Elicitor induction of the synthesis of a novel lectin-like arabinosylated hydroxyproline-rich glycoprotein in suspension cultures of Phaseolus vulgaris L.

Abstract

A novel lectin-like glycoprotein which accumulates in response to fungal elicitor action has been characterised in endomembranes from suspension cultures of French bean (Phaseolus vulgaris L.). The lectin, which has specificity towards N-acetylglucosamine oligomers, consists of a polypeptide of apparent molecular weight (Mr) 31 000 which is rich in glycine and contains 6.7% hydroxyproline O-linked to arabinose-containing oligosaccharides to give a glycoprotein of Mr 42500. A dual-labelling technique has been used to identify changes in the synthesis of the glycoprotein in cells exposed to fungal elicitor molecules. Thus, incorporation of [(14)C]proline into membranes in vivo and of [1(-3)H]arabinose from uridine 5'-diphosphate [1(-3)H]arabinose in vitro and analysis by isoelectric focussing-polyacrylamide gel electrophoresis gave absolute correspondence of the labelled isoform of the glycoprotein. Having established the absence of contaminating polypeptides, subsequent analysis of microsomal fractions bysodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the peak of sythesis of the Mr-42500 glycoprotein occurred 4 h after the addition of fungal elicitor. The changes in the level of incorporation into the glycoprotein monomers were concomitant with increases in the activity of prolyl hydroxylase (EC 1.14.11.2)Incorporation of [(14)C]proline and its subsequent post-translational modification to hydroxyproline in microsomal polypeptides was followed by rapid transfer into the wall with an average t 1/2 of about 7 min. The Mr-42500 glycoprotein was rapidly transferred out of the endomembrane fraction with a t 1/2 of 2 min and could be detected in wall fractions where it became progressively less extractable. The glycoprotein, which clearly differs from bean extensin, accounts for up to 40% of the hydroxyproline newly exported in response to elicitor action. The lectin, which resembles those found in the Solanaceae and which is coinduced with enzymes of phytoalexin synthesis, may play some role in disease resistance.