Kinesin-1 tail autoregulation and microtubule-binding regions function in saltatory transport but not ooplasmic streaming.

The N-terminal head domain of kinesin heavy chain (Khc) is well known for generating force for transport along microtubules in cytoplasmic organization processes during metazoan development, but the functions of the C-terminal tail are not clear. To address this, we studied the effects of tail mutations on mitochondria transport, determinant mRNA localization and cytoplasmic streaming in Drosophila. Our results show that two biochemically defined elements of the tail - the ATP-independent microtubule-binding sequence and the IAK autoinhibitory motif - are essential for development and viability. Both elements have positive functions in the axonal transport of mitochondria and determinant mRNA localization in oocytes, processes that are accomplished by biased saltatory movement of individual cargoes. Surprisingly, there were no indications that the IAK autoinhibitory motif acts as a general downregulator of Kinesin-1 in those processes. Time-lapse imaging indicated that neither tail region is needed for fast cytoplasmic streaming in oocytes, which is a non-saltatory bulk transport process driven solely by Kinesin-1. Thus, the Khc tail is not constitutively required for Kinesin-1 activation, force transduction or linkage to cargo. It might instead be crucial for more subtle elements of motor control and coordination in the stop-and-go movements of biased saltatory transport.

Three routes to suppression of the neurodegenerative phenotypes caused by kinesin heavy chain mutations.

Kinesin-1 is a motor protein that moves stepwise along microtubules by employing dimerized kinesin heavy chain (Khc) subunits that alternate cycles of microtubule binding, conformational change, and ATP hydrolysis. Mutations in the Drosophila Khc gene are known to cause distal paralysis and lethality preceded by the occurrence of dystrophic axon terminals, reduced axonal transport, organelle-filled axonal swellings, and impaired action potential propagation. Mutations in the equivalent human gene, Kif5A, result in similar problems that cause hereditary spastic paraplegia (HSP) and Charcot-Marie-Tooth type 2 (CMT2) distal neuropathies. By comparing the phenotypes and the complementation behaviors of a large set of Khc missense alleles, including one that is identical to a human Kif5A HSP allele, we identified three routes to suppression of Khc phenotypes: nutrient restriction, genetic background manipulation, and a remarkable intramolecular complementation between mutations known or likely to cause reciprocal changes in the rate of microtubule-stimulated ADP release by kinesin-1. Our results reveal the value of large-scale complementation analysis for gaining insight into protein structure-function relationships in vivo and point to possible paths for suppressing symptoms of HSP and related distal neuropathies.