Insights into the structural organization of the I1 inner arm dynein from a domain analysis of the 1 beta dynein heavy chain

To identify domains in the dynein heavy chain (Dhc) required for the assemb ly of an inner arm dynein, we characterized a new motility mutant (ida2-6) obtained by insertional mutagenesis. ida2-6 axonemes lack the polypeptides associated with the I1 inner arm complex. Recovery of genomic DNA flanking the mutation indicates that the defects are caused by plasmid insertion int o the Dhc10 transcription unit, which encodes the 1 beta Dhc of the I1 comp lex. Transformation with Dhc10 constructs encoding <20% of the Dhc can part ially rescue the motility defects by reassembly of an I1 complex containing an N-terminal 1 beta Dhc fragment and a full-length 1 alpha Dhc. Electron microscopic analysis reveals the location of the missing 1 beta Dhc motor d omain within the axoneme structure. These observations, together with recen t studies on the 1 alpha Dhc, identify a Dhc domain required. for complex a ssembly and further demonstrate that the intermediate and light chains are associated with the stem regions of the Dhcs in a distinct structural locat ion. The positioning of these subunits within the I1 structure has signific ant implications for the pathways that target the assembly of the I1 comple x into the axoneme and modify the activity of the I1 dynein during flagella r motility.