Molecular mechanism of calcium induced trimerization of C1q-like domain of otolin-1 from human and zebrafish

The C1q superfamily includes proteins involved in innate immunity, insulin sensitivity, biomineralization and more. Among these proteins is otolin-1, which is a collagen-like protein that forms a scaffold for the biomineralization of inner ear stones in vertebrates. The globular C1q-like domain (gC1q), which is the most conserved part of otolin-1, binds Ca(2+) and stabilizes its collagen-like triple helix. The molecular details of the assembly of gC1q otolin-1 trimers are not known. Here, we substituted putative Ca(2+)-binding acidic residues of gC1q otolin-1 with alanine to analyse how alanine influences the formation of gC1q trimers. We used human and zebrafish gC1q otolin-1 to assess how evolutionary changes affected the function of the protein. Surprisingly, the mutated forms of gC1q otolin-1 trimerized even in the absence of Ca(2+), although they were less stable than native proteins saturated with Ca(2+). We also found that the zebrafish gC1q domain was less stable than the human homologue under all tested conditions and became stabilized at higher concentrations of Ca(2+), which showed that specific interactions leading to the neutralization of the negative charge at the axis of a gC1q trimer by Ca(2+) are required for the trimers to form. Moreover, human gC1q otolin-1 seems to be optimized to function at lower concentrations of Ca(2+), which is consistent with reported Ca(2+) concentrations in the endolymphs of fish and mammals. Our results allow us to explain the molecular mechanism of assembly of proteins from the C1q superfamily, the modulating role of Ca(2+) and expand the knowledge of biomineralization of vertebrate inner ear stones: otoliths and otoconia.