The HhH(2)/NDD Domain of the Drosophila Nod Chromokinesin-Like Protein Is Required for Binding to Chromosomes in the Oocyte Nucleus

Nod is a chromokinesin-like protein that plays a critical role in segregating achiasmate chromosomes during female meiosis. The C-terminal half of the Nod protein contains two putative DNA binding domains. The first of these domains, known as the HMGN domain, consists of three tandemly repeated High Mobility Group-N motifs. This domain was previously shown to be both necessary and sufficient for binding of the C-terminal half of Nod to mitotic chromosomes in embryos. The second putative DNA binding domain, denoted HhH(2)/NDD, is a Helix-hairpin-Helix(2)/Nod-like DNA binding Domain. Although the HhH(2)/NDD domain is not required or sufficient for chromosome binding in embryos, several well-characterized nod mutations have been mapped in this domain. To characterize the role of the HhH(2)/NDD domain in mediating Nod function, we created a series of UAS-driven transgene constructs capable of expressing either a wildtype Nod-GFP fusion protein or proteins in which the HhH(2)/NDD domain had been altered by site-directed mutagenesis. Although wildtype Nod-GFP localizes to the oocyte chromosomes and rescues the segregation defect in nod mutant oocytes, two of three proteins carrying mutants in the HhH(2)/NDD domain fail to either rescue the nod mutant phenotype or to bind to oocyte chromosomes. However, these mutant proteins do bind to the polytene chromosomes in nurse cell nuclei and enter the oocyte nucleus. Thus, even though the HhH(2)/NDD domain is not essential for chromosome binding in other cell types, it is required for chromosome binding in the oocyte. These HhH(2)/NDD mutants also block the localization of Nod to the posterior pole of stage 9-10A oocytes, a process that is thought to facilitate the interaction of Nod with the plus ends of microtubules (Cui et al., 2005). This observation suggests that the Nod HhH2/NDD domain may play other roles in addition to binding Nod to meiotic chromosomes.

Key Words: Nod, chromokinesin, distributive pairing, meiosis

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