Structure and functional mechanism of porins

Cellular organisms such as gram-negative bacteria are enclosed by a dual lipid bilayer system. The outer membranes of the dual bilayer envelopes predominantly contain large numbers of water-filled transmembrane protein channels known as porins. The recent availability of the molecular structures of several bacterial porins has provided the opportunity for comparing the results of a wide range of functional studies with the atomic level structural details of these membrane channels. Taken together, the structure and function data present the most comprehensive set of boundary conditions available for the evaluation of theory and models predicting the characteristics of solute transport through membrane protein channels. In this paper, we review the high-resolution structure data from the bacterial porins, as well as recent theoretical studies, in the context of biophysical and biochemical observations and discuss the molecular mechanisms responsible for the transport of solutes through porin channels. Particular emphasis has been placed on the features and roles of common structural elements, channel sterics and electrostatics, and voltage-dependent gating. A model for water-coordinated transport, providing a qualitative view of the porin transport mechanism, is also described.