nucleoside transmembrane transporter activity porin activity nucleoside-specific channel forming porin activity Enables the energy independent passage of nucleoside, sized less than 1000 Da, across a membrane. 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. GO:0015471 molecular_function