Calcium- and Integrin-Binding Protein 2 (CIB2) in Physiology and Disease: Bright and Dark Sides

Abstract

Calcium- and integrin-binding protein 2 (CIB2) is a small EF-hand protein capable of binding Mg²⁺ and Ca²⁺ ions. While its biological function remains largely unclear, an increasing number of studies have shown that CIB2 is an essential component of the mechano-transduction machinery that operates in cochlear hair cells. Mutations in the gene encoding CIB2 have been associated with non-syndromic deafness. In addition to playing an important role in the physiology of hearing, CIB2 has been implicated in a multitude of very different processes, ranging from integrin signaling in platelets and skeletal muscle to autophagy, suggesting extensive functional plasticity. In this review, we summarize the current understanding of biochemical and biophysical properties of CIB2 and the biological roles that have been proposed for the protein in a variety of processes. We also highlight the many molecular aspects that remain unclarified and deserve further investigation.

Keywords: Usher syndrome; hearing; integrin signaling; mechanoelectrical transduction; non-syndromic deafness.

Figure 1

Sequence and structural properties of CIB2 and its pathogenetic variants. Top: multiple amino acid sequence alignment of human CIB1, CIB2, CIB3, and CIB4 performed by T–Coffee (https://tcoffee.crg.eu/apps/tcoffee/index.html, accessed on 22 February 2022). Consensus sequence and similarity descriptors are reported in grey text. Residues target of pathogenetic point mutations are grey-shaded in case of nonsense mutations or marked in red and blue-boxed in case of missense mutations (see Section 4.3). EF-hand motifs are displayed by colored boxes, and residues involved in Ca²⁺-coordination are indicated by letters referring to the canonical pentagonal bipyramidal geometry on the respective loops. Bottom: Representation of the three-dimensional structure of human CIB2 based on the homology model used in [14]. Cartoon and surface representation are superposed. The N-terminal region is colored gray, while the C-terminal region (helix 10) is colored yellow. EF1, EF2, EF3, and EF4 are colored green, slate blue, orange, and pink, respectively. Residue targets of pathogenic missense mutations are represented by magenta sticks, and Ca²⁺ ions are represented by red spheres.

Figure 2

Schematic representation of CIB2 cation sensitivity and structural transitions. (a) In the absence of cations, WT CIB2 is in a molten globule state (left). The addition of a physiological concentration of both Mg²⁺ and Ca²⁺ (middle top panel) triggers the binding of Mg²⁺ in EF3 and EF4. In the presence of physiological Ca²⁺ and absence of Mg²⁺ (middle bottom panel), Ca²⁺ binding is less favorable. When extra (non-physiological) Ca²⁺ concentrations are added, Ca²⁺ replaces Mg²⁺ in EF4 (upper right panel). (b) E64D CIB2 is in a molten globule state under physiological conditions (left). A well-defined three-dimensional structure is acquired upon the addition of extra (non-physiological) Mg²⁺ (upper right panel) or Ca²⁺ (bottom right panel), with a more pronounced effect for Ca²⁺. The Wi-Fi logo represents the strength of allosteric interactions between E64 and N121 residues.

Figure 3

Experimentally determined CIB2 interactors as classified based on cellular type. Direct interactors supported by experimental evidence are listed. WHRN, whirlin; TMC1/2, transmembrane channel-like protein 1/2; MYO7A, unconventional myosin-VIIa; RHEB, GTP-binding protein Rheb; BAIAP2L2, BAI1-associated protein 2-like 2; ITGA7, integrin alpha-7; SPHK1, sphingosine kinase 1; ITGA2B, integrin alpha-IIb.