Insight into the Unfolding Properties of Chd64, a Small, Single Domain Protein with a Globular Core and Disordered Tails.

Two major lipophilic hormones, 20-hydroxyecdysone (20E) and juvenile hormone (JH), govern insect development and growth. While the mode of action of 20E is well understood, some understanding of JH-dependent signalling has been attained only in the past few years, and the crosstalk of the two hormonal pathways remains unknown. Two proteins, the calponin-like Chd64 and immunophilin FKBP39 proteins, have recently been found to play pivotal roles in the formation of dynamic, multiprotein complex that cross-links these two signalling pathways. However, the molecular mechanism of the interaction remains unexplored. The aim of this work was to determine structural elements of Chd64 to provide an understanding of molecular basis of multiple interactions. We analysed Chd64 in two unrelated insect species, Drosophila melanogaster (DmChd64) and Tribolium castaneum (TcChd64). Using hydrogen-deuterium exchange mass spectrometry (HDX-MS), we showed that both Chd64 proteins have disordered tails that outflank the globular core. The folds of the globular cores of both Chd64 resemble the calponin homology (CH) domain previously resolved by crystallography. Monitoring the unfolding of DmChd64 and TcChd64 by far-ultraviolet (UV) circular dichroism (CD) spectroscopy, fluorescence spectroscopy and size-exclusion chromatography (SEC) revealed a highly complex process. Chd64 unfolds and forms of a molten globule (MG)-like intermediate state. Furthermore, our data indicate that in some conditions, Chd64 may exists in discrete structural forms, indicating that the protein is pliable and capable of easily acquiring different conformations. The plasticity of Chd64 and the existence of terminal intrinsically disordered regions (IDRs) may be crucial for multiple interactions with many partners.