Rearranging RNA structures at 75°C? Toward the molecular mechanism and physiological function of the Thermus thermophilus DEAD-box helicase Hera.

DEAD-box helicases catalyze the ATP-dependent destabilization of RNA duplexes. Hera is a DEAD-box helicase from Thermus thermophilus that consists of a helicase core, followed by a C-terminal extension comprising a dimerization domain and an RNA-binding domain. The combined structural information on individual Hera domains provides a molecular model of the Hera dimer. The modular architecture with flexible connections between individual domains affords different relative orientations of the RBD relative to the Hera helicase core, and of the two helicase cores within the dimer. Presumably, domain movements are intimately linked to RNA binding, to the interplay of the RBD and the helicase core, and to RNA unwinding, and may impact on the functional cooperation of the two helicase cores in RNA unwinding. The in vivo function of Hera is unknown. The Hera RBD recognizes two distinct elements in the RNA substrate, a single-stranded and a structured region. The helicase core then unwinds an adjacent RNA duplex in an ATP-dependent reaction. Overall, this mode of action is reminiscent of DEAD-box proteins that act as general RNA chaperones. This review summarizes the current knowledge on Hera structure and function, and discusses a possible role of Hera in the Thermus thermophilus cold-shock response.