Porin (protein)



Expert opinion needed: nucleoporin and porin are different structures according to "Molecular Biology of the Cell" (Alberts et al). This article only describes a porin.

Porins are 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 prevalent in the outer membrane of the mitochondria and Gram-negative bacteria.

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.

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

The term "nucleoporin" refers to porins facilitating transport through nuclear pores in the nuclear envelope.