Structural Basis for the Functional Diversity of Centrins: A Focus on Calcium Sensing Properties and Target Recognition

Malaria parasite centrins can assemble by Ca2+-inducible condensation

Centrins are small calcium-binding proteins that have a variety of roles and are universally associated with eukaryotic centrosomes. Rapid proliferation of the malaria-causing parasite Plasmodium falciparum in the human blood depends on a particularly divergent and acentriolar centrosome, which incorporates several essential centrins. Their precise mode of action, however, remains unclear. In this study calcium-inducible liquid-liquid phase separation is revealed as an evolutionarily conserved principle of assembly for multiple centrins from P. falciparum and other species. Furthermore, the disordered N-terminus and calcium-binding motifs are defined as essential features for reversible biomolecular condensation, and we demonstrate that certain centrins can form co-condensates. In vivo analysis using live cell STED microscopy shows liquid-like dynamics of centrosomal centrin. Additionally, implementation of an inducible protein overexpression system reveals concentration-dependent formation of extra-centrosomal centrin assemblies with condensate-like properties. The timing of foci formation and dissolution suggests that centrin assembly is regulated. This study thereby provides a new model for centrin accumulation at eukaryotic centrosomes.

Centrins control chicken cone cell lipid droplet dynamics through lipid-droplet-localized SPDL1

As evolutionarily conserved organelles, lipid droplets (LDs) carry out numerous functions and have various subcellular localizations in different cell types and species. In avian cone cells, there is a single apically localized LD. We demonstrated that CIDEA (cell death inducing DFFA like effector a) and microtubules promote the formation of the single LD in chicken cone cells. Centrins, which are well-known centriole proteins, target to the cone cell LD via their C-terminal calcium-binding domains. Centrins localize on cone cell LDs with the help of SPDL1-L (spindle apparatus coiled-coil protein 1-L), a previously uncharacterized isoform of the kinetochore-associated dynein adaptor SPDL1. The loss of CETN3 or overexpression of a truncated CETN1 abrogates the apical localization of the cone cell LD. Simulation analysis showed that multiple LDs or a single mispositioned LD reduces the light sensitivity. Collectively, our findings identify a role of centrins in the regulation of cone cell LD localization, which is important for the light sensitivity of cone cells.

Analysis of Predicted Amino Acid Sequences of Diatom Microtubule Center Components

Diatoms synthesize species-specific exoskeletons inside cells under the control of the cytoskeleton and microtubule center. Previous studies have been conducted with the visualization of the microtubule center; however, its composition has not been studied and reliably established. In the present study, several components of MTOC in diatoms, GCP (gamma complex proteins), Aurora A, and centrins have been identified. Analysis of the predicted amino acid sequences of these proteins revealed structural features typical for diatoms. We analyzed the conserved amino acids and the motives necessary for the functioning of proteins. Phylogenetic analysis of GCP showed that all major groups of diatoms are distributed over phylogenetic trees according to their systematic position. This work is a theoretical study; however, it allows drawing some conclusions about the functioning of the studied components and possible ways to regulate them.

Focus on centrin in normal and altered human spermatozoa

This review provides details on the role of centrin in human spermatozoa and in various forms of male infertility. Centrin is a calcium (Ca²⁺)-binding phosphoprotein that is located in the centrioles - which are typical structures of the sperm connecting piece and play a key role in centrosome dynamics during sperm morphogenesis - as well as in zygotes and early embryos during spindle assembly. In humans, three different centrin genes encoding three isoforms have been discovered. Centrin 1, the only one expressed in spermatozoa, seems to be lost inside the oocyte after fertilization. The sperm connecting piece is characterized by the presence of numerous proteins including centrin, that deserves particular attention because, in humans, it is enriched during maturation of the centrioles. In normal sperm, centrin 1 is visible as two distinct spots in the head-tail junction; however, in some defective spermatozoa, centrin 1 distribution is altered. Centrin has been studied in humans and animal models. Its mutations may lead to several structural alterations, such as serious defects in the connective piece and, subsequently, fertilization failure or incomplete embryonic development. However, the effects of these abnormalities on male fertility have not been fully studied. Because the presence and the function of centrin in the sperm connecting piece appears important for reproductive success, additional studies are needed to bring medical benefits in resolving some cases of idiopathic infertility.

Conformational Plasticity of Centrin 1 from Toxoplasma gondii in Binding to the Centrosomal Protein SFI1

Centrins are calcium (Ca²⁺)-binding proteins that are involved in many cellular functions including centrosome regulation. A known cellular target of centrins is SFI1, a large centrosomal protein containing multiple repeats that represent centrin-binding motifs. Recently, a protein homologous to yeast and mammalian SFI1, denominated TgSFI1, which shares SFI1-repeat organization, was shown to colocalize at centrosomes with centrin 1 from Toxoplasma gondii (TgCEN1). However, the molecular details of the interaction between TgCEN1 and TgSFI1 remain largely unknown. Herein, combining different biophysical methods, including isothermal titration calorimetry, nuclear magnetic resonance, circular dichroism, and fluorescence spectroscopy, we determined the binding properties of TgCEN1 and its individual N- and C-terminal domains to synthetic peptides derived from distinct repeats of TgSFI1. Overall, our data indicate that the repeats in TgSFI1 constitute binding sites for TgCEN1, but the binding modes of TgCEN1 to the repeats differ appreciably in terms of binding affinity, Ca²⁺ sensitivity, and lobe-specific interaction. These results suggest that TgCEN1 displays remarkable conformational plasticity, allowing for the distinct repeats in TgSFI1 to possess precise modes of TgCEN1 binding and regulation during Ca²⁺ sensing, which appears to be crucial for the dynamic association of TgCEN1 with TgSFI1 in the centrosome architecture.

Rapid kinetics of calcium dissociation from plant calmodulin and calmodulin-like proteins and effect of target peptides

Calcium (Ca²⁺) signaling represents a universal information code in plants, playing crucial roles spanning developmental processes to stress responses. Ca²⁺ signals are decoded into defined plant adaptive responses by different Ca²⁺ sensing proteins, including calmodulin (CaM) and calmodulin-like (CML) proteins. Although major advances have been achieved in describing how these Ca²⁺ decoding proteins interact and regulate downstream target effectors, the molecular details of these processes remain largely unknown. Herein, the kinetics of Ca²⁺ dissociation from a conserved CaM and two CML isoforms from A. thaliana has been studied by fluorescence stopped-flow spectroscopy. Kinetic data were obtained for the isolated Ca²⁺-bound proteins as well as for the proteins complexed with different target peptides. Moreover, the lobe specific interactions between the Ca²⁺ sensing proteins and their targets were characterized by using a panel of protein mutants deficient in Ca²⁺ binding at the N-lobe or C-lobe. Results were analyzed and discussed in the context of the Ca²⁺-decoding and Ca²⁺-controlled target binding mechanisms in plants.