HCN channels play important physiological functions in the brain and heart, from working memory formation, pain sensation, to cardiac pace making. HCN channels sense both electrical and chemical stimuli and bridges membrane excitability with intracellular signaling pathways. Dually regulated by voltage and ligand binding, HCN channel forms an elegant research target for protein allostery. Intracellular cAMP directly binds to and opens the channel. The basic question of how cAMP binding opens the channel remains elusive. We approach this research topic by following the research theme of structure, dynamics, and function. We have made significant progress by solving the crystal structures for the WT and a mutant form of human HCN4 C- terminal fragment, which contains the cyclic nucleotide binding domain (CNBD). To address the dynamic interaction between cAMP and the whole channel, we established the patch-clamp fluorometry technique that provides simultaneous recordings of channel activity and cAMP. We demonstrated that cAMP preferably binds to the channel in the open state. Then we went one step further and investigated how distributed sub-domains contribute to the global binding of cAMP. We found that the inner activation gate in the ion conducting pore remotely controls cAMP binding. This exciting discovery directly touches upon the very basics of how ligand- dependent regulation of protein functions is implemented. A fundamental understanding of the protein allostery in cAMP regulation of HCN channel is still missing. We are propelled to expand our study by the following two challenges. First, given the detailed understanding of isolated domains within the protein molecule, how they communicate with each other and the rest of the protein remains largely unknown. Secondly, for the study of protein allostery, it s challenging but critical to integrate the information from both structure and dynamics. To circumvent these difficulties, we have established and applied the techniques of electrophysiology, biophotonics, biochemistry, structural and computational biology. We have three specific aims: 1) Interpret allosteric ligand regulation at the level of liand - whole protein interaction. We will study the dynamic, cAMP - whole channel interaction in other HCN isoforms and the roles of important sub-domains, including the S4-S5 linker and C-linker, in remotely affecting cAMP binding. 2) Solve structures representing transitional states in cAMP gating. We will pursue the structure of the unliganded form and mutant forms of the protein and continue our effort in pursuing the full-length HCN structures. 3) Investigate the intriguing relationship between protein structure and dynamics. To this end, we will combine computational and experimental approaches for protein dynamics to address the molecular motions that define the direction of conformation changes during cAMP regulation of HCN channel. This proposal will lay a strong foundation for our long-ter research goals: 1) a fundamental understanding of protein allostery and protein folding, using ion channel proteins as a research platform~ 2) insights for the treatment of ion channel related neurological and cardiac disorders.
HCN channels play important physiological roles in the brain and heart. By investigating how intracellular cAMP binds to and subsequently opens the channel, this proposal will illuminate the basic molecular mechanism of ligand-dependent allosteric regulation of protein function and provide a solid foundation for designing better treatments for HCN channel related human diseases.