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 h ost 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 p urified from cell walls of pea roots using a bioassay based on suppres sion of rhicadhesin activity. This molecule appeared to be sensitive t o treatments with pronase or glycosidase. Its isoelectric point is 6.4 , and its apparent molecular mass was estimated to be 32 kDa before an d 29 kDa after glycosidase treatment, as determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and ultrafiltration. The s equence of the first 29 N-terminal amino acids was determined: 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 e xtracellular matrix protein vitronectin was reported to inhibit attach ment of A. tumefaciens to carrot cells [29]. A variety of adhesive pro teins, including vitronectin, contain a common cell attachment determi nant with the sequence R-G-D. Since we could not detect other cell wal l components able to suppress rhicadhesin activity, and since an R-G-D containing hexapeptide was also active as a receptor, we speculate th at the plant receptor for rhicadhesin is a glycoprotein containing an R-G-D attachment site.