Any of a class of proteins that form water-filled channels across cell membranes. In the outer membrane of Gram-negative bacteria, most porins consist of three identical subunits that associate and extend fully through the membrane to form channels about 1 nm in diameter. These permit the entry and exit of hydrophilic low-molecular-weight substances; some contain a specific binding site and allow the passage only of certain substances. The pores can be opened and closed, which has significance for antibiotic resistance in pathogenic bacteria. Similar porin proteins occur in the mitochondrial outer membrane of eukaryotic cells, and they are now known to exist in eukaryotic plasma membranes, where they may be responsible for controllable movements of water in or out of the cell associated with osmosis. Nucleoporins constitute the pores in the nuclear envelope surrounding the nucleus.