Structure of the N-terminal Calcium Sensor Domain of Centrin Reveals the Biochemical Basis for Domain-specific Function

Phosphorylation promotes the endonuclease-like activity of human centrin 2

Centrin is a member of the EF-hand superfamily of calcium-binding proteins, which is involved in the nucleotide excision repair (NER). Reversible phosphorylation of centrin is an important regulatory mechanism in vivo and is closely related to many physiological processes. To explore the possible role of centrin in NER, the endonuclease-like activity of human centrin 2 (HsCen2) regulated by phosphorylation in the absence or presence of Tb³⁺ was investigated by spectroscopy techniques, gel electrophoresis, and molecular docking simulation in 10 mM Hepes, pH 7.4. The results showed that phosphorylation weakened the binding of Tb³⁺ to HsCen2 and enhanced the binding of DNA to HsCen2. Phosphorylation improves the endonuclease-like activity of HsCen2. In addition, Tb³⁺ is favorable for DNA binding and endonuclease-like activity of HsCen2 before and after phosphorylation. These results provide clear insights into the effects of phosphorylation on the properties of HsCen2 and offer important clues for further exploration of how phosphorylation affects protein-driven functions.

Phosphorylation weakened the binding of Tb³⁺ to HsCen2, enhanced the binding of DNA to HsCen2; and improves the endonuclease-like activity of HsCen2; Additionally, the endonuclease-like activity of HsCen2 or HsCen2p is regulated up by Tb³⁺-binding.

NMR structure and dynamics of Q4DY78, a conserved kinetoplasid-specific protein from Trypanosoma cruzi

The 106-residue protein Q4DY78 (UniProt accession number) from Trypanosoma cruzi is highly conserved in the related kinetoplastid pathogens Trypanosoma brucei and Leishmania major. Given the essentiality of its orthologue in T. brucei, the high sequence conservation with other trypanosomatid proteins, and the low sequence similarity with mammalian proteins, Q4DY78 is an attractive protein for structural characterization. Here, we solved the structure of Q4DY78 by solution NMR and evaluated its backbone dynamics. Q4DY78 is composed of five α -helices and a small, two-stranded antiparallel β-sheet. The backbone RMSD is 0.22 ± 0.05 Å for the representative ensemble of the 20 lowest-energy structures. Q4DY78 is overall rigid, except for N-terminal residues (V⁸ to I¹⁰), residues at loop 4 (K⁵⁷ to G⁶⁵) and residues at the C-terminus (F⁸⁹ to F¹¹²). Q4DY78 has a short motif FPCAP that could potentially mediate interactions with the host cytoskeleton via interaction with EVH1 (Drosophila Enabled (Ena)/Vasodilator-stimulated phosphoprotein (VASP) homology 1) domains. Albeit Q4DY78 lacks calcium-binding motifs, its fold resembles that of eukaryotic calcium-binding proteins such as calcitracin, calmodulin, and polcacin Bet V4. We characterized this novel protein with a calcium binding fold without the capacity to bind calcium.

Distinct Calcium Binding and Structural Properties of Two Centrin Isoforms from Toxoplasma gondii

Centrins are calcium (Ca2+)-binding proteins that have been implicated in several regulatory functions. In the protozoan parasite Toxoplasma gondii, the causative agent of toxoplasmosis, three isoforms of centrin have been identified. While increasing information is now available that links the function of centrins with defined parasite biological processes, knowledge is still limited on the metal-binding and structural properties of these proteins. Herein, using biophysical and structural approaches, we explored the Ca2+ binding abilities and the subsequent effects of Ca2+ on the structure of a conserved (TgCEN1) and a more divergent (TgCEN2) centrin isoform from T. gondii. Our data showed that TgCEN1 and TgCEN2 possess diverse molecular features, suggesting that they play nonredundant roles in parasite physiology. TgCEN1 binds two Ca2+ ions with high/medium affinity, while TgCEN2 binds one Ca2+ with low affinity. TgCEN1 undergoes significant Ca2+-dependent conformational changes that expose hydrophobic patches, supporting a role as a Ca2+ sensor in toxoplasma. In contrast, Ca2+ binding has a subtle influence on conformational features of TgCEN2 without resulting in hydrophobic exposure, suggesting a different Ca2+ relay mode for this isoform. Furthermore, TgCEN1 displays a Ca2+-dependent ability to self-assemble, while TgCEN2 did not. We discuss our findings in the context of Ca2+ signaling in toxoplasma.

SAC3B is a target of CML19, the centrin 2 of Arabidopsis thaliana

Arabidopsis centrin 2, also known as calmodulin-like protein 19 (CML19), is a member of the EF-hand superfamily of calcium (Ca2+)-binding proteins. In addition to the notion that CML19 interacts with the nucleotide excision repair protein RAD4, CML19 was suggested to be a component of the transcription export complex 2 (TREX-2) by interacting with SAC3B. However, the molecular determinants of this interaction have remained largely unknown. Herein, we identified a CML19-binding site within the C-terminus of SAC3B and characterized the binding properties of the corresponding 26-residue peptide (SAC3Bp), which exhibits the hydrophobic triad centrin-binding motif in a reversed orientation (I8W4W1). Using a combination of spectroscopic and calorimetric experiments, we shed light on the SAC3Bp-CML19 complex structure in solution. We demonstrated that the peptide interacts not only with Ca2+-saturated CML19, but also with apo-CML19 to form a protein-peptide complex with a 1 : 1 stoichiometry. Both interactions involve hydrophobic and electrostatic contributions and include the burial of Trp residues of SAC3Bp. However, the peptide likely assumes different conformations upon binding to apo-CML19 or Ca2+-CML19. Importantly, the peptide dramatically increases the affinity for Ca2+ of CML19, especially of the C-lobe, suggesting that in vivo the protein would be Ca2+-saturated and bound to SAC3B even at resting Ca2+-levels. Our results, providing direct evidence that Arabidopsis SAC3B is a CML19 target and proposing that CML19 can bind to SAC3B through its C-lobe independent of a Ca2+ stimulus, support a functional role for these proteins in TREX-2 complex and mRNA export.

Calcium-induced human centrin 1 self-assembly and double-regulating the binding with peptide R18-Sfi1p

Centrin is a member of the EF-hand super-family that plays pivotal role in the centrosome duplication and separation. In the present paper, we characterized the properties of metal ions as well as peptide R18-Sfi1p binding to human centrin 1 (HsCen1) by fluorescence spectra and isothermal titration calorimetry (ITC). Four metal ions binding sites on HsCen1 were identified through ITC experiments. The conditional binding constants of the EF-hand domain on HsCen1 with Ca²⁺ were quantitatively calculated. In reversible manner, Ca²⁺ can induce HsCen1 self-assembly. In addition, HsCen1 bound with peptide R18-Sfi1p in calcium-dependent with middle-affinity. Phosphorylation at Ser170 weakened interaction HsCen1 with the substrate and removal calcium ions further weakened interactions of the two molecules. Hence, we inferred that centrin initiating downstream peptides may be a double-regulated process by calcium and phosphorylation. These results are of significance for understanding the relationship between PTM and metal regulation.

A role for Saccharomyces cerevisiae Centrin (Cdc31) in mitochondrial function and biogenesis

Centrins belong to a family of proteins containing calcium-binding EF-hand motifs that perform well-established roles in centrosome and spindle pole body (SPB) duplication. Yeast encodes a single Centrin protein (Cdc31) that binds components in the SPB. However, further studies revealed a role for Centrins in mRNA export, and interactions with contractile filaments and photoreceptors. In addition, human Centrin-2 can bind the DNA-lesion recognition factor XPC, and improve the efficiency of nucleotide excision repair. Similarly, we reported that yeast Cdc31 binds Rad4, a functional counterpart of the XPC DNA repair protein. We also found that Cdc31 is involved in the ubiquitin/proteasome system, and mutations interfere with intracellular protein turnover. In this report, we describe new findings that indicate a role for Cdc31 in the energy metabolism pathway. Cdc31 and cdc31 mutant proteins showed distinct interactions with proteins in energy metabolism, and mutants showed sensitivity to oxidative stress and poor growth on non-fermentable carbon. Significant alteration in mitochondrial morphology was also detected. Although it is unclear how Cdc31 contributes to so many unrelated mechanisms, we propose that by controlling SPB duplication Centrin proteins might link the cellular responses to DNA damage, oxidative load and proteotoxic stresses to growth control.

Calmodulin as a protein linker and a regulator of adaptor/scaffold proteins

Calmodulin (CaM) is a universal regulator for a huge number of proteins in all eukaryotic cells. Best known is its function as a calcium-dependent modulator of the activity of enzymes, such as protein kinases and phosphatases, as well as other signaling proteins including membrane receptors, channels and structural proteins. However, less well known is the fact that CaM can also function as a Ca²⁺-dependent adaptor protein, either by bridging between different domains of the same protein or by linking two identical or different target proteins together. These activities are possible due to the fact that CaM contains two independently-folded Ca²⁺ binding lobes that are able to interact differentially and to some degree separately with targets proteins. In addition, CaM can interact with and regulates several proteins that function exclusively as adaptors. This review provides an overview over our present knowledge concerning the structural and functional aspects of the role of CaM as an adaptor protein and as a regulator of known adaptor/scaffold proteins.

Electrostatics effects on Ca(2+) binding and conformational changes in EF-hand domains: Functional implications for EF-hand proteins

Mutations of Gln41 and Lys75 with nonpolar residues in the N-terminal domain of calmodulin (N-Cam) revealed the importance of solvation energetics in conformational change of Ca(2+) sensor EF-hand domains. While in general these domains have polar residues at these corresponding positions yet the extent of their conformational response to Ca(2+) binding and their Ca(2+) binding affinity can be different from N-Cam. Consequently, here we address the charge state of the polar residues at these positions. The results show that the charge state of these polar residues can affect substantially the conformational change and the Ca(2+) binding affinity of our N-Cam variants. Since all the variants kept their conformational activity in the presence of Ca(2+) suggests that the differences observed among them mainly originate from the difference in their molecular dynamics. Hence we propose that the molecular dynamics of Ca(2+) sensor EF-hand domains is a key factor in the multifunctional aspect of EF-hand proteins.

The E144 residue of Scherffelia dubia centrin discriminates between the DNA repair protein XPC and the centrosomal protein Sfi1

Centrins are members of the EF-hand family of calcium-binding proteins, which are highly conserved among eukaryotes. Centrins bind to several cellular targets, through a hydrophobic triad. However, the W(1)xxL(4)xxxL(8) triad in XPC (Xeroderma Pigmentosum Group C protein) is found in the reverse orientation, as in the L(8)xxxL(4)xxW(1) triad in Sfi1 (Suppressor of Fermentation-Induced loss of stress resistance protein 1). As shown by previous NMR studies of human centrin 2 in complex with XPC or Sfi1, the E148 residue of human centrin 2 is in contact with XPC but is pushed away from the triad of Sfi1. We corroborated these findings using site-directed mutagenesis to generate mutations in Scherffelia dubia centrin (SdCen) and by using isothermal titration calorimetry to analyze the binding affinity of these mutants to XPC and Sfi1. We mutated the F109 residue, which is the main residue involved in target binding regardless of triad orientation, and the E144 residue, which was thought to be involved only in XPC binding. The F109L mutation reduced the binding of SdCen to XPC and Sfi1 and the negative effect was greater upon temperature increase. By contrast, the E144A mutation reduced the binding to XPC but had no effect on Sfi1 binding. The F109L-E144A mutation enhanced the negative effect of the two single mutations on XPC binding. Sfi1 proteins from Ostreococcus lucimarinus and Ostreococcus tauri, which belong to the same clade as S. dubia, were also investigated. A comparative analysis shows that the triad residues are more conserved than those in human Sfi1.

Polo-like kinase phosphorylation of bilobe-resident TbCentrin2 facilitates flagellar inheritance in Trypanosoma brucei

In the protist parasite Trypanosoma brucei, the single Polo-like kinase (TbPLK) controls the inheritance of a suite of organelles that help position the parasite's single flagellum. These include the basal bodies, the bilobe, and the flagellar attachment zone (FAZ). TbCentrin2 was previously shown to be a target for TbPLK in vitro, and this is extended in this study to in vivo studies, highlighting a crucial role for serine 54 in the N-terminal domain. Duplication of the bilobe correlates with the presence of TbPLK and phospho-TbCentrin2, identified using phosphospecific antiserum. Mutation of S54 leads to slow growth (S54A) or no growth (S54D), the latter suggesting that dephosphorylation is needed to complete bilobe duplication and subsequent downstream events necessary for flagellum inheritance.

Hydrogen/Deuterium Exchange Reflects Binding of Human Centrin 2 to Ca(2+) and Xeroderma Pigmentosum Group C Peptide: An Example of EX1 Kinetics

Xeroderma pigmentosum (XP) is a genetic disease affecting 1 in 10,000-100,000 and predisposes people to early-age skin cancer, a disease that is increasing. Those with XP have decreased ability to repair UV-induced DNA damage, leading to increased susceptibility of cancerous non-melanomas and melanomas. A vital, heterotrimeric protein complex is linked to the nucleotide excision repair pathway for the damaged DNA. The complex consists of XPC protein, human centrin 2, and RAD23B. One of the members, human centrin 2, is a ubiquitous, acidic, Ca(2+)-binding protein belonging to the calmodulin superfamily. The XPC protein contains a sequence motif specific for binding to human centrin 2. We report here the Ca(2+)-binding properties of human centrin 2 and its interaction with the XPC peptide motif. We utilized a region-specific H/D exchange protocol to localize the interaction of the XPC peptide with the C-terminal domain of centrin, the binding of which is different than that of calmodulin complexes. The binding dynamics of human centrin 2 to the XPC peptide in the absence and presence of Ca(2+) are revealed by the observation of EX1 H/D exchange regime, indicating that a locally unfolded population exists in solution and undergoes fast H/D exchange.

Such small hands: the roles of centrins/caltractins in the centriole and in genome maintenance

Centrins are small, highly conserved members of the EF-hand superfamily of calcium-binding proteins that are found throughout eukaryotes. They play a major role in ensuring the duplication and appropriate functioning of the ciliary basal bodies in ciliated cells. They have also been localised to the centrosome, which is the major microtubule organising centre in animal somatic cells. We describe the identification, cloning and characterisation of centrins in multiple eukaryotic species. Although centrins have been implicated in centriole biogenesis, recent results have indicated that centrosome duplication can, in fact, occur in the absence of centrins. We discuss these data and the non-centrosomal functions that are emerging for the centrins. In particular, we discuss the involvement of centrins in nucleotide excision repair, a process that repairs the DNA lesions that are induced primarily by ultraviolet irradiation. We discuss how centrin may be involved in these diverse processes and contribute to nuclear and cytoplasmic events.

The interaction between lanthanide (III) and N-terminal domain of Euplotes octocarinatus centrin

Centrin, a member of calcium-binding proteins, is an essential component for microtubule-organizing center (MTOC). Lanthanide (Ln) ions can increase amounts, enhance stability and orderliness of microtubules which is an important component of cytoskeleton. To investigate the structural basis of the effect of Ln ions on orderliness of microtubules, we focused on the interactions between the isolated N-terminal domain of Euplotes centrin (N-EoCen) and Ln by some combined biophysical and biochemical methods. Our results suggest that Ln ions may bind to the canonical calcium binding sites on N-EoCen. Taking advantage of ligand competition, we first determined the metal-binding affinities of Nd(3+), Eu(3+), Gd(3+) and Tm(3+) with N-EoCen. Major changes of N-EoCen in secondary and tertiary structure are noted while Ln ions bind with N-EoCen through CD spectra and 2-p-toluidinylnaphthalene-6-sulfonate (TNS) fluorescence. N-EoCen exists in the form of monomer and dimer in the presence of Ln ions. These results can provide some insights into the structural basis of how Ln ions achieve biological effect in cell through the centrin protein.

An interplay between Centrin2 and Centrin4 on the bi-lobed structure in Trypanosoma brucei

Centrins are conserved calcium-binding proteins important for various regulatory functions. In procyclic Trypanosoma brucei, TbCentrin2 and TbCentrin4 have distinct effects on cell division but both localize to the basal bodies that seed the flagellum, and a bi-lobed structure important for organelle duplication and cell division. Here we show that TbCentrin2 and TbCentrin4 both bind to the basal bodies and bi-lobed structure through the conserved C-terminal domain. Molecular genetic manipulation of TbCentrin4 levels greatly affects TbCentrin2 association with the bi-lobed structure. Using established synchronization methods, TbCentrin2 expression level is shown to be relatively constant throughout the cell cycle while TbCentrin4 level fluctuates, decreasing most during early S-phase when the bi-lobe undergoes duplication. These results thus suggest a co-ordinated action between these two centrin proteins, where the cell cycle-dependent TbCentrin4 expression could regulate the abundance of TbCentrin2 on the bi-lobed structure.

Biophysical characterization of calmodulin and calmodulin-like proteins from rice, Oryza sativa L

Calmodulin (CaM) transduces the increase in cytosolic Ca(2+) concentrations by binding to and altering the activities of target proteins, thereby affecting the physiological responses to the vast array of stimuli. Here, we examined the purified recombinant proteins encoded by three Cam and eight Cam-like (CML) genes from rice. With the exception of one OsCML, all recombinant proteins could be purified by Ca(2+)-dependent hydrophobic chromatography and exhibited an electrophoretic mobility shift when incubated with Ca(2+). The three CaMs all bound CaM kinase II peptide, but none of the eight CMLs did, suggesting a possible differential target binding between the CaM and CML proteins. In addition, their conformational changes upon Ca(2+)-binding were evaluated by circular dichroism spectroscopy and fluorescence spectroscopy using 8-Anilino-1-naphthalene-sulfonic acid. Taken together, OsCMLs were found exhibiting a spectrum of both structural and functional characteristics that ranged from typical to atypical of CaMs. From structural comparison, the OsCMLs have overall main-chain conformation nearly identical to OsCaMs, but with distinct distribution of some charged and hydrophobic amino acids on their target-binding site. These results suggest that genetic polymorphism has promoted the functional diversity of the OsCML family, whose members possess modes of actions probably different from, though maybe overlapping with, those of OsCaMs.

Analysis of the role of Mg²⁺ on conformational change and target recognition by ciliate Euplotes octocarinatus centrin

The binding of Mg(2+) with the Euplotes octocarinatus centrin (EoCen) and the effect of Mg(2+) on the binding of EoCen with the peptide melittin were examined by spectroscopic methods. In this study, it was found that Mg(2+) may bind with Ca(2+)-binding sites, at least partly, on EoCen, which displays ∼10-fold weaker affinity than Ca(2+). In the presence of Mg(2+), Ca(2+)-saturated EoCen undergoes significant conformational changes resulting in decreased exposure of hydrophobic surfaces on the protein. Additionally, excess Mg(2+) did not change the stoichiometry, but rather reduced the affinity of EoCen to melittin. The Mg(2+)-dependent decrease in the affinities of EoCen to melittin is an intrinsic property of Mg(2+), rather than a nonspecific ionic effect. The inhibitory effect of Mg(2+) on the formation of complexes between EoCen and melittin may contribute to the specificity of EoCen in target activation in response to cellular Ca(2+) concentration fluctuations.

The structure, molecular dynamics, and energetics of centrin-melittin complex

Centrin is a calcium binding protein (CaBP) belonging to the EF-hand superfamily. As with other proteins within this family, centrin is a calcium sensor with multiple biological target proteins. We chose to study Chlamydomonas reinhardtii centrin (Crcen) and its interaction with melittin (MLT) as a model for CaBP complexes due to its amphipathic properties. Our goal was to determine the molecular interactions that lead to centrin-MLT complex formation, their relative stability, and the conformational changes associated with the interaction, when compared to the single components. For this, we determined the thermodynamic parameters that define Crcen-MLT complex formation. Two-dimensional infrared (2D IR) correlation spectroscopy were used to study the amide I', I'*, and side chain bands for (13)C-Crcen, MLT, and the (13)C-Crcen-MLT complex. This approach resulted in the determination of MLT's increased helicity, while centrin was stabilized within the complex. Herein we provide the first complete molecular description of centrin-MLT complex formation and the dissociation process. Also, discussed is the first structure of a CaBP-MLT complex by X-ray crystallography, which shows that MLT has a different binding orientation than previously characterized centrin-bound peptides. Finally, all of the experimental results presented herein are consistent with centrin maintaining an extended conformation while interacting with MLT. The molecular implications of these results are: (1) the recognition of hydrophobic contacts as requirements for initial binding, (2) minimum electrostatic interactions within the C-terminal end of the peptide, and (3) van der Waals interactions within MLTs N-terminal end are required for complex formation.

Solution NMR structure of Apo-calmodulin in complex with the IQ motif of human cardiac sodium channel NaV1.5

The function of the human voltage-gated sodium channel Na(V)1.5 is regulated in part by intracellular calcium signals. The ubiquitous calcium sensor protein calmodulin (CaM) is an important part of the complex calcium-sensing apparatus in Na(V)1.5. CaM interacts with an IQ (isoleucine-glutamine) motif in the large intracellular C-terminal domain of the channel. Using co-expression and co-purification, we have been able to isolate a CaM-IQ motif complex and to determine its high-resolution structure in absence of calcium using multi-dimensional solution NMR. Under these conditions, the Na(V)1.5 IQ motif interacts with the C-terminal domain (C-lobe) of CaM, with the N-terminal domain remaining free in solution. The structure reveals that the C-lobe adopts a semi-open conformation with the IQ motif bound in a narrow hydrophobic groove. Sequence similarities between voltage-gated sodium channels and voltage-gated calcium channels suggest that the structure of the CaM-Na(V)1.5 IQ motif complex can serve as a general model for the interaction between CaM and ion channel IQ motifs under low-calcium conditions. The structure also provides insight into the biochemical basis for disease-associated mutations that map to the IQ motif in Na(V)1.5.

The characterization for the binding of calcium and terbium to Euplotes octocarinatus centrin

Centrin is a member of the EF-hand superfamily that plays critical role in the centrosome duplication and separation. In the present paper, we characterized properties of metal ions binding to Euplotes octocarinatus centrin (EoCen) by fluorescence spectra and circular dichroism (CD) spectra. Changes of fluorescence spectra and alpha-helix contents of EoCen proved that Tb(3+) and Ca(2+) induced great conformational changes of EoCen resulting in exposing hydrophobic surfaces. At pH 7.4, Ca(2+) (and Tb(3+)) bond with EoCen at the ratio of 4:1. Equilibrium experiment indicated that Ca(2+) and Tb(3+) exhibited different binding capabilities for C- and N-terminal domains of protein. C-terminal domain bond with Ca(2+) or Tb(3+) approximately 100-fold more strongly than N-terminal. Aromatic residue-sensitized Tb(3+) energy transfer suggested that site IV bond to Tb(3+) or Ca(2+) more strongly than site III. Based on fluorescence titration curves, we reckoned the conditional binding constants of EoCen site IV quantitatively to be K(IV)=(1.23+/-0.51)x10(8)M(-1) and K(IV)=(6.82+/-0.33)x10(5)M(-1) with Tb(3+) and Ca(2+), respectively. Metal ions bond to EoCen in the order of IV>III>II, I.

Centrin4 coordinates cell and nuclear division in T. brucei

Centrins are Ca2+-binding proteins that have been implicated in a number of biological processes, including organelle duplication, mRNA export, DNA repair and signal transduction. In the protozoan parasite Trypanosoma brucei we have previously described TbCentrin2, which is present on a bi-lobed structure, and involved in the duplication and segregation of the Golgi complex. Recently, another centrin, TbCentrin4, was also found at the bi-lobe and has been implicated in organelle segregation and cytokinesis. We now show that cytokinesis is not inhibited, but that a dysregulation of nuclear and cell division leads to the production of zoids – daughter siblings that contain all organelles except the nucleus. Our results, therefore, suggest that TbCentrin4 is involved in processes that coordinate karyokinesis and cytokinesis.

Crystallization and preliminary X-ray characterization of full-length Chlamydomonas reinhardtii centrin

Chlamydomonas reinhardtii centrin is a member of the EF-hand calcium-binding superfamily. It is found in the basal body complex and is important for flagellar motility. Like other members of the EF-hand family, centrin interacts with and modulates the function of other proteins in a calcium-dependent manner. To understand how C. reinhardtii centrin interacts with its protein targets, it has been crystallized in the presence of the model peptide melittin and X-ray diffraction data have been collected to 2.2 A resolution. The crystals are orthorhombic, with unit-cell parameters a = 52.1, b = 114.4, c = 34.8 A, and are likely to belong to space group P2(1)2(1)2.

Centrins in retinal photoreceptor cells: regulators in the connecting cilium

Changes in the intracellular Ca2+ concentration regulate the visual signal transduction cascade directly or more often indirectly through Ca2+-binding proteins. Here we focus on centrins, which are members of a highly conserved subgroup of the EF-hand superfamily of Ca2+-binding proteins in photoreceptor cells of the vertebrate retina. Centrins are commonly associated with centrosome-related structures. In mammalian retinal photoreceptor cells, four centrin isoforms are expressed as prominent components in the connecting cilium linking the light-sensitive outer segment compartment with the metabolically active inner segment compartment. Our data indicate that Ca2+-activated centrin isoforms assemble into protein complexes with the visual heterotrimeric G-protein transducin. This interaction of centrins with transducin is mediated by binding to the betagamma-dimer of the heterotrimeric G-protein. More recent findings show that these interactions of centrins with transducin are reciprocally regulated via site-specific phosphorylations mediated by the protein kinase CK2. The assembly of centrin/G-protein complexes is a novel aspect of translocation regulation of signalling proteins in sensory cells, and represents a potential link between molecular trafficking and signal transduction in general.

Centrin/Cdc31 is a novel regulator of protein degradation

Rad23 is required for efficient protein degradation and performs an important role in nucleotide excision repair. Saccharomyces cerevisiae Rad23, and its human counterpart (hHR23), are present in a complex containing the DNA repair factor Rad4 (termed XPC, for xeroderma pigmentosum group C, in humans). XPC/hHR23 was also reported to bind centrin-2, a member of the superfamily of calcium-binding EF-hand proteins. We report here that yeast centrin, which is encoded by CDC31, is similarly present in a complex with Rad4/Rad23 (called NEF2). The interaction between Cdc31 and Rad23/Rad4 varied by growth phase and reflected oscillations in Cdc31 levels. Strikingly, a cdc31 mutant that formed a weaker interaction with Rad4 showed sensitivity to UV light. Based on the dual function of Rad23, in both DNA repair and protein degradation, we questioned if Cdc31 also participated in protein degradation. We report here that Cdc31 binds the proteasome and multiubiquitinated proteins through its carboxy-terminal EF-hand motifs. Moreover, cdc31 mutants were highly sensitive to drugs that cause protein damage, failed to efficiently degrade proteolytic substrates, and formed altered interactions with the proteasome. These findings reveal for the first time a new role for centrin/Cdc31 in protein degradation.

Calmodulin has the Potential to Function as a Ca-Dependent Adaptor Protein

Calmodulin (CaM) is a versatile Ca(2+)-binding protein that regulates the activity of numerous effector proteins in response to Ca(2+) signals. Several CaM-dependent regulatory mechanisms have been identified, including autoinhibitory domain displacement, sequestration of a ligand-binding site, active site reorganization, and target protein dimerization. We recently showed that the N- and C-lobes of animal and plant CaM isoforms could independently and sequentially bind to target peptides derived from the CaM-binding domain of Nicotiana tabacum mitogen-activated protein kinase phosphatase (NtMKP1), to form a 2:1 peptide:CaM complex. This suggests that CaM might facilitate the dimerization of NtMKP1, although the dimerization mechanism is distinct from the previously described simultaneous binding of other target peptides to CaM. The independent and sequential binding of the NtMKP1 peptides to CaM also suggests an alternative plausible scenario in which the C-lobe of CaM remains tethered to NtMKP1, and the N-lobe is free to recruit a second target protein to the complex, such as an NtMKP1 target. Thus, we hypothesize that CaM may be capable of functioning as a Ca(2+)-dependent adaptor or recruiter protein.

An Sfi1p-like centrin-binding protein mediates centrin-based Ca2+ -dependent contractility in Paramecium tetraurelia

The previous characterization and structural analyses of Sfi1p, a Saccharomyces cerevisiae centrin-binding protein essential for spindle pole body duplication, have suggested molecular models to account for centrin-mediated, Ca2+-dependent contractility processes (S. Li, A. M. Sandercock, P. Conduit, C. V. Robinson, R. L. Williams, and J. V. Kilmartin, J. Cell Biol. 173:867-877, 2006). Such processes can be analyzed by using Paramecium tetraurelia, which harbors a large Ca2+ -dependent contractile cytoskeletal network, the infraciliary lattice (ICL). Previous biochemical and genetic studies have shown that the ICL is composed of diverse centrin isoforms and a high-molecular-mass centrin-associated protein, whose reduced size in the démaillé (dem1) mutant correlates with defective organization of the ICL. Using sequences derived from the high-molecular-mass protein to probe the Paramecium genome sequence, we characterized the PtCenBP1 gene, which encodes a 460-kDa protein. PtCenBP1p displays six almost perfect repeats of ca. 427 amino acids (aa) and harbors 89 potential centrin-binding sites with the consensus motif LLX11F/LX2WK/R, similar to the centrin-binding sites of ScSfi1p. The smaller (260-kDa) protein encoded by the dem1 mutant PtCenBP1 allele comprises only two repeats of 427 aa and 46 centrin-binding sites. By using RNA interference and green fluorescent protein fusion experiments, we showed that PtCenBP1p forms the backbone of the ICL and plays an essential role in its assembly and contractility. This study provides the first in vivo demonstration of the role of Sfi1p-like proteins in centrin-mediated Ca2+-dependent contractile processes.

Calcium-dependent and -independent binding of soybean calmodulin isoforms to the calmodulin binding domain of tobacco MAPK phosphatase-1

The recent finding of an interaction between calmodulin (CaM) and the tobacco mitogen-activated protein kinase phosphatase-1 (NtMKP1) establishes an important connection between Ca(2+) signaling and the MAPK cascade, two of the most important signaling pathways in plant cells. Here we have used different biophysical techniques, including fluorescence and NMR spectroscopy as well as microcalorimetry, to characterize the binding of soybean CaM isoforms, SCaM-1 and -4, to synthetic peptides derived from the CaM binding domain of NtMKP1. We find that the actual CaM binding region is shorter than what had previously been suggested. Moreover, the peptide binds to the SCaM C-terminal domain even in the absence of free Ca(2+) with the single Trp residue of the NtMKP1 peptides buried in a solvent-inaccessible hydrophobic region. In the presence of Ca(2+), the peptides bind first to the C-terminal lobe of the SCaMs with a nanomolar affinity, and at higher peptide concentrations, a second peptide binds to the N-terminal domain with lower affinity. Thermodynamic analysis demonstrates that the formation of the peptide-bound complex with the Ca(2+)-loaded SCaMs is driven by favorable binding enthalpy due to a combination of hydrophobic and electrostatic interactions. Experiments with CaM proteolytic fragments showed that the two domains bind the peptide in an independent manner. To our knowledge, this is the first report providing direct evidence for sequential binding of two identical peptides of a target protein to CaM. Discussion of the potential biological role of this interaction motif is also provided.

Insights into functional aspects of centrins from the structure of N-terminally extended mouse centrin 1

Centrins are members of the family of Ca(2+)-binding EF-hand proteins. In photoreceptor cells, centrin isoform 1 is specifically localized in the non-motile cilium. This connecting cilium links the light-sensitive outer segment with the biosynthetic active inner segment of the photoreceptor cell. All intracellular exchanges between these compartments have to occur through this cilium. Three-dimensional structures of centrins from diverse organisms are known, showing that the EF-hand motifs of the N-terminal domains adopt closed conformations, while the C-terminal EF-hand motifs have open conformations. The crystal structure of an N-terminally extended mouse centrin 1 (MmCen1-L) resembles the overall structure of troponin C in its two Ca(2+) bound form. Within the N-terminal extension in MmCen1-L, residues W24 and R25 bind to the C-terminal domain of centrin 1 in a target-protein-like geometry. Here, we discuss this binding mode in connection with putative interaction sites of the target-protein transducin and the self-assembly of centrins.

Structural role of Sfi1p-centrin filaments in budding yeast spindle pole body duplication

Centrins are calmodulin-like proteins present in centrosomes and yeast spindle pole bodies (SPBs) and have essential functions in their duplication. The Saccharomyces cerevisiae centrin, Cdc31p, binds Sfi1p on multiple conserved repeats; both proteins localize to the SPB half-bridge, where the new SPB is assembled. The crystal structures of Sfi1p–centrin complexes containing several repeats show Sfi1p as an α helix with centrins wrapped around each repeat and similar centrin–centrin contacts between each repeat. Electron microscopy (EM) shadowing of an Sfi1p–centrin complex with 15 Sfi1 repeats and 15 centrins bound showed filaments 60 nm long, compatible with all the Sfi1 repeats as a continuous α helix. Immuno-EM localization of the Sfi1p N and C termini showed Sfi1p–centrin filaments spanning the length of the half-bridge with the Sfi1p N terminus at the SPB. This suggests a model for SPB duplication where the half-bridge doubles in length by association of the Sfi1p C termini, thereby providing a new Sfi1p N terminus to initiate SPB assembly.

The biochemical effect of Ser167 phosphorylation on Chlamydomonas reinhardtii centrin

Centrin is an EF-hand calcium-binding protein found in microtubule organizing centers of organisms ranging from algae and yeast to man. Phosphorylation in the centrin C-terminal domain occurs in mitosis and is associated with alterations in contractile fibers. To obtain insight into the structural basis for the functional effect of phosphorylation, Chlamydomonas reinhardtii centrin C-terminal domain phosphorylated at Ser167 (pCRC-C) has been produced and characterized. The structure of pCRC-C was compared to the unmodified protein by NMR spectroscopy. The effect of phosphorylation on target binding was examined for the complex of pCRC-C and a 19 residue centrin-binding fragment of Kar1. Remarkably, the efficient and selective phosphorylation by PKA was suppressed in the complex. Moreover, comparisons of NMR chemical shift differences induced by phosphorylation reveal a greater effect from phosphorylation in the context of the Kar1 complex than for the free protein. These results directly demonstrate that phosphorylation modulates the structure and biochemical activities of centrin.