ABSTRACT
The molecular biology of membranes has been studied for many years.1 Membrane proteins perform a range of functions from cell signaling to solute transport;1,2 although well characterized biochemically, these functions have not been studied greatly using the methods of proteomics. In principle, proteomics approaches offer the opportunity to identify the proteins involved in particular cellular processes, to study the context within which the proteins are expressed, and to monitor protein expression and turnover in relation to the dynamics of the cellular environment. The structure of the cell membrane consists of a lipid bilayer with which proteins are associated. Membrane proteins can be broadly divided into two categories: integral (or intrinsic) membrane proteins, which are inserted in the lipid bilayer, and membrane
associated (or peripheral) proteins, which are bound to the membrane through covalent or noncovalent interactions:
Integral or intrinsic membrane proteins contain hydrophobic transmembrane domains (TMDs), which pass through the lipid bilayer several times, anchoring the protein within the structure. Since water is absent in the membrane, peptide bonds in the bilayer form hydrogen bonds that are primarily intramolecular. This can be maximized if the polypeptide chain forms a tightly ordered secondary or tertiary structure as it crosses the bilayer.2 Many integral membrane proteins have TMDs of 15 to 25 amino acids that span the lipid bilayer in the form of an helix. Alternatively, porins, which are found in the outer membranes of bacteria, form sheets organized into a -barrel tertiary structures that can allow the passage of molecules across the membrane. Integral membrane proteins can be considered as true membrane proteins since they are embedded in the membrane itself. They will be present exclusively in membrane preparations and are the most challenging to characterize biochemically.
