Subunit structure and interactions of the phloem proteins of Cucurbita maxima (pumpkin)

The two major proteins from the phloem exudate of Cucurbita maxima (pumpkin), PP1 and PP2, were stable in the absence of reducing agents after modification of their accessible cysteine residues with iodoacetamide. This permitted their purification without precautions to prevent oxidation. PP2, a lectin specific for oligomers of N-acetyl-D-glucosamine, was shown by sedimentation-equilibrium ultracentrifugation to be a dimer of Mr of 48000. Neither dithiothreitol nor tri-(N-acetyl-D-glucosamine) altered this value. The constituent polypeptides were linked by two buried disulphide bridges. PP2 behaved aberrantly on gel-filtration on both Sephadex and Bio-Gel unless tri-(N-acetyl-D-glucosamine) was added to the elution buffer; the Mr was then measured as 46000. Other proteins which bind oligomers of N-acetyl-D-glucosamine are also retarded on gel-filtration. Soluble phloem filaments were prepared by collection of exudate into deaerated buffer containing iodoacetamide but no reducing agent. Oxidative gellation of the filaments was prevented by rapid modification of their many accessible cysteine residues, and is assumed to have maintained the degree of polymerisation found in vivo. Those disulphide bridges which were present allowed the incorporation of approximately 60% of the PP1 and 80% of the PP2 into polymeric material. It is concluded that PP1 and PP2 are both structural proteins present in the filaments observable in vivo. PP2 had an elongated binding-site for oligomers of N-acetyl-D-glucosamine. It is suggested that this lectin immobilises bacteria and fungi to the cross-linked filaments which seal wounded phloem sieve-tubes, and thus maintains sterility.