Further studies on knockout mice lacking a functional dynein heavy chain (MDHC7). 1. Evidence for a structural deficit in the axoneme.

Male mice had previously been generated in which the inner dynein arm heavy chain 7 gene (MDHC7) was inactivated by the substitution of four exons encoding the ATP-binding site (P1-loop) with the neomycin resistance gene, giving a putative non-functional gene product. We have used additional techniques of electron microscopy to determine what effect the truncated, non-functional heavy chain has on the assembly of the inner dynein arm complex. From a comparison of MDHC7-/- with the wild-type morphology, we have found that the expected loss of a C-terminal (globular) domain is associated with inner dynein arm 3, a change from two visible "heads" to one. This deficit was seen in replicas of rapidly-frozen, deeply-etched spermatozoa, and was confirmed in filtered images of 20-nm-thin sections, cut in longitudinal planes. Assembly of the other IDAs appeared unaffected. This study is the first to reveal the location of a specific dynein heavy chain within the 96-nm repeat pattern of the inner dynein arms of the mammalian axoneme.