Two bean cell wall proteins more abundant during water deficit are high inproline and interact with a plasma membrane protein

Two antigenically related glycoproteins, called p33 and p36, accumulate in the soluble fraction of the cell wall in response to water deficit in Phase olus vulgaris. In this report, we show that p33 and p36 are able to adhere to leaf protoplasts, and that they bind to plasma membrane (PM) vesicles in a divalent cation-dependent manner. Data from the partial amino acid seque nce of the p33 and p36 proteins indicate that they contain repeats of the d ecapeptide POVYKPOVEK; therefore, they are related to proline-rich proteins . Binding assays demonstrate that both proteins specifically bind to an 80 kDa PM protein. This binding is competed with a peptide that contains the R GD motif, as well as with fibronectin, which also includes this sequence, s uggesting that the 80 kDa PM protein has an integrin-like function whose na tural ligands are p33 and p36. This is the first case where a PM ligand for a higher plant cell wall protein has been identified.