Zebrafish mitotic kinesin-like protein 1 (Mklp1) functions in embryonic cytokinesis

To understand the functions of microtubule motors in vertebrate development, we are investigating the kinesin-like proteins (KLPs) of the zebrafish, Danio rerio. Here we describe the structure, intracellular distribution, and function of zebrafish mitotic KLP1 (Mklp1). The zebrafish mklp1 gene that encodes this 867-amino acid protein maps to a region of zebrafish linkage group 18 that is syntenic with part of human chromosome 15. In zebrafish AB9 fibroblasts and in COS-7 cells, the zebrafish Mklp1 protein decorates spindle microtubules at metaphase, redistributes to the spindle midzone during anaphase, and becomes concentrated in the midbody during telophase and cytokinesis. The motor is detected consistently in interphase nuclei of COS cells and occasionally in those of AB9 cells. Nuclear targeting of Mklp1 is conferred by two basic motifs located in the COOH terminus of the motor. In cleaving zebrafish embryos, green fluorescent protein (GFP)-tagged Mklp1 is found in the nucleus in interphase and associates with microtubules of the spindle midbody in cytokinesis. One- or two-cell embryos injected with synthetic mRNAs encoding dominant-negative variants of GFP-Mklp1 frequently fail to complete cytokinesis during cleavage, resulting in formation of multinucleated blastomeres. Our results indicate that the zebrafish Mklp1 motor performs a critical function that is required for completion of embryonic cytokinesis.