AN UPDATE ON FIBROUS FLAGELLAR ROOTS IN GREEN-ALGAE

In flagellate green algae two types of fibrous flagellar roots can be distinguished: system I fibres, cross-striated bundles of 2 nm filaments (striation periodicity about 30 nm), which are associated with flagellar root microtubules, and system II fibres, contractile bundles of 4-8 nm filaments which are often cross-striated (striation periodicity variable but greater than 80 nm). The major protein of system II fibres is centrin, a Ca2+-modulated phosphoprotein, which is a member of the EF-hand protein family. The major protein of system I fibres (of several Chlamydomonas-type green algae) is a 34 kDa phosphoprotein, named assemblin. Because of the solubility characteristics of system I fibres and the properties of their major protein (paracrystal-formation in vitro, several isoelectric variants, heptad motifs in parts of the amino acid sequence), assemblin is, presumably related to the k-m-e-f class of a-helical fibrous proteins.