Structural studies of the C-terminal tail of polycystin-2 (PC2) reveal insights into the mechanisms used for the functional regulation of PC2

Abstract

Mutations in polycystin-2 (PC2) lead to autosomal dominant polycystic kidney disease (ADPKD). The molecular mechanism linking mutations in PC2 and the pathogenesis of ADPKD is not well understood. Therefore, understanding the functional regulation of PC2 and its interaction with other proteins under both physiological and pathogenic conditions is important for elucidating the disease mechanism and identifying potential molecular targets for treatment. Normally, PC2 functions as a calcium-permeable channel whose activity is regulated by calcium binding to the C-terminal domain of PC2 (PC2 Cterm). The PC2 Cterm is also involved in the PC2 channel assembly and hetero-oligomerization with other binding partners in cells. Different functional domains of the PC2 Cterm have been studied using structural approaches. Within the PC2 Cterm, there is a calcium-binding EF-hand domain, crucial for the calcium-dependent activity of the PC2 channel. Downstream of the EF-hand domain lies a coiled-coil region, which is involved in the assembly and hetero-interaction of the PC2 protein. The PC2 Cterm can form an oligomer, mediated by the coiled-coil region. Although PC2 Cterm has been extensively studied for its relationship with ADPKD and its importance in PC2 regulation, there are misunderstandings with respect to the definition of the domain topology within the PC2 Cterm and the functional role of each domain. Here, we review previous studies that connect the molecular properties of the domains of PC2 Cterm to distinct aspects of PC2 functions and regulation.

Figure 1. Mutations in PC1 and PC2 can lead to autosomal dominant polycystic disease

When genetic mutations affect the functionally important regions of PC1 and PC2 proteins, there will be progressive cyst growth in the kidney, leading to a dysfunctional polycystic kidney.

Figure 2. The topology of full length PC2 and the EF‐hand domain sequence alignment

A, PC2 contains six transmembrane helices and two cytosolic termini. The C‐terminal tail of PC2 contains a calcium‐binding EF‐hand, a coiled‐coil domain, and three connecting linker regions (linker 1, linker 2 and linker 3). Helices are numbered and N and C‐termini are labelled according to the residue number of the full length PC2 sequence. B, alignment of the EF‐hand domain sequence of human (HU, upper lines) and sea urchin (SU, lower lines) PC2 homologues. Both EF‐hand domains contain two helix–loop–helix (HLH) motifs. The human PC2 EF‐hand (HU, upper lines) has a four residue truncation within the first helix–loop–helix motif, whereas the sea urchin PC2 EF‐hand (SU, lower lines) contains two intact helix–loop–helix motifs.

Figure 3. Schematic hypothesis for how calcium binding within the PC2 Cterm can regulate PC2 channel gating

We hypothesize that calcium binding to the EF‐hand domain of PC2 can alter the conformation and dynamics of the Cterm. The conformational changes can be propagated to the transmembrane helices due to the decreased protein dynamics in sub‐micromolar calcium levels and cause the channel to be opened.