RapA2 is a calcium binding lectin that specifically recognizes Rhizobium leguminosarum acidic exopolysaccharides

ABDIAN, P; CARAMELO, J.; AUSMEES, N.; ZORREGUIETA, A

Mar del Plata

Congreso; VIII Congreso Argentino de Microbiología General, organizado por la Sociedad Argentina de Microbiología General (SAMIGE); 2012

RapA are extracellular proteins secreted by the type I PrsDE system of the closely related Rhizobium leguminosarum biovars and Rhizobium etli. At least three isoforms have been found in R. leguminosarum bv. trifolii R200. RapA1 isoform was first described as an agglutinin that localizes at one cell pole. The exact role of RapA proteins has not been determined yet. Structural similarity with eukaryotic cadherins suggested that RapA proteins could be involved in cellular aggregation by protein-protein interactions. A detailed biophysical characterization showed that RapA2 does not mediate cellular interactions through a cadherin-like mechanism. We have proposed that RapA might be involved in the modulation of the acidic exopolysaccharide (EPS) matrix structure during biofilm formation in static or continuous flow conditions. Here we report a biochemical characterization of RapA2, the only isoform present in R. leguminosarum bv. viciae 3841. Supporting previous evidence, we showed that RapA2 interacts specifically with the acidic exopolysaccharides produced by R. leguminosarum, and that binding activity is completely dependent on the presence of calcium ions. Inhibition binding assays indicate that glucuronic acid residues may be implicated in the structure recognized by RapA2 on EPS and CPS (capsular polysaccharide), both of which have similar  structures. The results presented here suggest that RapA proteins are calcium-binding lectins and sustain a role of these proteins in the development of the biofilm matrix made of EPS.