The Nodulin 26 Intrinsic Protein Subfamily
Nodulin intrinsic proteins (NIPs) represent a land plant specific subfamily of the major intrinsic protein/aquaporin superfamily. NIPs are named for the first member of the family discovered, soybean nodulin 26 of symbiotic nitrogen fixing root nodules. Evolutionarily, NIPs appear in early nonvascular and vascular land plant lineages, with the family undergoing substantial diversification and sub functionalization during subsequent evolution of seed plants. Structurally, most NIPs can be divided into three "pore" families based on the composition of amino acids comprising the predicted aromatic arginine selectivity region of the channel pore. Functionally, two of these families (NIP II and NIP III) serve as channels for metalloid nutrients (boric acid and silicic acid respectively) while the biological role of NIP I channels remains more open. Biochemical functions for NIP proteins are diverse, with transport selectivities ranging from metalloid hydroxides to glycerol, lactic acid, urea, and hydrogen peroxide. Some NIPs retain their aquaporin function, while others have lost this signature activity of the aquaporin family. In the present chapter, the evolutionary origins, structural and functional properties, and potential biological functions, particularly beyond their roles as metalloid facilitators, are reviewed.