Author:
Leineweber Kirsten,Schulz Alexander,Thompson Gary A.
Abstract
This paper originates from a presentation at the International Conference on Assimilate Transport and Partitioning, Newcastle, NSW, August 1999
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.
Subject
Plant Science,Agronomy and Crop Science
Cited by
19 articles.
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