Dynamic transitions in the translocated phloem filament protein

Recent evidence suggests that the P-proteins of Cucurbita maxima exist in at least two structural states: large polymers that are immobilized in individual sieve elements and small polymers or individual subunits that are translocated over long distances. We investigated variation in the structure of the phloem filament protein (phloem protein 1 or PP1) to determine the translocated form of the protein and its relationship to the polymerized state. It was demonstrated that the stability, folding state and assembly of the phloem filament protein rely on distinct intramolecular disulfide bonds. Acid trapping experiments combined with intergeneric grafts revealed that the phloem filament protein is translocated as an 88 kDa globular protein. By altering the pH of the collection buffer (pH 2-10), four individual conformational isoforms of PP1 with molecular masses of 81, 83, 85 and 88 kDa were consistently observed. The 81 kDa isoform represents the totally reduced phloem filament protein, the 83 and 85 kDa isoforms folding intermediates, and the 88 kDa its native soluble translocated form. The 83 and 85 kDa folding intermediates are susceptible to aggregation causing the gelation and formation of P-protein filaments in oxidized phloem sap. In contrast to the 88 kDa globular transport form, the 81, 83 and 85 kDa isoforms possibly exhibit lower stability, and therefore a higher sensitivity to proteolytic digestion.