Structure of striated microtubule-associated fibers of flagellar roots. Comparison of native and reconstituted states.

Several fiber systems are associated with the flagella and basal bodies of eukaryotic cells. Apart from the contractile and Ca(2+)-sensitive system-II fibers, these include the noncontractile system-I fibers that run parallel to flagellar root microtubules. Using electron microscopy and image reconstruction, we have investigated the structure of the system-I fibers of the flagellate green alga Spermatozopsis similis. The fibers were observed in three different states: (1) in situ, (2) after isolation of the intact fibers, (3) after disassembly and reconstitution of fibers in vitro from their 34 kDa subunit protein. The fibers are highly ordered; they show a constant repeat of 28 nm, they are polar, and they contain several transverse and longitudinal striations. A model is discussed showing the system-I fiber to be built from rod-like molecules with a staggered arrangement and identical polarities.