Porin (protein)

Porins are beta barrel proteins which cross a cellular membrane and act as a pore through which molecules can diffuse. Unlike other membrane transport proteins, porins are large enough to allow passive diffusion - i.e. they act as channels which are specific to different types of molecules. They are present in the outer membrane of Gram-negative bacteria, the mitochondria, and the chloroplast.

Structure
Porins are composed of beta sheets these are generally linked together by beta turns on the cytoplasmic side and long loops of amino acids on the other. The beta sheets lie in an antiparallel fashion and form a cylindrical tube, called a beta barrel. The amino acid composition of the porin beta sheets is unique in that polar and nonpolar residues alternate along them. This means the nonpolar residues face outwards so as to interact with the nonpolar lipid membrane, whilst the polar residues face inwards into the centre of the beta barrel to interact with the aqueous channel.

The porin channel is partially blocked by a loop, called the eyelet, that projects into the cavity. It is generally found between strands 5 and 6 of each barrel, and it defines the size of solute that can traverse the channel. It is lined almost exclusively with charged amino acids arranged on opposite sides of the channel, creating a transversal electric field across the pore. The eyelet has a local surplus of negative charges from four glutamic acid and seven aspartic acid residues (in contrast to one histidine, two lysine and three arginine residues) is partially compensated for by  two bound calcium atoms, and this asymmetric arrangement of molecules is thought to have an influence in the selection of molecules that can pass through the channel. Branden and Toozle, Introduction to Protein Structure, second edition

Cellular roles
Porins typically control the diffusion of small metabolites like sugars, ions, and amino acids.

In gram-negative bacteria the inner membrane is the major permeability barrier, while the outer membrane contains porins which render it largely permeable to molecules less than about 1500 daltons.

The term "nucleoporin" refers to porins facilitating transport through nuclear pores in the nuclear envelope. However, they are often considered distinct from other porins (they are not classified as porins in MeSH.)