Legumes form a unique subcellular compartment to house nitrogen- fixing endosymbiotic bacteria (Rhizobium species). Integrity of the membrane enclosing this compartment, the peribacteroid membrane, is essential for the maintenance of the symbiotic state. However, little is known about the nature, function and biogenesis of this membrane. Originally derived from the plasma membrane, the peribacteriod membrane incorporates a number of nodule-specific proteins (nodulins) which play new roles imposed on this subcellular compartment. Several soybean genes that encode peribacteriod membrane nodulins have been identified and characterized. The peribacteroid membrane nodulins are specifically targeted to this compartment and are not integrated into the plasma membrane of the infected cells. The goal of this research is to identify sequences and the properties of the peribacteroid membrane proteins necessary for correct targeting of nodulins to the peribacteroid membrane. Deletion analysis of soybean nodulin sequences followed by fusion to reporter genes and transformation of a heterologous legume, Lotus corniculatus, will be used to determine the mechanism of targeting. The effect of integration of complete soybean peribacteroid membrane nodulins into the Lotus peribacteroid membrane will be studied using specific antibodies raised against individual nodulins, either synthetic peptides or nodulins from cDNA clones expressed in E. coli. This study will shed light not only on the cellular mechanisms that permit nitrogen fixing bacteria to live in nodules on the roots of legumes, but more generally on the mechanisms by which cell membranes differentiate to allow compartmentalization of vital cellular functions.
