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♦ See referenced article, J. Biol. Chem. 2010, 285, 23880–23888
The formation of N2-fixing rhizobia colonies in the root nodules of legumes leads to the development of a novel organelle known as a “symbiosome,” which houses these endosymbiotic bacteria. The symbiosome is delimited by the symbiosome membrane (SM), which controls the transport of metabolites between the symbiont and the plant host. Nodulin 26 is the primary protein component on the SM of N2-fixing soybean nodules that serves as a low energy transport pathway for water, osmolytes, and NH3 across the membrane. Recently, there has been evidence suggesting that nodulin 26 and related major intrinsic proteins (MIPs), in addition to their transport duties, also act as scaffolds for recruiting metabolic enzymes. Now, in this Paper of the Week, Pintu Masalkar and colleagues show that the C-terminal domain peptide of nod26 interacts with 1 at a 1:1 stoichiometry. Additional β-cytosolic glutamine synthetase GS1 experiments using a yeast two-hybrid system or bimolecular fluorescence complementation revealed that in fact all four cytosolic GS isoforms expressed in soybean root nodules interact with nod26. GS is the principal ammonia assimilatory enzyme in soybean, and its interaction with nod26, a transporter of NH3, likely promotes the efficient assimilation of fixed nitrogen while also preventing potential ammonia toxicity by localizing GS to the cytosolic side of the SM.