cell does likewise with its CD4 receptors. This way there can be a lot stronger signal than there would be if the corresponding molecules were equally distributed along the cell membrane (which I think is normally the case). Also, the cells can secrete cytokines more specifically by this method, and bystander cells do not get activated unnecessarily. This phenomenon is called an immunological synapse. Somewhat similar synapse is also formed by cytotoxic (CD8) lymphocytes and MHC I-expressing cells.
What this has to do with lipid rafts? If I recall correctly, the cell uses lipid rafts to cluster these molecules. I don't know if the MHC or TCR molecules are actively targeted to the rafts, however - the process may be passive, but the clustering of MHC or T cell receptor containing lipid rafts when the immunological synapse is formed is an active process. At least the TCR movement is guided by actin filaments, maybe also by microtubules.
Oh, and even though I talk about MHC:TCR complexes, I think there are several co-receptors involved like often is the case when it comes to cell signalling.
oh I did not contest that the proteins specifically associate with lipid rafts. this is a common phenomenon in membrane biochemistry. I just don't see how a cell could specifically target proteins at lipid rafts. well. I can imagine an abstract way: receptor protein associates specifically with rafts, then the protein is targeted at the receptor in the raft. However this doesn't make much sense: if the receptor can associate with rafts, why can't the actual protein?
7 posts • Page 1 of 1
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