Protein particles in Chlamydomonas flagella undergo a transport cycle consisting of four phases

We used an improved procedure to analyze the intraflagellar transport (IFT) of protein particles in Chlamydomonas and found that the frequency of the particles. not only the velocity, changes at each end of the flagella. Thus , particles undergo structural remodeling at both flagellar locations. Ther efore, we propose that the IFT consists of a cycle composed of at least fou r phases: phases II and IV, in which particles undergo anterograde and retr ograde transport, respectively, and phases I and III, in which particles ar e remodeled/ exchanged at the proximal and distal end of the flagellum, res pectively. In support of our model, we also identified 13 distinct mutants of flagellar assembly (fla), each defective in one or two consecutive phase s of the IFT cycle. The phase I-II mutant fla10-1 revealed that cytoplasmic dynein requires the function of kinesin II to participate in the cycle, Ph ase I and II mutants accumulate complex A, a particle component, near the b asal bodies, In contrast, phase III and IV mutants accumulate complex B, a second particle component, in flagellar bulges. Thus, fla mutations affect the function of each complex at different phases of the cycle.