MadSci Network: Cell Biology
Query:

Re: why is the concave part of the cisternae always facing the nucleus?

Date: Fri May 25 10:44:42 2007
Posted By: Stephanie Levi, Grad student, Molecular Genetics and Cell Biology, University of Chicago
Area of science: Cell Biology
ID: 1179925900.Cb
Message:

Excellent question. First, let's go over what the endoplasmic reticulum (ER) and Golgi do in the cell. Among its duties, the ER is responsible for folding newly-made proteins. The new proteins need to be shuttled to their destinations, and that's the Golgi's job (it also acts like a carbohydrate factory for the cell). Proteins leave the ER in little bubbles of membrane called vesicles, and they move between the ER to the Golgi. When the arrive at the Golgi, the vesicles fuse with the first cisterna, called the cis-cisterna, and the new proteins are delivered to the first part of the Golgi. A sugar molecule, called a glycan, is added to the new protein there. As the protein proceeds through the Golgi, this complex branched glycan's shape is changed by enzymes, so that when the protein is ready to leave, the sugar molecule can act like a molecular zipcode that tells the cell where the protein needs to go. So, with all this going on, it makes sense that the Golgi would be right near the ER. The ER is an extension of the nuclear membrane, so the spots on the ER where proteins leave for the Golgi (called ER exit sites [ERES] in mammals and transitional ER [tER] sites in yeast), are often seen right along small parts of the nuclear membrane. The Golgi is therefore positioned right near the ERES (or tER) site, and thus appears to be right next to the nucleus. The exact reason for the Golgi cisternae's curvature always facing away from the ER so that the concave part faces the ERES is unknown, but probably has something to do with the distribution of proteins throughout the cisternae and the distribution of lipids in the membrane, plus the biophysics of receiving a load of new proteins and lipids as membrane-bound cargo vesicles make their way to the Golgi. I am including a paper with some pictures of a yeast Golgi in which you can see the curvature of the Golgi cisternae near a tER site in a budding yeast, which is very neat to see. Look at Figures 1, 5, 7, and 14. Figure 14 is a model of a Golgi stack near a tER site (the tER site's at the top, the first cisterna is right below that, marked C1). Oh, and make sure to capitalize the word Golgi! If you have more questions, write in, because this is my area of research, I'm happy to help you. Resources: http://www.molbiolcell.org/cgi/ content/full/14/6/2277 http:// www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.TOC&depth=2 Use this link to get the cell bible, _The Cell_ by Alberts et al. Search for Golgi and you'll find an in depth overview of ER sites and the Golgi. I would prefer if you started with a first-year biology text book, but this will give you a good overview, just very detailed.


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