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Cell membrane targeting signals.

Sat May 08, 2010 9:31 pm

Hey guys, first post here :)

So, at the moment I'm going over the signalling of newly synthesised proteins and my lecturer's notes are confusing me a little. How are proteins destined for the cell membrane targeted? Are they synthesised by the Rough ER and make their way to the membrane through the excretory pathway, or are they simply synthesised in the cytosol with signal sequences to translocate straight into the membrane?

EDIT: Ok, so I've googled a bit more and I think proteins destined for the cell membrane are (post-translation) sent to the ER. So, do they get sent along with all the proteins for secretion and stick around with a stop-transfer sequence?

Sun May 09, 2010 2:44 am

proteins that are destined for the surface of the cell are translated in the rough ER and in that process they are stuck in between pieces of cell membrane. The "pieces" of cell membrane then fuse with the cell membrane so they don't have to get processed by the golgi at all.

For cellular proteins there is a signal that is located after the protein has been translated at the end of the protein. the protein then travels from the ribosome where it was just translated to the golgi (random bumping into) where the golgi reads the signal and clips it off, packaging the protein in a membrane bound vessel that is "labeled" to travel to whatever area needs to get it.

Re: Cell membrane targeting signals.

Sun May 09, 2010 3:26 pm

Ok, that makes sense. I think that's what my lecturer's notes were saying too :) Thanks for the help.

Re: Cell membrane targeting signals.

Tue Sep 06, 2011 9:39 am

Proteins destined for secretion or residence within the lumen of the ER, Golgi apparatus, or lysosomes are translocated across the ER membrane and released into the lumen of the ER as already described. However, proteins destined for incorporation into the plasma membrane or the membranes of the ER, Golgi, or lysosomes are initially inserted into the ER membrane instead of being released into the lumen. From the ER membrane, they proceed to their final destination along the same pathway as that of secretory proteins: ER→ Golgi→ plasma membrane or lysosomes. These proteins are transported along this pathway as membrane components, however, rather than as soluble proteins.

Integral membrane proteins are embedded in the membrane by hydrophobic regions that span the phospholipid bilayer. The membrane-spanning portions of these proteins are usually α-helical regions consisting of 20 to 25 hydrophobic amino acids. The formation of an α helix maximizes hydrogen bonding between the peptide bonds, and the hydrophobic amino acid side chains interact with the fatty acid tails of the phospholipids. However, different integral membrane proteins differ in how they are inserted . For example, whereas some integral membrane proteins span the membrane only once, others have multiple membrane-spanning regions. In addition, some proteins are oriented in the membrane with their amino terminus on the cytosolic side; others have their carboxy terminus exposed to the cytosol. These orientations of proteins inserted into the ER, Golgi, lysosomal, and plasma membranes are established as the growing polypeptide chains are translocated into the ER. The lumen of the ER is topologically equivalent to the exterior of the cell, so the domains of plasma membrane proteins that are exposed on the cell surface correspond to the regions of polypeptide chains that are translocated into the ER
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