Structure of avian thymic hormone, a high-affinity avian beta-parvalbumin, in the Ca2+-free and Ca2+-bound states

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

Parvalbumins are small molecules with important functions in Ca²⁺ signaling, but their sequence comparisons to date, especially in fish, have been relatively poor. We here, characterize sequence motifs that distinguish parvalbumin subfamilies across vertebrate species, as well as those that distinguish individual parvalbumins (orthologues) in fish, and map them to known parvalbumin structures. As already observed by others, all classes of jawed vertebrates possess parvalbumins of both the α-parvalbumin and oncomodulin subfamilies. However, we could not find convincing phylogenetic support for the common habit of classifying all non-α-parvalbumins together as "β-parvalbumins." In teleost (modern bony) fish, we here distinguish parvalbumins 1-to-10, of which the gene copy number can differ between species. The genes for α-parvalbumins (pvalb6 and pvalb7) and oncomodulins (pvalb8 and pvalb9) are well conserved between teleost species, but considerable variation is observed in their copy numbers of the non-α/non-oncomodulin genes pvalb1-to-5 and pvalb10. Teleost parvalbumins 1-to-4 are hardly distinguishable from each other and are highly expressed in muscle, and described allergens belong to this subfamily. However, in some fish species α-parvalbumin expression is also high in muscle. Pvalb5 and pvalb10 molecules form distinct lineages, the latter even predating the origin of teleosts, but have been lost in some teleost species. The present study aspires to be a frame of reference for future studies trying to compare different parvalbumins.

The Highly Conservative Cysteine of Oncomodulin as a Feasible Redox Sensor

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 pK_a 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.

Characterization of avian thymic hormone and chicken parvalbumin 3 target cells

Avian thymic hormone (ATH) is a β-parvalbumin produced by epithelial cells in the thymic cortex and in the eyes of chickens. Chicken parvalbumin 3 (CPV3) is a homologous protein produced in the thymus and in hair cells of the chicken ear. ATH circulates in the blood on a five-day cycle and stimulates cell-mediated immunity when administered to young chickens. We report the identification of target cells for ATH and CPV3 and the immunophenotype of target cells for ATH. Newly hatched chicks were injected intracoelomically with ATH and killed 5, 10, 15 or 20 min later. Naïve chickens also were killed at 1, 7 and 14 days of age. Various tissues were examined by EM for the presence of either ATH or CPV3 using colloidal gold labeling. Gold particles were initially present on plasma membranes of lymphocytes in T cell areas of spleen and cecal tonsils from the chicks injected with ATH, internalized within 10 min, and accumulated in nuclei by 20 min. Immunofluorescence staining also identified the presence of ATH in T cell areas of spleen and cecal tonsils. Target cells labeled for ATH were immunophenotyped by double labeling. They were positive for CD3, CD8 and the lymphocyte receptor TCR-1, a phenotype characteristic of cytotoxic γδ T cells. Some of the target cells in the spleen were TCR-3 positive. Targeting of lymphocytes by CPV3 indicated that it may also be an immunomodulating hormone.