A DYNEIN LIGHT-CHAIN IS ESSENTIAL FOR THE RETROGRADE PARTICLE MOVEMENT OF INTRAFLAGELLAR TRANSPORT (IFT)

Several enzymes, including cytoplasmic and flagellar outer arm dynein, share an M(r)8,000 light chain termed LC8. The function of this chain is unknown, but it is highly conserved between a wide variety of orga nisms. We have identified deletion alleles of the gene (fla14) encodin g this protein in Chlamydomonas reinhardtii. These mutants have short, immotile flagella with deficiencies in radial spokes, in the inner an d outer arms, and in the beak-like projections in the B tubule of the outer doublet microtubules. Most dramatically, the space between the d oublet microtubules and the flagellar membrane contains an unusually h igh number of rafts, the particles translocated by intraflagellar tran sport (IFT) (Kozminski, K.G., P.L. Beech, and J.L. Rosenbaum. 1995. J. Cell Biol. 131:1517-1527). IFT is a rapid bidirectional movement of r afts under the flagellar membrane along axonemal microtubules. Anterog rade IFT is dependent on a kinesin whereas the motor for retrograde IF T is unknown. Anterograde IFT is normal in the LC8 mutants but retrogr ade IFT is absent; this undoubtedly accounts for the accumulation of r afts in the flagellum. This is the first mutation shown to specificall y affect retrograde IFT; the fact that LC8 loss affects retrograde IFT strongly suggests that cytoplasmic dynein is the motor that drives th is process. Concomitant with the accumulation of rafts, LC8 mutants ac cumulate proteins that are components of the 15-16S IFT complexes (Col e, D.G., D.R. Deiner, A.L. Himelblau, P.L. Beech, J.C. Fuster, and J.L . Rosenbaum. 1998. J. Cell Biol. 141:993-1008), confirming that these complexes are subunits of the rafts. Polystyrene microbeads are still translocated on the surface of the flagella of LC8 mutants, indicating that the motor for flagellar surface motility is different than the m otor for retrograde IFT.