8. Analysis of thermal-stress induced nucleocytoplasmic transport carrier Hikeshis

Naoko Imamoto
Cellular Dynamics Laboratory, RIKEN

Cellular stresses affect many aspects of cell physiology, including dynamic redistribution of nucleocytoplasmic localization of various proteins. We have identified a nuclear import pathway that becomes active during thermal stress. A protein named Hikeshi mediates this pathway as a nuclear import receptor for molecular chaperone Hsp70. Hikeshi is essential for attenuation and reversal of the thermal stress response in HeLa cells.

Hikeshi is an evolutionarily conserved protein in eukaryotic cells, but its function has not been characterized. We attempted to characterize a function of Hikeshi homologues in different organisms. In fission yeast, Hikeshi homologue localizes to the nuclear rim, interacts with Hsp70 homolog Ssa2, and mediates its nuclear import in a reconstituted mammalian nuclear transport system. These results show Hikeshi homolog possesses biochemical properties similar with human Hikeshi. However, in fission yeast, Hikeshi homologue is not required for heat stress response and survival after heat stress. Instead, fission yeast Hikeshi homologue was required for the normal expression of stress response genes under optimal conditions and for cell growth during glucose deprivation. siRNA depletion of Hikeshi in Caenorhabditis elegans reduces its life span and affects its thermal resistance. Hikeshi knockout is lethal in mice.

A single amino acid substitution in Hikeshi cause human disease. Through collaboration with Israel scientists, we found leukoencephalopathy, which is a genetic white-matter disorders in humans, is associated with Hikeshi point mutation. The mutation was associated with undetectable level of Hikeshi in the patients’ fibroblasts and with lack of nuclear Hsp70 during heat shock stress, a phenomenon which was reversed upon the introduction of normal human Hikeshi to the patients cells. These data underscore the importance of activity of Hikeshi for disease development. The patients die due to febrile illness and the life- threatening myo-pericarditis, suggesting roles of Hikeshi in inflammatory processes.

Although Hikeshi was initially found to be activated in response to thermal stress, this evolutionarily conserved protein is involved in more general stress responses. Studies attempting to reveal mechanistic action of Hikeshi in cells are now undertaken. Crystal structure studies of Hikeshi and interaction properties of Hikeshi with Hsp70 suggest that the exclusive cellular target of Hikeshi is Hsp70.

【References】
[1] Song, J., Kose, S. et al (2015). Structural and functional analysis of Hikeshi, a new nuclear transport receptor of Hsp70s. Acta Crystallogr. D Biol. Crystallogr. 71, 473-483.
[2] Edvardson, S., Kose, S., et al (2016). Leukoencephalopathy and early death associated with an Ashkenazi-Jewish founder mutation in the Hikeshi gene. J. Med. Genet. 53, 132-7.