The Cysteine Scanning Mutagenesis Method for eejits: Part The First (in which I don't actually talk about the Cysteine Scanning Mutagenesis Method)

Figure 1. Topology of a P2X receptor subunitWhen a protein strongly suspected of being a plasma membrane ion channel is isolated and cloned, one of the first things we want to find out about it is which parts form the water-filled pore responsible for conducting ion passage across the otherwise water-unfriendly lipid bilayer into and out of the cell.In the absence of a 3-dimensional crystal structure for the ion channel, this requires a bit of detective work. First, various techniques are used to determine the approximate topology of individual ion channel subunits, telling us which parts of the protein are extracellular, which parts are intracellular and which parts span the biological membrane in which the channel is usually found (e.g. the plasma membrane). The example we'll keep returning to for reasons of author bias is the ATP-gated P2X receptor family, of which one of the three subunits required to form a functional channel is shown in Figure 1. Intuitively, whatever part of the protein transports ions across a biological membrane must itself be within the membrane, either as a transmembrane spanning section of peptide crossing from one side to the other, or a loop that dips part of thew way in to the membrane and back out the same side of the membrane. Now, it happens that many ion channels have lots of peptide sections, almost always in the form of alpha-helices, threading to and fro across the membrane, and only some of these are going to be involved in forming...
Source: Across the Bilayer - Category: Medical Scientists Source Type: blogs