Just a quick lab advertisement, my bay mate Yoko and my boss Tom have a review article in Cell about how morphological differences between various regions of the endoplasmic reticulum (ER) are representative of functional differences.
You can divide the ER into 3 morphological parts.
- The Nuclear Envelope (which can be further divided into an outer nuclear membrane and inner nuclear membrane)
- Peripheral ER sheets
- Peripheral ER tubes
As I've mentioned before, a main focus of our lab (the Rapoport lab) concerns the mechanism of how newly synthesized protein is translocated into the ER. Here is the supper dumb down version.
- mRNA is translated by the ribosome
- out of the ribosome the nascent polypeptide chain emerges
- if the nascent chain includes a "signal sequence" (BTW a clue on yesterday's Map that Campus) or a transmembrane domain, then the signal recognition particle (SRP) binds. SRP inhibits the ribosome from translating the rest of the mRNA.
- SRP then binds to the SRP receptor on the surface of the ER.
- The nascent chain/ transmembrane domain is inserted into sec61 (i.e. the translocon) a pore within the ER. The ribosome then binds to sec61, SRP is released and protein synthesis resumes. This is called coinsertional translocation. ER membranes that are dedicated to this process are studded with ribosomes (see panel B) and are hence called rough ER. Portions of the ER that are devoid of ribosomes are called smooth ER.
Well much evidence points to protein translocation being restricted to ER sheets (see B) and absent from ER tubes (see C). Functions such as lipid biosynthesis and calcium storage are distributed throughout the rough and smooth ER.
Why is protein translocation restricted to sheets? Well imagine a long mRNA being translated by many ribosomes ... such a huge super complex (called a polysome) could not fit properly of a tube. In fact when ER surfaces are imaged polysomes are often arranged in spiral like formations. (I'll try to a get a pic for you ...) Such structures would naturally redistribute to the sheet "phase" of the ER. But as all you cell biologist know, the cell is smarter than that - this distribution might be highly regulated by some unknown class of proteins ...
In any case, go read the review.