The Highly Conservative Cysteine of Oncomodulin as a Feasible Redox Sensor

Abstract

Oncomodulin (Ocm), or parvalbumin β, is an 11–12 kDa Ca2+-binding protein found inside and outside of vertebrate cells, which regulates numerous processes via poorly understood mechanisms. Ocm consists of two active Ca2+-specific domains of the EF-hand type (“helix-loop-helix” motif), covered by an EF-hand domain with inactive EF-hand loop, which contains a highly conservative cysteine with unknown function. In this study, we have explored peculiarities of the microenvironment of the conservative Cys18 of recombinant rat Ocm (rWT Ocm), redox properties of this residue, and structural/functional sensitivity of rWT Ocm to the homologous C18S substitution. We have found that pKa of the Cys18 thiol lays beyond the physiological pH range. The measurement of redox dependence of rWT Ocm thiol–disulfide equilibrium (glutathione redox pair) showed that redox potential of Cys18 for the metal-free and Ca2+-loaded protein is of −168 mV and −176 mV, respectively. Therefore, the conservative thiol of rWT Ocm is prone to disulfide dimerization under physiological redox conditions. The C18S substitution drastically reduces α-helices content of the metal-free and Mg2+-bound Ocm, increases solvent accessibility of its hydrophobic residues, eliminates the cooperative thermal transition in the apo-protein, suppresses Ca2+/Mg2+ affinity of the EF site, and accelerates Ca2+ dissociation from Ocm. The distinct structural and functional consequences of the minor structural modification of Cys18 indicate its possible redox sensory function. Since some other EF-hand proteins also contain a conservative redox-sensitive cysteine located in an inactive EF-hand loop, it is reasonable to suggest that in the course of evolution, some of the EF-hands attained redox sensitivity at the expense of the loss of their Ca2+ affinity.

Disulfide Dimerization of Neuronal Calcium Sensor-1: Implications for Zinc and Redox Signaling

“…Disulfide bonds in these proteins are commonly formed by conserved cysteine residues localized in their non-functional EF-hands. Consistently, it has been suggested that in the course of evolution, they sacrificed the ability to bind calcium to acquire redox sensitivity [60]. However, such a tendency is not always observed, even within the NCS family: NCS-1 and recoverin form disulfide dimers via their single conservative cysteine in EF1, whereas VILIP-1 uses its unique C-terminal cysteine [28,30,31].…”

“…This study is the first to discover that ubiquitous neuronal Ca 2+ -sensor protein NCS-1 possesses redox sensitivity in response to oxidizing conditions by the formation of disulfide dimers with altered structural properties and functional activity. In this respect, NCS-1 resembles some members of the NCS family (recoverin and VILIP-1) and several other EF-hand proteins, such as S100 proteins, oncomodulin, secretagogin, and others [27,28,[30][31][32][33][59][60][61]. Disulfide bonds in these proteins are commonly formed by conserved cysteine residues localized in their non-functional EF-hands.…”

Comprehensive Sequence Analysis of Parvalbumins in Fish and Their Comparison with Parvalbumins in Tetrapod Species

“…Another pronounced difference from other parvalbumins is the possession of F18 instead of C18, although a very few α-parvalbumins do have C18 and a very few non-α parvalbumins lost C18 ( Supplementary File S3 ). The C18 residue can be used for cysteine-bridged homodimer formation [ 99 , 100 ], but that does not seem to be the natural situation [ 10 , 95 , 101 ], and, instead, C18 may have a redox sensory function [ 102 ]. Figure 9 D shows that both C18 and F18 insert into the space between the α-helices A and B, and especially F18 may stabilize the contact between these helices.…”

Parvalbumin Role in Epilepsy and Psychiatric Comorbidities: From Mechanism to Intervention

“…The α-parvalbumin isoform is found in skeletal muscles, GABAergic neurons, and the outer and inner hair cells of the cochlea. Although β-parvalbumin is restricted to the outer hair cells of the cochlea, it is expressed and secreted by macrophages and neutrophils, serving as a neuronal growth factor (Vologzhannikova et al, 2021). Most of the GABAergic synaptic inhibition throughout the neocortex and hippocampal formation is thought to originate from a heterogeneous population of locally projecting interneurons (Ben-Ari et al, 2021), including parvalbuminexpressing cells (Tremblay et al, 2016).…”

“…The α-parvalbumin isoform is found in skeletal muscles, GABAergic neurons, and the outer and inner hair cells of the cochlea. Although β-parvalbumin is restricted to the outer hair cells of the cochlea, it is expressed and secreted by macrophages and neutrophils, serving as a neuronal growth factor ( Vologzhannikova et al, 2021 ).…”