NATIONAL INSITUTE FOR BASIC BIOLOGY

National Institute for Basic Biology

Division of Cellular Communication

(Adjunct)

Professor:
Ritsu Kamiya
Associate Professor:
to be appointed
Research Associate:
to be appointed


The research in this laboratory, started in November, 1996, is aimed at an understanding of the molecular mechanisms that regulate the assembly and function of cytoskeletal proteins. Current research effort is centered on the function of axonemal dyneins, microtubule-based motor proteins that produce force for flagellar beating. The organism employed is Chlamydomonas, a biflagellate green alga particularly suited for genetic and molecular biological studies.

A single flagellar axoneme contains at least eleven kinds of dynein heavy chains in inner and outer arms. To understand the specific function of each heavy chain, we have been isolating and characterizing mutants that lack different kinds of axonemal dyneins. Their motility phenotypes have indicated that different dynein species differ in function in a fundamental manner. For example, the outer arm heavy chains are important for flagellar beating at high frequency, whereas most of the inner arm heavy chains are important for producing proper waveforms. In addition, indirect evidence has suggested that the force generation properties differ greatly among different heavy chains. We are currently trying to measure the force production by different dyneins using micro-physiological techniques.

The inner dynein arms are known to contain actin as a subunit. Thus the two independent motility systems of eukaryotes - the actin-based and microtubule-based motility systems - should somehow cooperate in the inner arm, although the function of actin in dynein arms is unknown at present. The mutant ida5 we isolated as a mutant that lacks four subspecies of inner-arm dynein was recently found to have a mutation in the actin-encoding gene. This mutant as well as another independently isolated mutant (ida5-t) has a deletion in the actin gene and express no conventional actin. The cytoplasm and axonemes of these mutants lack the conventional actin entirely but contain two novel proteins which are immunologically distinct from, but related to, actin. Since Chlamydomonas has been shown to have only a single copy of actin-encoding gene, it is likely that the two novel proteins in these mutants originate from other gene(s) that codes for an actin-like protein which has hitherto been undetected in wild-type cells. The net growth rate of the mutants did not differ from that of wild type, but the mating efficiency was greatly reduced because of their deficient fertilization tubule growth. These results raise the possibility that Chlamydomonas can live without conventional actin.

The discovery of the mutants that lack the conventional actin entirely should open the way to studying the function of actin in the cell as well as in the dynein arm. We have succeeded in transforming the mutant Chlamydomonas with cloned actin gene and found that inner dynein arms become restored upon transformation. Studies with artificially mutated actin gene will enable us to determine whether actin is really essential for cytokinesis or other fundamental processes in Chlamydomonas, and what functions are carried out by actin in the axoneme and cytoplasm. A most important immediate project will be to characterize the novel proteins found only in the mutants that lack actin.

Fig. 1
Wild-type and mutant axonemes of Chlamydomonas that lack different heavy chains of outer arm dynein. Wild-type outer arm contains three heavy chains, a, b and g, of which the mutant oda4-s7 lacks b and oda11 lacks a. Lower photographs are the averaged images of the outer doublet microtubules, showing the structural defect in each mutant (arrowhead).


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Last Modified: 12:00, June 27, 1997