On the definition, nomenclature and classification of water channel proteins (aquaporins and relatives)

A water channel protein (WCP) or a water channel can be defined as a transmembrane protein that has a specific three-dimensional structure with a pore that provides a pathway for water permeation across biological membranes. The pore is formed by two highly conserved regions in the amino acid sequence, called NPA boxes (or motifs) with three amino acid residues (asparagine-proline-alanine, NPA) and several surrounding amino acids. The NPA boxes have been called the "signature" sequence of WCPs. WCPs are a family of proteins belonging to the Membrane Intrinsic Proteins (MIPs) superfamily. In addition, in the MIP superfamily (with more than 1000 members) there are also proteins with no channel activity. The WCP family include three subfamilies: aquaporins, aquaglyceroporins and S-aquaporins. (1) The aquaporins (AQPs) are water selective or specific water channels, also named by various authors as "orthodox", "ordinary", "conventional", "classical", "pure", "normal", or "sensu strictu" aquaporins); (2) The aquaglyceroporins are permeable to water, but also to other small uncharged molecules, in particular glycerol; this family includes the glycerol facilitators, abbreviated as GlpFs, from glycerol permease facilitators. The "signature" sequence for aquaglyceroporins is the aspartic acid residue (D) in the second NPA box. (3) The third subfamily of WCPs have little conserved amino acid sequences around the NPA boxes, unclassifiable to the first two subfamilies. I recommend to use always for this subfamily the name S-aquaporins. They are also named "superaquaporins", "aquaporins with unusual (or deviated) NPA boxes", "subcellular aquaporins", or "sip-like aquaporins". I also recommend to use always the spelling aquaporin (not aquaporine), and, for various AQPs, the abbreviation AQP followed immediately by the number, (e.g. AQP1), with no space or - which might create confusions with "minus".