STRUCTURE AND FUNCTIONAL MECHANISM OF PORINS

Cellular organisms such as gram-negative bacteria are enclosed by a du al lipid bilayer system. The outer membranes of the dual bilayer envel opes predominantly contain large numbers of water-filled transmembrane protein channels known as porins. The recent availability of the mole cular structures of several bacterial porins has provided the opportun ity for comparing the results of a nide range of functional studies wi th the atomic level structural details of these membrane channels. Tak en together the structure and function data present the most comprehen sive 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-resoluti on structure data from the bacterial porins, as well as recent theoret ical studies. in the context of biophysical and biochemical observatio ns and discuss the molecular mechanisms responsible for tile transport of solutes through porin channels. Particular emphasis has been place d on the features and roles of common structural elements, channel ste rics and electrostatics, and voltage-dependent gating. A model for wat er-coordinated transport, providing a qualitative view of the porin tr ansport mechanism, is also described.