Distinct mutants of retrograde intraflagellar transport (IFT) share similar morphological and molecular defects

A microtubule-based transport of protein complexes, which is bidirectional and occurs between the space surrounding the basal bodies and the distal pa rt of Chlamydomonas flagella, is referred to as intraflagellar transport (I FT). The IFT involves molecular motors and particles that consist of 17S pr otein complexes, To identify the function of different components of the IF T machinery, we isolated and characterized four temperature-sensitive (ts) mutants of flagellar assembly that represent the loci FLA15, FLA16, and FLA 17, These mutants were selected among other ts mutants of flagellar assembl y because they displayed a characteristic bulge of the flagellar membrane a s a non-conditional phenotype. Each of these mutants was significantly defe ctive for the retrograde velocity of particles and the frequency of bidirec tional transport but not for the anterograde velocity of particles, as reve aled by a novel method of analysis of IFT that allows tracking of single pa rticles in a sequence of video images, Furthermore, each mutant was defecti ve for the same four subunits of a 17S complex that was identified earlier as the IFT complex A. The occurrence of the same set of phenotypes, as the result of a mutation in any one of three loci, suggests the hypothesis that complex A is a portion of the IFT particles specifically involved in retro grade intraflagellar movement.