Extracellular Loops are Essential for the Assembly and Function of TRPP/PKD Complexes

Transient receptor potential channel polycystin subfamily (TRPP) proteins assemble with polycystic kidney disease (PKD) proteins to form functionally important complexes. For example, the TRPP2/PKD1 receptor-ion channel complex plays a critical role in renal physiology. Mutations in either protein cause autosomal dominant polycystic kidney disease (ADPKD), one of the most common genetic diseases in humans. A similar complex, assembled by TRPP3 and PKD1L3, is a candidate for the sour taste receptor. The TRPP2/PKD1 complex contains three TRPP2 subunits and one PKD1 subunit and the interaction between their C-termini is crucial for the complex assembly. The TRPP3/PKD1L3 complex has the same subunit stoichiometry but its assembly involves interactions between the transmembrane segments of both proteins. These interactions have been shown to be essential for the assembly, surface expression and function of the complexes. Here we find another novel binding site between these proteins. When co-expressed in HEK293T cells, the extracellular loops between the first and second transmembrane segments (I-II loop) of TRPP2 and TRPP3 associate with the extracellular loops between the sixth and seventh transmembrane segments (VI-VII loop) of PKD1 and PKD1L3 respectively. These loops can also associate with their binding partners when the latter is expressed as full-length proteins. The loop-loop associations are functionally crucial since expression of either the TRPP3 I-II loop or the PKD1L3 VI-VII loop shows dominate negative effect on the acid-induced current of TRPP3/PKD1L3 complex. These results demonstrate, for the first time, the previously unknown essential role that these extracellular loops play in the assembly of TRPP/PKD complexes.