Light-dependent CK2-mediated phosphorylation of centrins regulates complex formation with visual G-protein

Protein kinase CK2 modulates the activity of Maf-family bZIP transcription factor NRL in rod photoreceptors of mammalian retina

Maf-family basic motif leucine zipper protein NRL specifies rod photoreceptor cell fate during retinal development and, in concert with homeodomain protein CRX and other regulatory factors, controls the expression of most rod-expressed genes including the visual pigment gene Rhodopsin (Rho). Transcriptional regulatory activity of NRL is modulated by post-translational modifications, especially phosphorylation, and mutations at specific phosphosites can lead to retinal degeneration. During our studies to elucidate NRL-mediated transcriptional regulation, we identified protein kinase CK2 in NRL-enriched complexes bound to Rho promoter-enhancer regions and in NRL-enriched high molecular mass fractions from the bovine retina. The presence of CK2 in NRL complexes was confirmed by co-immunoprecipitation from developing and adult mouse retinal extracts. In vitro kinase assay and bioinformatic analysis indicated phosphorylation of NRL at Ser117 residue by CK2. Co-transfection of Csnk2a1 cDNA encoding murine CK2 with human NRL and CRX reduced the bovine Rho promoter-driven luciferase expression in HEK293 cells and mutagenesis of NRL-Ser117 residue to Ala restored the reporter gene activity. In concordance, overexpression of CK2 in the mouse retina in vivo by electroporation resulted in reduction of Rho promoter-driven DsRed reporter expression as well as the transcript level of many phototransduction genes. Thus, our studies demonstrate that CK2 can phosphorylate Ser117 of NRL. Modulation of NRL activity by CK2 suggests intricate interdependence of transcriptional and signaling pathways in maintaining rod homeostasis.

Protein kinase CK2: a potential therapeutic target for diverse human diseases

CK2 is a constitutively active Ser/Thr protein kinase, which phosphorylates hundreds of substrates, controls several signaling pathways, and is implicated in a plethora of human diseases. Its best documented role is in cancer, where it regulates practically all malignant hallmarks. Other well-known functions of CK2 are in human infections; in particular, several viruses exploit host cell CK2 for their life cycle. Very recently, also SARS-CoV-2, the virus responsible for the COVID-19 pandemic, has been found to enhance CK2 activity and to induce the phosphorylation of several CK2 substrates (either viral and host proteins). CK2 is also considered an emerging target for neurological diseases, inflammation and autoimmune disorders, diverse ophthalmic pathologies, diabetes, and obesity. In addition, CK2 activity has been associated with cardiovascular diseases, as cardiac ischemia-reperfusion injury, atherosclerosis, and cardiac hypertrophy. The hypothesis of considering CK2 inhibition for cystic fibrosis therapies has been also entertained for many years. Moreover, psychiatric disorders and syndromes due to CK2 mutations have been recently identified. On these bases, CK2 is emerging as an increasingly attractive target in various fields of human medicine, with the advantage that several very specific and effective inhibitors are already available. Here, we review the literature on CK2 implication in different human pathologies and evaluate its potential as a pharmacological target in the light of the most recent findings.

Post-translational modification enzymes as key regulators of ciliary protein trafficking

Primary cilia are evolutionarily conserved microtubule-based organelles that protrude from the surface of almost all cell types and decode a variety of extracellular stimuli. Ciliary dysfunction causes human diseases named ciliopathies, which span a wide range of symptoms, such as developmental and sensory abnormalities. The assembly, disassembly, maintenance and function of cilia rely on protein transport systems including intraflagellar transport (IFT) and lipidated protein intraflagellar targeting (LIFT). IFT is coordinated by three multisubunit protein complexes with molecular motors along the ciliary axoneme, while LIFT is mediated by specific chaperones that directly recognize lipid chains. Recently, it has become clear that several post-translational modification enzymes play crucial roles in the regulation of IFT and LIFT. Here, we review our current understanding of the roles of these post-translational modification enzymes in the regulation of ciliary protein trafficking as well as their regulatory mechanisms, physiological significance and involvement in human diseases.

Cul3-Klhl18 ubiquitin ligase modulates rod transducin translocation during light-dark adaptation

Adaptation is a general feature of sensory systems. In rod photoreceptors, light-dependent transducin translocation and Ca²⁺ homeostasis are involved in light/dark adaptation and prevention of cell damage by light. However, the underlying regulatory mechanisms remain unclear. Here, we identify mammalian Cul3-Klhl18 ubiquitin ligase as a transducin translocation modulator during light/dark adaptation. Under dark conditions, Klhl18^-/- mice exhibited decreased rod light responses and subcellular localization of the transducin α-subunit (Tα), similar to that observed in light-adapted Klhl18^+/+ mice. Cul3-Klhl18 promoted ubiquitination and degradation of Unc119, a rod Tα-interacting protein. Unc119 overexpression phenocopied Tα mislocalization observed in Klhl18^-/- mice. Klhl18 weakly recognized casein kinase-2-phosphorylated Unc119 protein, which is dephosphorylated by Ca²⁺ -dependent phosphatase calcineurin. Calcineurin inhibition increased Unc119 expression and Tα mislocalization in rods. These results suggest that Cul3-Klhl18 modulates rod Tα translocation during light/dark adaptation through Unc119 ubiquitination, which is affected by phosphorylation. Notably, inactivation of the Cul3-Klhl18 ligase and calcineurin inhibitors FK506 and cyclosporine A that are known immunosuppressant drugs repressed light-induced photoreceptor damage, suggesting potential therapeutic targets.

Farnesylation of the Transducin G Protein Gamma Subunit Is a Prerequisite for Its Ciliary Targeting in Rod Photoreceptors

Primary cilia are microtubule-based organelles, which protrude from the plasma membrane and receive a wide range of extracellular signals. Various cilia use G protein-coupled receptors (GPCRs) for the detection of these signals. For instance, vertebrate rod photoreceptors use their cilia (also called outer segments) as antennae detecting photons by GPCR rhodopsin. Rhodopsin recognizes incoming light and activates its G protein, transducin, which is composed of three subunits α, β, and γ. Similar to all G protein γ subunits, the transducin Gγ1 subunit undergoes C-terminal prenylation resulting in the addition of an isoprenoid farnesyl; however, the significance of this posttranslational modification is unclear. To study the role of the farnesyl group, we genetically introduced a mutant Gγ1 that lacked the prenylation site into the retinal photoreceptors of mice. The biochemical and physiological analyses of these mice revealed that mutant Gγ1 dimerizes with the endogenous transducin Gβ1 subunit and that the resulting Gβγ dimers display reduced hydrophobicity. Although mutant Gβγ dimers could form a heterotrimeric G protein, they could not mediate phototransduction. This deficiency was due to a strong exclusion of non-farnesylated Gβγ complexes from the cilia (rod outer segments). Our results provide the first evidence that farnesylation is required for trafficking of G-protein βγ subunits to the cilium of rod photoreceptors.

Cilia - The sensory antennae in the eye

Cilia are hair-like projections found on almost all cells in the human body. Originally believed to function merely in motility, the function of solitary non-motile (primary) cilia was long overlooked. Recent research has demonstrated that primary cilia function as signalling hubs that sense environmental cues and are pivotal for organ development and function, tissue hoemoestasis, and maintenance of human health. Cilia share a common anatomy and their diverse functional features are achieved by evolutionarily conserved functional modules, organized into sub-compartments. Defects in these functional modules are responsible for a rapidly growing list of human diseases collectively termed ciliopathies. Ocular pathogenesis is common in virtually all classes of syndromic ciliopathies, and disruptions in cilia genes have been found to be causative in a growing number of non-syndromic retinal dystrophies. This review will address what is currently known about cilia contribution to visual function. We will focus on the molecular and cellular functions of ciliary proteins and their role in the photoreceptor sensory cilia and their visual phenotypes. We also highlight other ciliated cell types in tissues of the eye (e.g. lens, RPE and Müller glia cells) discussing their possible contribution to disease progression. Progress in basic research on the cilia function in the eye is paving the way for therapeutic options for retinal ciliopathies. In the final section we describe the latest advancements in gene therapy, read-through of non-sense mutations and stem cell therapy, all being adopted to treat cilia dysfunction in the retina.

New insights into the interaction of centrin with Sfi1

Centrin binds to Rad4(XPC) and Sfi1 through the hydrophobic motif W(1)xxL(4)xxxL(8) in the opposite orientation. Rad4 has one motif, but Sfi1 has approximately 20 repeats, each of which interacts with a centrin molecule. To investigate the parameters involved in centrin binding, we purified a ScSfi1 domain containing 6 repeats complexed with either yeast centrin Cdc31 or human centrin 1. The present study was performed using mutagenesis of centrin and of Sfi1 residues involved in centrin binding and the stability of the centrin-centrin complexes was assessed using thermal denaturation and CD. Calcium stabilized these complexes, as indicated by the Tm increases measured by circular dichroism. The complexes, which were composed of Sfi1 variants and yeast centrin, were analysed in the presence of EDTA. The replacement of W with F within the repeat region yielded a functional repeat (Tm 45°C). The replacement of W with A in two adjacent Sfi1 repeats reduced the thermal stability of the Sfi1-centrin complexes (40°C). We analysed three HsCen1 variants that were homologous to the yeast mutants and induced cell cycle arrest during the G2/M transition. The HsCen1 variants E105K and F113L reduced the thermal stability (50°C, 50°C) of the ScSfi1-HsCen1 complexes; in contrast, the A109T variant exhibited no change in thermal stability relative to the wild-type (60°C). Conversely to ScCdc31, there were no apparent centrin-centrin interactions with wild-type HsCen1, but they did occur for the S170D mutation that mimics PKA phosphorylation at the S170 residue.

KIAA0556 is a novel ciliary basal body component mutated in Joubert syndrome

BACKGROUND

Joubert syndrome (JBTS) and related disorders are defined by cerebellar malformation (molar tooth sign), together with neurological symptoms of variable expressivity. The ciliary basis of Joubert syndrome related disorders frequently extends the phenotype to tissues such as the eye, kidney, skeleton and craniofacial structures.

RESULTS

Using autozygome and exome analyses, we identified a null mutation in KIAA0556 in a multiplex consanguineous family with hallmark features of mild Joubert syndrome. Patient-derived fibroblasts displayed reduced ciliogenesis potential and abnormally elongated cilia. Investigation of disease pathophysiology revealed that Kiaa0556 (-/-) null mice possess a Joubert syndrome-associated brain-restricted phenotype. Functional studies in Caenorhabditis elegans nematodes and cultured human cells support a conserved ciliary role for KIAA0556 linked to microtubule regulation. First, nematode KIAA0556 is expressed almost exclusively in ciliated cells, and the worm and human KIAA0556 proteins are enriched at the ciliary base. Second, C. elegans KIAA0056 regulates ciliary A-tubule number and genetically interacts with an ARL13B (JBTS8) orthologue to control cilium integrity. Third, human KIAA0556 binds to microtubules in vitro and appears to stabilise microtubule networks when overexpressed. Finally, human KIAA0556 biochemically interacts with ciliary proteins and p60/p80 katanins. The latter form a microtubule-severing enzyme complex that regulates microtubule dynamics as well as ciliary functions.

CONCLUSIONS

We have identified KIAA0556 as a novel microtubule-associated ciliary base protein mutated in Joubert syndrome. Consistent with the mild patient phenotype, our nematode, mice and human cell data support the notion that KIAA0556 has a relatively subtle and variable cilia-related function, which we propose is related to microtubule regulation.

CK2 phosphorylation of human centrins 1 and 2 regulates their binding to the DNA repair protein XPC, the centrosomal protein Sfi1 and the phototransduction protein transducin β

Centrins are calcium-binding proteins that can interact with several cellular targets (Sfi1, XPC, Sac3 and transducin β) through the same hydrophobic triad. However, two different orientations of the centrin-binding motif have been observed: W(1)xxL(4)xxxL(8) for XPC (xeroderma pigmentosum group C protein) and the opposite orientation L(8)xxxL(4)xxW(1) for Sfi1 (suppressor of fermentation-induced loss of stress resistance protein 1), Sac3 and transducin β. Centrins are also phosphorylated by several protein kinases, among which is CK2. The purpose of this study was to determine the binding mechanism of human centrins to three targets (transducin β, Sfi1 and XPC), and the effects of in vitro phosphorylation by CK2 of centrins 1 and 2 with regard to this binding mechanism. We identified the centrin-binding motif at the COOH extremity of transducin β. Human centrin 1 binds to transducin β only in the presence of calcium with a binding constant lower than the binding constant observed for Sfi1 and for XPC. The affinity constants of centrin 1 were 0.10 10(6) M(-1), 249 10(6) M(-1) and 52.5 10(6) M(-1) for Trd, R17-Sfi1 and P17-XPC respectively. CK2 phosphorylates human centrin 1 at residue T138 and human centrin 2 at residues T138 and S158. Consequently CK2 phosphorylation abolished the binding of centrin 1 to transducin β and reduced the binding to Sfi1 and XPC. CK2 phosphorylation of centrin 2 at T138 and S158 abolished the binding to Sfi1 as assessed using a C-HsCen2 T138D-S158D phosphomimetic form of centrin 2.

Phosphorylation of the Usher syndrome 1G protein SANS controls Magi2-mediated endocytosis

The human Usher syndrome (USH) is a complex ciliopathy with at least 12 chromosomal loci assigned to three clinical subtypes, USH1-3. The heterogeneous USH proteins are organized into protein networks. Here, we identified Magi2 (membrane-associated guanylate kinase inverted-2) as a new component of the USH protein interactome, binding to the multifunctional scaffold protein SANS (USH1G). We showed that the SANS-Magi2 complex assembly is regulated by the phosphorylation of an internal PDZ-binding motif in the sterile alpha motif domain of SANS by the protein kinase CK2. We affirmed Magi2's role in receptor-mediated, clathrin-dependent endocytosis and showed that phosphorylated SANS tightly regulates Magi2-mediated endocytosis. Specific depletions by RNAi revealed that SANS and Magi2-mediated endocytosis regulates aspects of ciliogenesis. Furthermore, we demonstrated the localization of the SANS-Magi2 complex in the periciliary membrane complex facing the ciliary pocket of retinal photoreceptor cells in situ. Our data suggest that endocytotic processes may not only contribute to photoreceptor cell homeostasis but also counterbalance the periciliary membrane delivery accompanying the exocytosis processes for the cargo vesicle delivery. In USH1G patients, mutations in SANS eliminate Magi2 binding and thereby deregulate endocytosis, lead to defective ciliary transport modules and ultimately disrupt photoreceptor cell function inducing retinal degeneration.

The role of primary cilia in the development and disease of the retina

The normal development and function of photoreceptors is essential for eye health and visual acuity in vertebrates. Mutations in genes encoding proteins involved in photoreceptor development and function are associated with a suite of inherited retinal dystrophies, often as part of complex multi-organ syndromic conditions. In this review, we focus on the role of the photoreceptor outer segment, a highly modified and specialized primary cilium, in retinal health and disease. We discuss the many defects in the structure and function of the photoreceptor primary cilium that can cause a class of inherited conditions known as ciliopathies, often characterized by retinal dystrophy and degeneration, and highlight the recent insights into disease mechanisms.

Calcium-binding capacity of centrin2 is required for linear POC5 assembly but not for nucleotide excision repair

Centrosomes, the principal microtubule-organising centres in animal cells, contain centrins, small, conserved calcium-binding proteins unique to eukaryotes. Centrin2 binds to xeroderma pigmentosum group C protein (XPC), stabilising it, and its presence slightly increases nucleotide excision repair (NER) activity in vitro. In previous work, we deleted all three centrin isoforms present in chicken DT40 cells and observed delayed repair of UV-induced DNA lesions, but no centrosome abnormalities. Here, we explore how centrin2 controls NER. In the centrin null cells, we expressed centrin2 mutants that cannot bind calcium or that lack sites for phosphorylation by regulatory kinases. Expression of any of these mutants restored the UV sensitivity of centrin null cells to normal as effectively as expression of wild-type centrin. However, calcium-binding-deficient and T118A mutants showed greatly compromised localisation to centrosomes. XPC recruitment to laser-induced UV-like lesions was only slightly slower in centrin-deficient cells than in controls, and levels of XPC and its partner HRAD23B were unaffected by centrin deficiency. Interestingly, we found that overexpression of the centrin interactor POC5 leads to the assembly of linear, centrin-dependent structures that recruit other centrosomal proteins such as PCM-1 and NEDD1. Together, these observations suggest that assembly of centrins into complex structures requires calcium binding capacity, but that such assembly is not required for centrin activity in NER.

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.

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.

Direct interaction of the Usher syndrome 1G protein SANS and myomegalin in the retina

The human Usher syndrome (USH) is the most frequent cause of combined hereditary deaf-blindness. USH is genetically heterogeneous with at least 11 chromosomal loci assigned to 3 clinical types, USH1-3. We have previously demonstrated that all USH1 and 2 proteins in the eye and the inner ear are organized into protein networks by scaffold proteins. This has contributed essentially to our current understanding of the function of USH proteins and explains why defects in proteins of different families cause very similar phenotypes. We have previously shown that the USH1G protein SANS (scaffold protein containing ankyrin repeats and SAM domain) contributes to the periciliary protein network in retinal photoreceptor cells. This study aimed to further elucidate the role of SANS by identifying novel interaction partners. In yeast two-hybrid screens of retinal cDNA libraries we identified 30 novel putative interacting proteins binding to the central domain of SANS (CENT). We confirmed the direct binding of the phosphodiesterase 4D interacting protein (PDE4DIP), a Golgi associated protein synonymously named myomegalin, to the CENT domain of SANS by independent assays. Correlative immunohistochemical and electron microscopic analyses showed a co-localization of SANS and myomegalin in mammalian photoreceptor cells in close association with microtubules. Based on the present results we propose a role of the SANS-myomegalin complex in microtubule-dependent inner segment cargo transport towards the ciliary base of photoreceptor cells.

Adhesion and degranulation promoting adapter protein (ADAP) is a central hub for phosphotyrosine-mediated interactions in T cells

TCR stimulation leads to an increase in cellular adhesion among other outcomes. The adhesion and degranulation promoting adapter protein (ADAP) is known to be rapidly phosphorylated after T cell stimulation and relays the TCR signal to adhesion molecules of the integrin family. While three tyrosine phosphorylation sites have been characterized biochemically, the binding capabilities and associated functions of several other potential phosphotyrosine motifs remain unclear. Here, we utilize in vitro phosphorylation and mass spectrometry to map novel phosphotyrosine sites in the C-terminal part of human ADAP (486–783). Individual tyrosines were then mutated to phenylalanine and their relevance for cellular adhesion and migration was tested experimentally. Functionally important tyrosine residues include two sites within the folded hSH3 domains of ADAP and two at the C-terminus. Furthermore, using a peptide pulldown approach in combination with stable isotope labeling in cell culture (SILAC) we identified SLP-76, PLCγ, PIK3R1, Nck, CRK, Gads, and RasGAP as phospho-dependent binding partners of a central YDDV motif of ADAP. The phosphorylation-dependent interaction between ADAP and Nck was confirmed by yeast two-hybrid analysis, immunoprecipitation and binary pulldown experiments, indicating that ADAP directly links integrins to modulators of the cytoskeleton independent of SLP-76.

Retinophilin is a light-regulated phosphoprotein required to suppress photoreceptor dark noise in Drosophila

Photoreceptor cells achieve high sensitivity, reliably detecting single photons, while limiting the spontaneous activation events responsible for dark noise. We used proteomic, genetic, and electrophysiological approaches to characterize Retinophilin (RTP) (CG10233) in Drosophila photoreceptors and establish its involvement in dark-noise suppression. RTP possesses membrane occupation and recognition nexus (MORN) motifs, a structure shared with mammalian junctophilins and other membrane-associated proteins found within excitable cells. We show the MORN repeats, and both the N- and C-terminal domains, are required for RTP localization in the microvillar light-gathering organelle, the rhabdomere. RTP exists in multiple phosphorylated isoforms under dark conditions and is dephosphorylated by light exposure. An RTP deletion mutant exhibits a high rate of spontaneous membrane depolarization events in dark conditions but retains the normal kinetics of the light response. Photoreceptors lacking neither inactivation nor afterpotential C (NINAC) myosin III, a motor protein/kinase, also display a similar dark-noise phenotype as the RTP deletion. We show that NINAC mutants are depleted for RTP. These results suggest the increase in dark noise in NINAC mutants is attributable to lack of RTP and, furthermore, defines a novel role for NINAC in the rhabdomere. We propose that RTP is a light-regulated phosphoprotein that organizes rhabdomeric components to suppress random activation of the phototransduction cascade and thus increases the signaling fidelity of dark-adapted photoreceptors.

Dephosphorylation of Centrins by Protein Phosphatase 2C α and β

In the present study, we identified protein phosphatases dephosphorylating centrins previously phosphorylated by protein kinase CK2. The following phosphatases known to be present in the retina were tested: PP1, PP2A, PP2B, PP2C, PP5, and alkaline phosphatase. PP2C α and β were capable of dephosphorylating P-Thr¹³⁸-centrin1 most efficiently. PP2Cδ was inactive and the other retinal phosphatases also had much less or no effect. Similar results were observed for centrins 2 and 4. Centrin3 was not a substrate for CK2. The results suggest PP2C α and β to play a significant role in regulating the phosphorylation status of centrins in vivo.

The translocation of signaling molecules in dark adapting mammalian rod photoreceptor cells is dependent on the cytoskeleton

In vertebrate rod photoreceptor cells, arrestin and the visual G-protein transducin move between the inner segment and outer segment in response to changes in light. This stimulus dependent translocation of signalling molecules is assumed to participate in long term light adaptation of photoreceptors. So far the cellular basis for the transport mechanisms underlying these intracellular movements remains largely elusive. Here we investigated the dependency of these movements on actin filaments and the microtubule cytoskeleton of photoreceptor cells. Co-cultures of mouse retina and retinal pigment epithelium were incubated with drugs stabilizing and destabilizing the cytoskeleton. The actin and microtubule cytoskeleton and the light dependent distribution of signaling molecules were subsequently analyzed by light and electron microscopy. The application of cytoskeletal drugs differentially affected the cytoskeleton in photoreceptor compartments. During dark adaptation the depolymerization of microtubules as well as actin filaments disrupted the translocation of arrestin and transducin in rod photoreceptor cells. During light adaptation only the delivery of arrestin within the outer segment was impaired after destabilization of microtubules. Movements of transducin and arrestin required intact cytoskeletal elements in dark adapting cells. However, diffusion might be sufficient for the fast molecular movements observed as cells adapt to light. These findings indicate that different molecular translocation mechanisms are responsible for the dark and light associated translocations of arrestin and transducin in rod photoreceptor cells.

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.