Identification of dynein as the outer arms of sea urchin sperm axonemes

The location of dynein, the main flagellar ATPase, within the sea urchin sperm axoneme was investigated by the use of immunofluorescence and immunoelectron microscopy, employing an antiserum against a tryptic fragment of dynein 1 (Fragment 1A) purified from sea urchin sperm flagella. The axonemes were found to be stained with the antiserum when examined by an indirect immunofluorescence technique. Immunoelectron microscopy with the antiserum and a ferritin-conjugated IgG fraction of goat antiserum to rabbit IgG revealed that, among the structures within the axoneme, only the outer arms were labeled with ferritin particles. With either the normal serum or antiserum absorbed with Fragment 1A, there were no ferritin particles within the axonemes. When the outer arms were extracted with 0.5 M NaCl, leaving the inner arms intact, again no ferritin dots were detected. Furthermore, it was found that the outer arm on the no. 5 doublet microtubule, which connects with the extra arm projection backward from the no. 6 doublet, had no attached ferritin particles. From these observations, it can be concluded that the outer arm consists of dynein (at least dynein 1) and that Fragment 1A, containing the active site for ATPase activity of dynein 1, is located at the distal end of the outer arms. The significance of the present findings is considered in connection with flagellar movement.