α-Type channels. Transmembrane channel proteins of this class are ubiquitously found in the membranes of all types of organisms from bacteria to higher eukaryotes. These transporters usually catalyze movement of solutes by an energy-independent process by passage through a transmembrane aqueous pore or channel without evidence for a carrier-mediated mechanism. These channel proteins usually consist largely of α-helical spanners, although β-strands may be present and may even contribute to the channel. Outer membrane porin-type channel proteins are excluded from this class and are instead included in class 1.B. Transmembrane communication via the M2 proton channel, K⁺ channels, and integrin receptors has been discussed by Grigoryan et al. (2011).