2-  Transport across membranes: Similar to how transport functions via the nucleus, most proteins are transported across the biomembrane with the help of receptor proteins which form a complex with the import molecule.

Protein translocators located between the biomembrane (transmembrane protein or integrins) identifies the signal sequence by unfolding the protein and transports it across the membrane into the organelle or cell (similar to the way NPCs do).

Once the protein enters the organelle or cell, its signal sequence is cut off by a peptidase enzyme and the protein is folded into its final 3-D shape again with the help of helper molecules.

 

In the case of proteins intended for secretion out of the cell, as soon as their N-terminal contains the signal sequence that will indicate this, a signal recognition particle binds to it and forms a protein complex causing synthesis to slow down. The SRP-protein complex will travel and bind to a SRP-receptor on a rough endoplasmic reticulum (RER) membrane. Once the protein gets to the RER membrane, its synthesis will continue at the normal rate until it is complete. During translocation out of the cell, the signal sequence is cut off by a signal peptidase located in the RER membrane and released from the translocation channel and degraded to amino acids.

RER serves as an entering point for proteins that have to go to other organelles like the Golgi complex and lysosome. They can also ferry proteins out of the cell by transport vesicles formed from the budding off of their biomembrane.

 

There are three mechanisms in which newly synthesized proteins are inserted into the RER membrane (the mechanisms vary with the type of protein);

  • Type I; signal sequence on N-terminals enters first and continues to elongate until a hydrophobic stop-sequence is reached, and then inserted in the membrane and forms the anchor for that protein. Of course then signal sequence is cut by protease.
  • Type II; these proteins have rather long hydrophobic regions that will be anchored in the membrane with the C-terminal leading. Protein continues to be inserted until it reaches the hydrophobic stop-sequence, but the signal sequence is not cut.
  • Type III; same as type I, the only difference is that the signal sequence is not cut.

 

And according to the number of times that protein passes through the membrane, the proteins of the ER membrane can be divided into single pass transmembrane protein, double pass transmembrane protein, and multi-pass transmembrane protein.

 

I understand the information here may appear bland. This is because it has been generalized for most process within the cell and cell biologists study the various pathways that exist which can be very overwhelming. Here a brief overview is being discussed such that when you choose to do more detailed study, this generic pattern would have provided you with a basic background.