|MadSci Network: Cell Biology|
1. Actually, the cell really is extremely crowded. Your library will likely have a copy of "The Machinery of Life" by David S. Goodsell (Springer-Verlag, 1992). Goodsell draws cells and cell components to scale and based on real data. The cover alone ought to convince you how crowded a cell is. Now to protein trafficking. 2. Soluble proteins. Soluble proteins destined for the cytosol are simply synthesized and possibly directly released off the endoplasmic reticulum (ER). Those that are modified by addition of carbohydrate or lipid may sometime go to the Golgi apparatus through the soluble interior of the ER which is directly connected to the soluble space of the Golgi. However, they can usually be modified directly by soluble enzymes. 3. Membrane proteins. Membrane proteins are inserted into the ER membrane AS THEY ARE BEING MADE. They achieve their final conformation/topology while in the ER, or at least a topology very close to final. They then move laterally through the ER membrane to the Golgi. This likely involves interactions with some set of cytoskeletal components that act as attachment points and motors. Once in the Golgi, they get modified if that's what that particular protein needs. How do the various membrane proteins get to their respective organelles? For the most part, membrane proteins appear to have specific short amino acids sequences that direct that protein to a particular organ. For example, a sequence for translocation to the nucleus is 5 a.a. in length and contains all aspartates and glutamates. Other signals are present which will send a protein to a mitochondrion or back to the ER, etc. In the Golgi, proteins bound, for example, to the mitochondrion end up being sorted out as they pass through the various layers of the Golgi until they are together. Once together and when they have reached the final membranes of the Golgi, they "bud" off as a vesicle. The localization signal of the proteins appears to somehow tell the machinery to take that vesicle to the mitocondrion where it fuses and thus inserts new protein into the membrane. Localization signals may or may not (usually) be cleaved off. Switching the short localization sequence on a protein from, for example, nucleus to mitochondrial will cause that protein to be directed to the wrong organ where it may or may not function properly or possibly even be toxic. 4. Now finally to bacteria. Membrane proteins in a bacterium are inserted into the membrane DURING SYNTHESIS. If, for example, amino acids 10-30 form a membrane domain, that segment is being inserted into the membrane by the time the ribosome has attached a.a. 40 or 45. For insight into the insertion process, see Heinrich et al., Cell (2000) 102:233-244. How are they sorted? Good question. We don't know for most proteins but the answer I think everyone assumes is that localization to a particular part of the cell is due to specific protein-protein interactions. This is however quite dynamic and localization can change very quickly. For insertion into the outer membrane, the same machinery that inserts proteins into the plasma/cell membrane probably takes outer membrane proteins also but there is an additional component(s) (probably) that help guide insertion into the outer membrane (of the Gram negatives). I suspect that the machinery used for this purpose has somehow been coopted by evolution to form the Type III secretion systems (there are 4 systems known) that pathogenic bacteria use to insert proteins not only through their inner and outer membranes but directly into the plasma membrane or even into the cytosol of the cell they are invading. For a general review of secretion systems use PubMed or some other search program and find reviews by Tony Pugsley. For some insight into the various specific proteins and some good pictures, see papers by Joe Lutkenhaus, Piet de Boer, Janine Maddock and Lucy Shapiro in addition to Pugsley.
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