How Membrane Proteins Are Assembled

 

Constitutive membrane proteins, i.e. those that are encoded in a normal cell's genome and are responsible for vital physiological activities, are assembled by means of a complex process involving synthesis of membrane proteins by ribosomes attached transiently to a complex of proteins referred to as a translocon located within the cell membrane (below). This translocon provides a transmembrane "tunnel" into which the newly synthesized protein can be injected.  After the synthesis is complete, the ribosome disengages from the translocon (which enters a closed state) and the protein is released into the membrane bilayer where it assumes (in an unknown way) its final folded three-dimensional structure.

Non-constitutive membrane proteins are generally proteins such as toxins and antimicrobial peptides that incorporate themselves into membranes of cells without passing through the constitutive pathway.  A good example is staphylococcal alpha-hemolysin (below) whose crystallographic structure was solved by Song et al.

Non-constitutive membrane proteins usually insert themselves into target membranes by physicochemical processes and are therefore of particular value in understanding membrane protein stability.