Kinesin-1 is a molecular transporter that trafficks along microtubules. There is some evidence that kinesin-1 targets specific cellular sites, but it is unclear how this spatial regulation is achieved. To investigate this process, we used a combination of in vivo imaging of kinesin heavy-chain Kif5c (an isoform of kinesin-1) fused to GFP, in vitro analyses and mathematical modelling. GFP-Kif5c fluorescent puncta localised to a subset of microtubules in live cells. These puncta moved at speeds of up to 1 m second -1 and exchanged into cortically labelled clusters at microtubule ends. This behaviour depended on the presence of a functional motor domain, because a rigor-mutant GFP-Kif5c bound to microtubules but did not move along them. Further analysis indicated that the microtubule subset decorated by GFP-Kif5c was highly stable and primarily composed of detyrosinated tubulin. In vitro motility assays showed that the motor domain of Kif5c moved detyrosinated microtubules at significantly lower velocities than tyrosinated (unmodified) microtubules. Mathematical modelling predicted that a small increase in detyrosination would bias kinesin-1 occupancy towards detyrosinated microtubules. These data suggest that kinesin-1 preferentially binds to and trafficks on detyrosinated microtubules in vivo, providing a potential basis for the spatial targeting of kinesin-1-based cargo transport. Supplementary material available online at

SummaryChromosome segregation in metazoans requires the alignment of sister-kinetochores onto the metaphase plate. During chromosome alignment, bioriented kinetochores move chromosomes by regulating the plus-end dynamics of the attached microtubules. The bundles of kinetochore-bound microtubules alternate between growth and shrinkage, leading to regular oscillations along the spindle axis. However, the molecular mechanisms that coordinate microtubule plus-end dynamics remain unknown. Here we show that CENP-H, a subunit of the CENP-A NAC/CAD kinetochore complex, is essential for this coordination, as kinetochores lacking CENP-H establish bioriented attachments, but fail to generate regular oscillations, due to an uncontrolled rate of microtubule plus-end turnover. These alterations lead to rapid erratic movements that disrupt metaphase plate organization. Moreover, we show that the abundance of the CENP-A NAC/CAD subunits CENP-H and CENP-I dynamically change on individual sister-kinetochores in vivo, as they preferentially bind the sister-kinetochore attached to growing microtubules, and that one other subunit, CENP-Q, binds microtubules in vitro. Thus, we propose that CENP-A NAC/CAD is a direct regulator of kinetochore-microtubule dynamics, which physically links centromeric DNA to microtubule plusends.

The achaete‐scute complex (AS‐C) and the daughterless (da) genes encode helix‐loop‐helix proteins which have been shown to interact in vivo and to be required for neurogenesis. We show in vitro that heterodimers of three AS‐C products with DA bind DNA strongly, whereas DA homodimers bind weakly and homo or heterocombinations of AS‐C products not at all. Proteins unable to dimerize did not bind DNA. Target sequences for the heterodimers were found in the promoters of the hunchback and the achaete genes. Using sequences of the former we show that the DNA binding results obtained in vitro fully correlate with the ability of different combinations to activate the expression of a reporter gene in yeast. Embryos deficient for the lethal of scute gene fail to activate hunchback in some neural lineages in a pattern consistent with the lack of a member of a multigene family.

We have determined the nucleotide sequence of two genes of the achaete ‐ scute complex (AS‐C) and show that they are homologous to two previously sequenced members of the same locus. These four genes are interspersed with other transcription units of unknown function. We also study the expression of one of these genes by in situ hybridization and compare it with the other three genes. We suggest that the complete function ascribed to the AS‐C by genetic experiments is carried out by the four homologous genes. We discuss the possible biochemical function of the AS‐C in neurogenesis in the light of the homologies of the four genes with the mammalian myc family.