Structure and protein composition of the striated flagellar rootlets of some protists

The striated rootlets of different protists are extremely diverse and, on the basis of structural organization, can be assigned to no fewer than four major types. In light of this extreme variation in fine-structure is it reasonable to expect that all striated rootlets may share common protein species? Using the flagellar rootlet of Naegleria gruberi strain NB-1 as reference, we compared rootlet size, structure, and protein composition to another Naegleria strain, another amoebo-flagellate, Tetramitus rostratus, and to the ciliate Tetrahymena pyriformis. Although differing in size and distribution in the cells, the rootlets of all three amoebo-flagellates appeared very similar in structure, periodicity, and in the presence of a common 170 000 Dalton subunit. Kinetodesmal fibres of Tetrahymena differed markedly in detailed fine-structure, in periodicity, and in the apparent absence of the 170 000 Dalton subunit as tested either by SDS gel electrophoresis or by indirect immunofluorescence staining using a specific antiserum directed against the NB-1 major rootlet protein. Consideration of literature describing striated rootlet structure in a wide variety of ciliated and flagellated cells led to the speculation that striated rootlets arose subsequent to primitive flagella and likely evolved along two major pathways: a narrow-period rootlet similar to those discussed above, which developed from the microtubule rootlets of algal flagellates, and a wide-band, contractile rootlet which originated from the primitive interbasal body connector prominent in both algal and protozoan species.