PURIFICATION AND PARTIAL CHARACTERIZATION OF A GLYCOPROTEIN FROM PEA (PISUM-SATIVUM) WITH RECEPTOR ACTIVITY FOR RHICADHESIN, AN ATTACHMENT PROTEIN OF RHIZOBIACEAE

Attachment of Rhizobium and Agrobacterium bacteria to cells of their host plants is a two-step process. The first step, direct attachment of bacteria to the plant cell wall, is mediated by the bacterial protein rhicadhesin. A putative plant receptor molecule for rhicadhesin was purified from cell walls of pea roots using a bioassay based on suppression of rhicadhesin activity. This molecule appeared to be sensitive to treatments with pronase or glycosidase. Its isoelectric point is 6.4, and its apparent molecular mass was estimated to be 32 kDa before and 29 kDa after glycosidase treatment, as determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and ultrafiltration. The sequence of the first 29 N-terminal amino acids was determined: A-D-A-D-A-L-Q-D-L-C(?)-V-A-D-Y-A-S-V-I-L-V-N-G-F-A-S-K(Q)-(P/Q)- (L)-(I). No homology with known proteins was found. In the course of this research project the extracellular matrix protein vitronectin was reported to inhibit attachment of A. tumefaciens to carrot cells [29]. A variety of adhesive proteins, including vitronectin, contain a common cell attachment determinant with the sequence R-G-D. Since we could not detect other cell wall components able to suppress rhicadhesin activity, and since an R-G-D containing hexapeptide was also active as a receptor, we speculate that the plant receptor for rhicadhesin is a glycoprotein containing an R-G-D attachment site.