Structural and biochemical characterisation of the paraflagellar rod of Crithidia fasciculata.

The trypanosomatid Crithidia fasciculata possesses an intraflagellar structure known as the paraflagellar or paraxial rod which runs from a point 1 to 2 micrometer distal to the basal body to the flagellar tip. In longitudinal section the paraflagellar rod was composed of three "sets" of parallel filaments arranged in a lattice. In cross section it consisted of two electron dense "plaques", one near the flagellar membrane, the other near the axoneme, separated by 6 to 7 fibrous elements. The position of the paraflagellar rod in relation to the axonemal central pair remained static along the length of the flagellum and was the same in all flagella examined. The paraflagellar rod was anchored to the axoneme by a regular array of 5 to 7 nm diameter links. These rod/axoneme links were sensitive to trypsin digestion enabling the rod to be separated from the axoneme. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated that the paraflagellar rod consisted mainly of two proteins, PFR1 (76 000 Daltons) and PFR2 (68 000 Daltons). The isoelectric points of these two proteins were remarkably similar. A PFR-enriched fraction was obtained by prolonged dialysis of demembranated flagella against a low concentration buffer. The paraflagellar rod and the central pair of singlet microtubules went into solution, leaving only the outer doublets intact. The relevance of these results to the study of the role of the paraflagellar rod in flagellar motility were discussed.