GO:0015288

Catalysis of the transfer of substances, sized less than 1000 Da, from one side of the membrane to the other. The transmembrane portions of porins consist exclusively of beta-strands which form a beta-barrel. They are found in the outer membranes of Gram-negative bacteria, mitochondria, plastids and possibly acid-fast Gram-positive bacteria. [GOC:mtg_transport, ISBN:0815340729, PMID:10839820, TC:1.B.1.-.-]