A CHLAMYDOMONAS OUTER ARM DYNEIN MUTANT WITH A TRUNCATED BETA-HEAVY CHAIN

A new allele of the Chlamydomonas oda4 flagellar mutant (oda4-s7) poss essing abnormal outer dynein arms was isolated. Unlike the previously described oda4 axoneme lacking all three (alpha, beta, and gamma) oute r-arm dynein heavy chains, the oda4-s7 axoneme contains the alpha and gamma heavy chains and a novel peptide with a molecular mass of approx imately 160 kD. The peptide reacts with a mAb (18betaB) that recognize s an epitope on the NH2-terminal part of the beta heavy chain. These o bservations indicate that this mutant has a truncated beta heavy chain , and that the NH2-terminal part of the beta heavy chain is important for the stable assembly of the outer arms. In averaged electron micros copic images of outer arms from cross sections of axonemes, the mutant outer arm lacks its mid-portion, producing a forked appearance. Toget her with our previous finding that the mutant oda11 lacks the alpha he avy chain and the outermost portion of the arm (Sakakibara, H., D. R. Mitchell, and R. Kamiya. 1991. J. Cell Biol. 113:615-622), this result defines the approximate locations of the three outer arm heavy chains in the axonemal cross section. The swimming velocity of oda4-s7 is 65 +/- 8 mum/s, close to that of oda4 which lacks the entire outer arm ( 62 +/- 8 mum/s) but significantly lower than the velocities of wild ty pe (194 +/- 23 mum/s) and oda11 (119 +/- 17 mum/s). Thus, the lack of the beta heavy chain impairs outer-arm function more seriously than do es the lack of the alpha heavy chain, suggesting that the alpha and be ta heavy chains play different roles in outer arm function.