THE 78,000-M(R) INTERMEDIATE CHAIN OF CHLAMYDOMONAS OUTER ARM DYNEIN IS A MICROTUBULE-BINDING PROTEIN

A previous study (King et al., 1991. J. Biol. Chem. 266:8401-8407) sho wed that the 78,000-M(r) intermediate chain (IC78) from the Chlamydomo nas outer arm dynein is in direct contact with alpha-tubulin in situ, suggesting that this protein may be involved in binding the dynein to the doublet microtubules. Molecular genetic analysis of this chain rec ently demonstrated that it is a WD repeat protein essential for outer arm assembly (Wilkerson et al., 1995. J. Cell Biol. 129:169-178). We h ave now transcribed and translated IC78 in vitro, and demonstrate that this molecule binds axonemes and microtubules, whereas a homologous p rotein (the 69,000-M(r) intermediate chain [IC69] of Chlamydomonas out er arm dynein) does not. Thus, IC78 is a bona fide microtubule-binding protein. Taken together with the previous results, these findings ind icate that IC78 is likely to provide at least some of the adhesive for ce that holds the dynein to the doublet microtubule, and support the g eneral hypothesis that the dynein intermediate chains are involved in targeting different dyneins to the specific cell organelles with which they associate. Analysis of the binding activities of various IC78 de letion constructs translated in vitro identified discrete regions of I C78 that affected the binding to microtubules; two of these regions ar e specifically missing in IC69. Previous studies also showed that IC78 is in direct contact with IC69; the current work indicates that the r egion of IC78 that mediates this interaction is coincident with two of IC78's WD repeats. This supports the hypothesis that these repeats ar e involved in protein-protein interactions within the dynein complex.