Structural and functional analysis of Hikeshi, a new nuclear transport receptor of Hsp70s.

Hikeshi is a nuclear transport receptor required for cell survival after stress. It mediates heat-shock-induced nuclear import of 70 kDa heat-shock proteins (Hsp70s) through interactions with FG-nucleoporins (FG-Nups), which are proteins in nuclear pore complexes (NPCs). Here, the crystal structure of human Hikeshi is presented at 1.8 Å resolution. Hikeshi forms an asymmetric homodimer that is responsible for the interaction with Hsp70s. The asymmetry of Hikeshi arises from the distinct conformation of the C-terminal domain (CTD) and the flexibility of the linker regions of each monomer. Structure-guided mutational analyses showed that both the flexible linker region and the CTD are important for nuclear import of Hsp70. Pull-down assays revealed that only full-length Hsp70s can interact with Hikeshi. The N-terminal domain (NTD) consists of a jelly-roll/β-sandwich fold structure which contains hydrophobic pockets involved in FG-Nup recognition. A unique extended loop (E-loop) in the NTD is likely to regulate the interactions of Hikeshi with FG-Nups. The crystal structure of Hikeshi explains how Hikeshi participates in the regulation of nuclear import through the recognition of FG-Nups and which part of Hikeshi affects its binding to Hsp70. This study is the first to yield structural insight into this highly unique import receptor.