Date of Award
Yong YY Yu
Ales AV Vancura
Yan YZ Zhu
Polycystin complexes, or TRPP/PKD complexes, made of transient receptor potential channel polycystin subfamily proteins (TRPP) and polycystic kidney disease (PKD) proteins, play key roles in coupling extracellular stimuli with intracellular Ca2+ signals. For example, the TRPP2/PKD1 complex has a crucial function in renal physiology, with mutations in either protein causing autosomal dominant polycystic kidney disease (ADPKD). In contrast, the TRPP3/PKD1L3 complex responds to low pH and plays a potential role in sour taste. It has been previously shown that the protein partners interact via association of the C-terminal or transmembrane segments, with consequences for the assembly, surface expression, and function of the polycystin complexes. However, the roles of extracellular components, especially the loops that connect the transmembrane segments, in the assembly and function of the polycystin complex, are largely unknown. Here, with the immunoprecipitation method, we found that extracellular loops between the first and second transmembrane segments of TRPP2 and TRPP3 associate with the extracellular loops between the sixth and seventh transmembrane segments of PKD1 and PKD1L3, respectively. Immunofluorescence and electrophysiology data further confirm that the association between these loops is essential for the trafficking and function of the complexes. Interestingly, most of the extracellular loops are also found to be involved in the homomeric assembly. Furthermore, ADPKD-associated TRPP2 mutant T448K significantly weakens TRPP2 homomeric assembly but has no obvious effect on TRPP2-PKD1 heteromeric assembly. Our results demonstrate a crucial role of these functionally underexplored extracellular loops in the assembly and function of the polycystin complexes. Otopetrin 1 (Otop1), a member of the recently identified Otop proton channel family, has been shown to be the sour taste receptor in mouse. Otop proteins are homodimers, and each subunit contains structurally similar six transmembrane N and C domains. Although cyro-EM structures of several Otop proteins have been published, how their functions are defined by structure are largely unknown. Especially, the key functional proterties such how these channel sense extracellular protons to initiate gating and how they conduct protons once channels are activated are elusive. In this study, we show that both the N and C domains of human Otop1 are needed for channel function. More interestingly, Otop1 protein with the order of the N and C domains reversed in protein sequence is still functional as long as a linker with proper length is inserted between the two domains, suggesting that the N and C domains of Otop1 fold separately during synthesis and assemble into a functional structure by forming the critical intrasubunit interface. The extracellular S5-S6 and S11-S12 loops, one in each domain, are found to be essential for channel function. Our results suggest that H229 in the S5-S6 loop is involved in proton sensation and D570 in the S11-S12 loop regulates proton permeation into the pore formed by the C domain. This study sheds light on the molecular mechanism of the structure and function of this new ion channel family.
Li, Bin, "Assembly and Function of the Polycystin Receptor-Ion Channel Complexes and the Otop1 Proton Channel" (2021). Theses and Dissertations. 380.
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