Abstract
NIPs show structural similarity to the family archetype soybean nodulin 26. Soybean nodulin 26 is the major protein of the symbiosome membrane of nitrogen fixing nodules. It confers upon the symbiosome the ability to transport water, glycerol and ammonia. It is regulated by phosphorylation of Ser262 by a symbiosome-membrane associated CDPK. Phosphorylation enhances water permeability and is regulated in a developmental fashion, as well as by osmotic signals, suggesting a potential osmoregulatory role. In addition, the C-terminal ser 262 epitope is a site for protein-protein interaction with nitrogen assimilatory enzymes, suggesting a possible metabolic "funnel" role in nitrogen fixation. Structural analysis of various other NIPs in other plant species suggest segregation into two phylogenetic subgroups, NIP I and NIP II, with different structures within the pore selectivity filter. This difference in pore architecture is reflected in distinct transport selectivity and function for NIP I and NIP II proteins.
