Effects of trypsin-digested outer-arm dynein fragments on the velocity of microtubule sliding in elastase-digested flagellar axonemes

Flagellar movement is caused by the coordinated activity of outer and inner dynein arms, which induces sliding between doublet microtubules. In trypsin-treated flagellar axonemes, microtubule sliding induced by ATP is faster in the presence than in the absence of the outer arms. To elucidate the mechanism by which the outer arms regulate microtubule sliding, we studied the effect of trypsin-digested outer-arm fragments on the velocity of microtubule sliding in elastase-treated axonemes of sea urchin sperm flagella. We found that microtubule sliding was significantly slower in elastase-treated axonemes than in trypsin-treated axonemes, and that this difference disappeared after the complete removal of the outer arms. After about 95% of the outer arms were removed, however, the velocity of sliding induced by elastase and ATP increased significantly by adding outer arms that had been treated with trypsin in the presence of ATP. The increase in sliding velocity did not occur in the elastase-treated axonemes from which the outer arms had been completely removed. Among the outer arm fragments obtained by trypsin treatment, a polypeptide of about 350 kDa was found to be possibly involved in the regulation of sliding velocity. These results suggest that the velocity of sliding in the axonemes with only inner arms is similar to that in the axonemes with both inner and outer arms, and that the 350 kDa fragment, probably of the alpha heavy chains, increases the sliding activity of the intact outer and inner arms on the doublet microtubules.