Synergetic effect of recoverin and calmodulin on regulation of rhodopsin kinase.

Phosphorylation of photoactivated rhodopsin by rhodopsin kinase (RK or GRK1), a first step of the phototransduction cascade turnoff, is under the control of Ca(2+)/recoverin. Here, we demonstrate that calmodulin, a ubiquitous Ca(2+)-sensor, can inhibit RK, though less effectively than recoverin does. We have utilized the surface plasmon resonance technology to map the calmodulin binding site in the RK molecule. Calmodulin does not interact with the recoverin-binding site within amino acid residues M1-S25 of the enzyme. Instead, the high affinity calmodulin binding site is localized within a stretch of amino acid residues V150-K175 in the N-terminal regulatory region of RK. Moreover, the inhibitory effect of calmodulin and recoverin on RK activity is synergetic, which is in agreement with the existence of separate binding sites for each Ca(2+)-sensing protein. The synergetic inhibition of RK by both Ca(2+)-sensors occurs over a broader range of Ca(2+)-concentration than by recoverin alone, indicating increased Ca(2+)-sensitivity of RK regulation in the presence of both Ca(2+)-sensors. Taken together, our data suggest that RK regulation by calmodulin in photoreceptor cells could complement the well-known inhibitory effect of recoverin on RK

Light-Induced Thiol Oxidation of Recoverin Affects Rhodopsin Desensitization

The excessive light illumination of mammalian retina is known to induce oxidative stress and photoreceptor cell death linked to progression of age-related macular degeneration. The photochemical damage of photoreceptors is suggested to occur via two apoptotic pathways that involve either excessive rhodopsin activation or constitutive phototransduction, depending on the light intensity. Both pathways are dramatically activated in the absence of rhodopsin desensitization by GRK1. Previously, we have shown that moderate illumination (halogen lamp, 1,500 lx, 1–5 h) of mammalian eyes provokes disulfide dimerization of recoverin, a calcium-dependent regulator of GRK1. Here, we demonstrate under in vivo conditions that both moderate long-term (metal halide lamp, 2,500 lx, 14 h, rat model) and intense short-term (halogen lamp, 30,000 lx for 3 h, rabbit model) illumination of the mammalian retina are accompanied by accumulation of disulfide dimer of recoverin. Furthermore, in the second case we reveal alternatively oxidized derivatives of the protein, apparently including its monomer with sulfinic group. Histological data indicate that thiol oxidation of recoverin precedes apoptosis of photoreceptors. Both disulfide dimer and oxidized monomer (or oxidation mimicking C39D mutant) of recoverin exhibit lowered α-helical content and thermal stability of their apo-forms, as well as increased Ca2+ affinity. Meanwhile, the oxidized monomer and C39D mutant of recoverin demonstrate impaired ability to bind photoreceptor membranes and regulate GRK1, whereas disulfide dimer exhibits notably improved membrane binding and GRK1 inhibition in absence of Ca2+. The latter effect is expected to slow down rhodopsin desensitization in the light, thereby favoring support of the light-induced oxidative stress, ultimately leading to photoreceptor apoptosis. Overall, the intensity and duration of illumination of the retina affect thiol oxidation of recoverin likely contributing to propagation of the oxidative stress and photoreceptor damage

Parvalbumin as a Pleomorphic Protein

Parvalbumin (PA) is a classical small, mostly cytosolic Ca2+-binding protein of the EF-hand superfamily expressed in vertebrates in a tissue- and cell-specific manner, serving as a magnesium/ calcium buffer. In the last decade novel data were published on structural peculiarities of PA, likely affecting its functionality. This review sums up these findings and discusses their potential physiological significance

The structural model of L35Ae 6X mutant

<p>The structural model of L35Ae 6X mutant built based on PDB entry 2lp6</p

Thermal denaturation of L35Ae samples, monitored by DSC

<p>The temperature dependencies of excess specific heat capacity for rWT L35Ae and L35Ae 10X, fitted by a single Gaussian function (J/(g*K) units). Protein concentrations were 0.65 mg/ml and 1.5 mg/ml, for rWT L35Ae and L35Ae 10X samples, respectively. Buffer conditions: 20 mM H3BO3–NaOH, 300 mM NaCl, pH 8.5. Heating rate was 1 K/min.</p

The structural model of L35Ae 10X mutant

<p>The structural model of L35Ae 10X mutant built based on PDB entry 2lp6</p

The structural model of wild-type L35Ae from P. horikoshii

<p>The structural model of wild-type L35Ae from <em>Pyrococcus horikoshii</em> built based on PDB entry 2lp6</p

Interleukin-11: A Multifunctional Cytokine with Intrinsically Disordered Regions

Cytokine interleukin-11 (IL-11) is a multifunctional protein with diverse roles in the normal cell signaling and in various pathologies. The structure of IL-11 is characterized by a four-helix bundle motif comprising two pairs of antiparallel α-helices arranged in an up–up–down–down configuration. Evaluation of the intrinsic disorder predisposition of human IL-11 by several computational tools clearly shows that this protein is predicted to have functional disordered regions potentially involved in interaction with natural binding partners. Signaling by IL-11 proceeds via an interaction of the protein with its membrane-specific receptor IL-11Rα and a subsequent interaction of the complex with the transmembrane signal-transducing receptor GP130. Cytoplasmic domain of IL-11Rα is predicted to be very disordered, and noticeable amount of disorder is present even in the large extracellular domain of the protein. GP130 is also predicted to have long disordered region that is located at the C-terminal of the protein and is expected to have several disorder-based binding sites. It shows that intrinsic disorder might play an important role in functioning of this signaling machine. A specific subset of the calcium sensor proteins (calmodulin, S100P, S100B, NCS-1, GCAP-1/2) exhibits metal-dependent binding of IL-11 with dissociation constants in a range of 1–19 μM, and the structural features of their hinge regions likely ensure selectivity and calcium sensitivity of IL-11 binding to the EF-hand proteins studied. IL-11 exhibits multiple effects on hematopoietic and non-hematopoietic systems. It plays a major role in orchestrating complex processes of tumor development and progression

Intrinsically Disordered Caldesmon Binds Calmodulin via the “buttons on a string” Mechanism

We show here that chicken gizzard caldesmon (CaD) and its C-terminal domain (residues 636–771, CaD136) are intrinsically disordered proteins. The computational and experimental analyses of the wild type CaD136 and series of its single tryptophan mutants (W674A, W707A, and W737A) and a double tryptophan mutant (W674A/W707A) suggested that although the interaction of CaD136 with calmodulin (CaM) can be driven by the non-specific electrostatic attraction between these oppositely charged molecules, the specificity of CaD136-CaM binding is likely to be determined by the specific packing of important CaD136 tryptophan residues at the CaD136-CaM interface. It is suggested that this interaction can be described as the “buttons on a charged string” model, where the electrostatic attraction between the intrinsically disordered CaD136 and the CaM is solidified in a “snapping buttons” manner by specific packing of the CaD136 “pliable buttons” (which are the short segments of fluctuating local structure condensed around the tryptophan residues) at the CaD136-CaM interface. Our data also show that all three “buttons” are important for binding, since mutation of any of the tryptophans affects CaD136-CaM binding and since CaD136 remains CaM-buttoned even when two of the three tryptophans are mutated to alanines