A unipolarly located, cell-surface-associated agglutinin, RapA, belongs toa family of Rhizobium-adhering proteins (Rap) in Rhizobium leguminosarum bv. trifolii

The phage-display cloning technique was used to find rhizobial proteins tha t bind to receptors located on the bacterial cell surface. The aim was to c lone the gene(s) encoding rhicadhesin, a universal rhizobial adhesion prote in, and/or other cell-surface-binding proteins. Four such Rhizobium-adherin g proteins (Rap) were revealed in Rhizobium leguminosarum by. trifolii stra in R200. The binding is mediated by homologous Ra domains in these proteins . One member of the Rap protein family, named RapA1, is a secreted calcium- binding protein, which are also properties expected for rhicadhesin. Howeve r, the size of the protein (24 kDa instead of 14 kDa) and its distribution among different rhizobia (present in only Rhizobium leguminosarum biovars a nd R. etli instead of all members of Rhizobiaceae) argue against RapA1 bein g rhicadhesin. Protein RapA1 consists of two homologous Pa domains and aggl utinates R200 cells by binding to specific receptors located at one cell po le during exponential growth. Expression of these cell-surface receptors wa s detected only in rhizobia that produce the RapA proteins. The authors pro pose that the homologous Pa domains, found to be present also in other prot eins with different structure, represent lectin domains, which confer upon these proteins the ability to recognize their cognate carbohydrate structur es.