Fine structure of P-protein filaments from Ricinus communis

Exudate from the phloem of Ricinus communis L. was negatively stained, examined in the electron microscope, and the filamentous components compared with those in fixed, sectioned material. In the exudate, two main fibrillar components were observed. One component has a diameter of 20±0.35 (standard error) nm, the other of 14.1±0.34 nm. This second compoent has projections along its length measuring 5 by 14 nm and spaced at intervals of 6.5-10 nm. Fibrils have been found possessing characteristics of both fibril types, suggesting some structural relationship between the two, possibly an interconvertibility. Several other types of fibrils occurred less frequently in the exudate. The exudate also contains torus-shaped structures measuring 13.5-15 nm in diameter. Sections of mature sieve elements of Ricinus and Acer rubrum L. contain fibrils structurally similar to the 14-nm fibrils from the exudate of Ricinus. Ricinus exudate was also fixed and pelleted in the ultracentrifuge. Thin sections of the pellet afforded cross-sectional views of the 20-nm fibrils, and showed that these fibrils apparently have a solid core. Possible models for the structure of the 20-nm filaments are described.