SOME PROPERTIES OF PHLOEM PROTEINS FROM PUMPKIN EXUDATE

Phloem exudate from Cucurbita pepo L. was collected in a medium containing 20 mM dithiothreitol (DTT). Filaments of phloem protein (P-protein) passed over into solution in the process. Formation of bundles of parallel filaments and paracrystals of P-protein occurred when 50 MM MgCl2 was added to a solution of exudate. Phalloidin (20 muM) altered the shape of paracrystals. Formation of paracrystals was also possible in the absence of high concentrations of MgCl2 in the event that polylysine was added to a solution of exudate (0.3 mg.ml-1) or the osmotic potential of the medium was raised by adding 5% PEG 6000. Bundles and paracrystals of P-protein consisted of two types of fibers: fibers with diameter of 5-7 nm and ones with diameter of 20-40 nm. Thick filaments consisted of tightly packed thin filaments partly oriented along the main axis and partly convoluted into a helix. The detected ability of P-protein to form different structures depending on environmental conditions possibly constitutes the basis of its polymorphism in young and mature sieve elements. All of the reagents affecting formation of P-protein structures that were used in the present study induce generation of paracrystal forms of muscle actin. The possibility that actin exists among exudate P-proteins is discussed.