Accurate chromosome alignment at metaphase and subsequent segregation of condensed chromosomes is a complex process involving elaborate and only partially characterized molecular machinery. Although several spindle associated molecular motors have been shown to be essential for mitotic function, only a few chromosome arm–associated motors have been described. Here, we show that human chromokinesin human HKIF4A (HKIF4A) is an essential chromosome-associated molecular motor involved in faithful chromosome segregation. HKIF4A localizes in the nucleoplasm during interphase and on condensed chromosome arms during mitosis. It accumulates in the mid-zone from late anaphase and localizes to the cytokinetic ring during cytokinesis. RNA interference–mediated depletion of HKIF4A in human cells results in defective prometaphase organization, chromosome mis-alignment at metaphase, spindle defects, and chromosome mis-segregation. HKIF4A interacts with the condensin I and II complexes and HKIF4A depletion results in chromosome hypercondensation, suggesting that HKIF4A is required for maintaining normal chromosome architecture. Our results provide functional evidence that human KIF4A is a novel component of the chromosome condensation and segregation machinery functioning in multiple steps of mitotic division.

Embryonic development in Drosophila melanogaster begins with a rapid series of mitotic nuclear divisions, unaccompanied by cytokinesis, to produce a multi-nucleated single cell embryo, the syncytial blastoderm. The syncytium then undergoes a process of cell formation, in which the individual nuclei become enclosed in individual cells. This process of cellularization involves integrating mechanisms of cell polarity, cell-cell adhesion and a specialized form of cytokinesis. The detailed molecular mechanism, however, is highly complex and, despite extensive analysis, remains poorly understood. Nevertheless, new insights are emerging from recent studies on aspects of membrane polarization and insertion, which show that membrane components from intracellular organelles are involved. In addition, actin and actin-associated proteins have been heavily implicated while new evidence shows that microtubule cytoskeletal elements are mechanistically involved in all aspects of cellularization. This review will draw on both the traditional models and the new data to provide a current perspective on the nature of cellular blastoderm formation in Drosophila melanogaster.

The hepatitis E virus (HEV) is the causative agent of hepatitis E, an acute form of viral hepatitis. The biology and pathogenesis of HEV remain poorly understood. We have used in vitro binding assays to show that the HEV ORF3 protein (pORF3) binds to a number of cellular signal transduction pathway proteins. This includes the protein tyrosine kinases Src, Hck, and Fyn, the p85␣ regulatory subunit of phosphatidylinositol 3-kinase, phospholipase C␥, and the adaptor protein Grb2. A yeast two-hybrid assay was used to further confirm the pORF3-Grb2 interaction. The binding involves a proline-rich region in pORF3 and the src homology 3 (SH3) domains in the cellular proteins. Competition assays and computer-assisted modeling was used to evaluate the binding surfaces and interaction energies of the pORF3⅐SH3 complex. In pORF3-expressing cells, pp60 src was found to associate with an 80-kDa protein, but no activation of the Src kinase was observed in these cells. However, there was increased activity and nuclear localization of ERK in the pORF3-expressing cells. These studies suggest that pORF3 is a viral regulatory protein involved in the modulation of cell signaling. The ORF3 protein of HEV appears to be the first example of a SH3 domain-binding protein encoded by a virus that causes an acute and primarily self-limited infection.Hepatitis E virus (HEV), 1 the causative agent for hepatitis E, is a waterborne pathogen endemic to much of the developing world where it causes rampant sporadic infections and large scale epidemics (1-4). While the infection is self-limited with no associated chronicity, a fraction of the patients progress to fulminant hepatitis (5, 6), the most severe form of acute hepatitis. High mortality rates of 20 -30% reported for HEV infection during pregnancy (7,8) are also the result of fulminant hepatitis. The reasons for this and the mechanisms of viral pathogenesis are not known. The studies on HEV biology and pathogenesis have been severely restricted by the lack of a reliable cell culture system and small animal models of viral infection. We have used subgenomic expression strategies to study the properties and functions of individual HEV gene products toward understanding viral replication and pathogenicity (9 -12).The HEV genome is a ϳ7.5-kilobase polyadenylated, positive-sense RNA that contains three open reading frames (ORFs) designated ORF1, ORF2, and ORF3 (13). The ORF3 of HEV encodes a protein of ϳ13.5 kDa, called pORF3, for which no function has been assigned. When expressed in animal cells, pORF3 is phosphorylated at a single serine residue (Ser 80 ) in its 123-amino acid primary sequence (11). In vitro phosphorylation experiments suggested that pORF3 may be a substrate for the mitogen-activated protein (MAP) kinase, and subcellular fractionation revealed its association with the cytoskeleton (11). Recent results using inhibitors, activators, and dominant negative alleles show that pORF3 is a substrate for the extracellular signal-regulated kinase (ERK) as well as the stressactivated pr...