Structure–Function Diversity of Calcium-Binding Proteins (CaBPs): Key Roles in Cell Signalling and Disease

Figures

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  Figure 1. Cartoon representation of the 3D structures of apo/CaM and apo/CaBP1. (a) Full-length apo/CaM (PDB 1DMO), (b) N-domain of apo/CaM (PDB 1CMF), (c) N-domain of apo/CaBP1 (PDB 2K7B), and (d) C-domain of apo/CaBP1 (PDB 2K7C). apo/CaBP1 does not structurally align with apo/CaM. Mg2+ ion is represented by yellow sphere.

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  Figure 2. Cartoon representation of the 3D structures of Ca2+/CaBPs and their comparison with Ca2+/CaM. (a) Full-length Ca2+/CaM (PDB 1CLL), (b) C-domain of Ca2+/CaM (PDB 1CMF), (c) N-domain of Ca2+/CaBP1 (PDB 2LAN), (d) C-domain of Ca2+/CaBP1 (PDB 2LAP, PDB 2K7D), (e) structural alignment of Ca2+/CaM and Ca2+/CaBP1 structures, (f) full-length Ca2+/CaBP1 (PDB 3OX5), (g) N-domain of Ca2+/CaBP4 (PDB 2M29), (h) C-domain of Ca2+/CaBP4 (PDB 2M28), (i) structural alignment of Ca2+/CaM and Ca2+/CaBP4 structures, (j) full-length Ca2+/CaBP5 (PDB 4OCI), (k) N-domain of Ca2+/CaBP7 (PDB 2LV7), and (l) structural alignment of Ca2+/CaM and Ca2+/CaBP7 structures. Ca2+ ions are represented by orange sphere.

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  Figure 3. Ca2+-dependent inactivation (CDI) model of Cav regulation by CaM and CaBP1. (a) Ca2+-free (apo) CaM and CaBP1 compete to be pre-associated with the IQ domain (red) of the closed Cav channel (pink) under resting conditions ([Ca2+]i = 100 nM). Next, membrane depolarisation causes an influx of Ca2+ through the open Cav, with initial low Ca2+ levels meaning that the channel’s open state is stabilised by CaM. Once cytosolic Ca2+ reaches 1 µM, Ca2+ binds to CaM, which causes channel inactivation (CDI), further stabilised by Ca2+/CaMs N-lobe binding to the NSCaTE (orange) domain of Cav1.2. (b) Alternatively, apo-CaBP1 is pre-associated with the IQ motif of Cav under resting conditions ([Ca2+] i = 100 nM). Next, when cytosolic Ca2+ reaches higher levels due to membrane depolarisation, Ca2+ binds to CaBP1, which can displace CaM and reduce or abolish CDI by stabilising the open state of the channel, leading to CDF. Created in BioRender.

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  Figure 4. Topology of the transient receptor potential canonical receptor type 5 (TRPC5). S1–6 indicate transmembrane regions, CaM- and IP3-binding sites on the C-terminal, with the second CaM site further down. It is suggested that there are multiple sites for CaBP1 binding, with the sites being very close to or even overlapping the CaM sites, similar to Cav1.2 channels. Created in BioRender.

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  Figure 5. Topology of the inositol triphosphate receptor (IP3R). (a) Side-on view of two subunits of an active IP3R showing a membrane-inserted channel domain with cytosolic N terminals (NTs, residues 1–604), which contain the suppressor domain (SD, residues 1–223), IP3-binding core (IBC, residues 224–604), and the CaBP1-binding sites. (b) Tetrameric IP3R receptor in the ER membrane. (c) Closed and open state of an IP3R channel from a bird’s-eye view. The closed state is bound by four Ca2+/CaBP1s within the IBC, clamping intrasubunit movement between the β-domain of the IBC and the SD and stabilising the closed state. The open state is bound by four IP3 molecules allowing for Ca2+ movement through the channel. Created in BioRender.

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  Figure 6. Primary structure of CaBP1–2, highlighting human pathogenic mutations. The primary structure of (a) CaBP1 and (b) CaBP2, focusing on the amino acid sequence around the four EF-hands, is shown in blue. The residues involved in Ca2+ coordination are indicated by dashed lines. Disease-associated mutations that have been identified in humans are illustrated as alternatively coloured circles at the relevant labelled positions. CaBP2 has been strongly linked to autosomal recessive hearing loss.

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  Figure 7. Primary structure of CaBP4–5, highlighting human pathogenic mutations. The primary structure of (a) CaBP4 and (b) CaBP5, focusing on the amino acid sequence around the four EF-hands, is shown in blue. The residues involved in Ca2+ coordination are indicated by dashed lines. Disease-associated mutations that have been identified in humans are illustrated as alternatively coloured circles at the relevant labelled positions. CaBP4 has the most pathogenic mutations and has been associated with several disease conditions. CaBP5 has had no relevant pathogenic mutations found.

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  Figure 8. Primary structure of CaBP7–8, highlighting human pathogenic mutations. The primary structure of (a) CaBP7 and (b) CaBP8, focusing on the amino acid sequence around the four EF-hands, is shown in blue. The residues involved in Ca2+ coordination are indicated by dashed lines. Disease-associated mutations that have been identified in humans are illustrated as alternatively coloured circles at the relevant labelled positions. CaBP8 has had no relevant pathogenic mutations found.