Isolation and characterization of calmodulin from the motile green alga Chlamydomonas reinhardtii

Cloning and Stress-Induced Expression Analysis of Calmodulin in the Antarctic Alga Chlamydomonas sp. ICE-L

Calmodulin (CaM) is a Ca²⁺-binding protein that plays a role in several Ca²⁺ signaling pathways, which dynamically regulates the activities of hundreds of proteins. The ice alga Chlamydomonas sp. ICE-L, which has the ability to adapt to extreme polar conditions, is a crucial primary producer in Antarctic ecosystem. This study hypothesized that Cam helps the ICE-L to adapt to the fluctuating conditions in the polar environment. It first verified the overall length of Cam, through RT-PCR and RACE-PCR, based on partial Cam transcriptome library of ICE-L. Then, the nucleotide and predicted amino acid sequences were, respectively, analyzed by various bioinformatics approaches to gain more insights into the computed physicochemical properties of the CaM. Potential involvements of Cam in responding to certain stimuli (i.e., UVB radiation, high salinity, and temperature) were investigated by differential expression, measuring its transcription levels by means of quantitative RT-PCR. Results showed that CaM was indeed inducible and regulated by high UVB radiation, high salinity, and nonoptimal temperature conditions. Different conditions had different expression tendencies, which provided an important basis for investigating the adaptation mechanism of Cam in ICE-L.

Villidin, a novel WD-repeat and villin-related protein from Dictyostelium, is associated with membranes and the cytoskeleton

Villidin is a novel multidomain protein (190 kDa) from Dictyostelium amoebae containing WD repeats at its N-terminus, three PH domains in the middle of the molecule, and five gelsolin-like segments at the C-terminus, followed by a villin-like headpiece. Villidin mRNA and protein are present in low amounts during growth and early aggregation, but increase during development and reach their highest levels at the tipped mound stage. The protein is present in the cytosol as well as in the cytoskeletal and membrane fractions. GFP-tagged full-length villidin exhibits a similar distribution as native villidin, including a distinct colocalization with Golgi structures. Interestingly, GFP fusions with the gelsolin/villin-like region are uniformly dispersed in the cytoplasm, whereas GFP fusions of the N-terminal WD repeats codistribute with F-actin and are associated with the Triton-insoluble cytoskeleton. Strains lacking villidin because of targeted deletion of its gene grow normally and can develop into fruiting bodies. However, cell motility is reduced during aggregation and phototaxis is impaired in the mutant strains. We conclude that villidin harbors a major F-actin binding site in the N-terminal domain and not in the villin-like region as expected; association of villidin with vesicular membranes suggests that the protein functions as a linker between membranes and the actin cytoskeleton.

Characterization of the calcium-binding contractile protein centrin from Tetraselmis striata (Pleurastrophyceae)

Centrin is a major protein of the contractile striated flagellar roots of the green alga Tetraselmis striata. We present a newly modified procedure for the preparation of centrin in sufficient quantity and purity to allow for detailed biochemical characterization. We establish that centrin purified by differential solubility, followed by phenyl-Sepharose and DEAE-Sephacel chromatography is identical with the protein extracted directly from striated flagellar roots with regard to molecular weight, isoelectric point, and calcium-dependent behavior in SDS-PAGE. We also compare the biochemical properties of purified centrin with calmodulin isolated from Tetraselmis and calmodulin isolated from mammalian brain. Centrin can be fully distinguished from either algal or mammalian calmodulin on the basis of molecular weight, isoelectric point, calcium-dependent behavior in SDS-PAGE, proteolytic peptide maps, amino acid composition, ability to activate bovine brain phosphodiesterase, and reactivity with specific antibodies.

Substitution at position 116 of Schizosaccharomyces pombe calmodulin decreases its stability under nitrogen starvation and results in a sporulation-deficient phenotype

We constructed Schizosaccharomyces pombe strains that carry phenylalanine, instead of arginine, as residue 116 of calmodulin by site-directed mutagenesis of the cam1 gene. Whereas haploid strains carrying the mutant allele, designated cam1-F116, exhibit no defects in growth and mating, diploid strains homozygous for cam1-F116 are deficient in sporulation. The four nuclei generated by the two serial meiotic divisions are not encapsulated in these diploids. The mutation is recessive. Semiquantitative analysis using polyclonal antibodies showed that vegetatively growing cam1-F116 cells have a smaller amount of calmodulin than wild-type cells. The quantitative difference becomes more remarkable if the cells are starved for nitrogen, which is a condition for induction of sporulation. In addition to this in vivo observation, we showed in vitro that the mutant protein is susceptible to a proteolytic activity induced by nitrogen starvation that hardly affects the wild-type calmodulin. Thus, the sporulation deficiency of the cam1-F116 mutant may be ascribed to shortage of calmodulin due to proteolysis of the mutant molecules under nitrogen starvation. Two other mutations at position 116 resulted in similar but leakier Spo- phenotypes.

Stimulation of the erythrocyte Ca2+-ATPase and of bovine brain cyclic nucleotide phosphodiesterase by chemically modified calmodulin

Chemically modified calmodulins have been used to investigate structural features which are important for the interaction of the activator with targets. Carbamoylation of lysine residues had no influence on the ability of calmodulin to stimulate the plasma membrane Ca2+-ATPase whereas the stimulation of the bovine brain cyclic-nucleotide phosphodiesterase was reduced up to 50%. Different species of carbamoylated calmodulin have been isolated but no differences were detected in their interaction with the cyclic-nucleotide phosphodiesterase. Modification of arginine residues by 1,2-cyclohexanedione had no effect of the stimulation of the phosphodiesterase but reduced by 40% the stimulation of the erythrocyte Ca2+ ATPase. Mild oxidation of methionines by N-chlorosuccinimide produced a number of differently modified calmodulins. The different species have been purified and the modified residues have been identified. They affected the two different test enzymes to different extents indicating that methionines in the central helix of calmodulin are of greater importance for the interaction with the phosphodiesterase, whereas methionines located in the C-terminal half of calmodulin are more important for the interaction with the Ca2+-ATPase.

Analysis and in vivo disruption of the gene coding for calmodulin in Schizosaccharomyces pombe

Calmodulin is a low molecular weight calcium-binding protein that modulates many enzyme systems in eukaryotes. We have cloned the gene encoding calmodulin from the fission yeast, Schizosaccharomyces pombe, by using synthetic oligonucleotide probes that correspond to three distinct regions of Tetrahymena calmodulin. A 1.6-kilobase (kb) DNA fragment that hybridized to all of them contains a gene whose deduced product possesses 74% amino acid homology with bovine calmodulin. This gene, which is unique in the S. pombe genome and is named cam1, encodes 149 amino acids excluding the first methionine and is transcribed into mRNA of 1.2-kb length. It has an intron that apparently starts immediately after the initiation codon and is 126 bp long. S. pombe calmodulin exhibits more homology to vertebrate calmodulin than to that of the budding yeast, Saccharomyces cerevisiae. Gene disruption experiments revealed that cam1 gene function is essential for vegetative growth of S. pombe. Spores bearing disrupted cam1 halt growth soon after germination and rarely carry out the first cell division, indicating that calmodulin does not exist in excess in those cells.

Purification and properties of calmodulin-lysine N-methyltransferase from rat brain cytosol

A S-adenosylmethionine:protein-lysine N-methyltransferase (EC 2.1.1.43) has been purified from rat brain cytosol 7,080-fold with a yield of 8%, using octopus calmodulin as a substrate. It contains a lysine residue that is not fully methylated. The enzyme was purified by ammonium sulfate fractionation, Sephacryl S-200 gel filtration, and phosphocellulose and octopus calmodulin-Sepharose affinity chromatographies. Among protein substrates, it was highly specific toward octupus calmodulin. The Km values for octopus calmodulin and S-adenosyl-L-methionine were found to be 2.2 X 10(-8) M and 0.8 X 10(-6) M, respectively. The molecular weight was estimated to be 57,000 by gel filtration and the pH optimum was between 7.5 and 8.5. The enzyme was stimulated in the presence of 10(-7) M Mn2+ and 10(-4) M Ca2+. HPLC of the acid hydrolysate of methyl-3H-labeled calmodulin showed the formation of epsilon-N-mono, epsilon-N-di, and epsilon-N-trimethyllysine. Reverse-phase HPLC of tryptic peptides of the methyl-3H-labeled calmodulin demonstrated that the labeled N-methyllysine lies in the 107-126 peptide. These findings suggest that this enzyme methylated a specific lysine residue of octopus calmodulin.

Two calmodulins in Naegleria flagellates: characterization, intracellular segregation, and programmed regulation of mRNA abundance during differentiation

Flagellates of Naegleria gruberi contain two calmodulins that differ in apparent molecular weight and intracellular location. Calmodulin-1, localized in flagella, has an apparent molecular weight of approximately 16,000, approximately the size of other protozoan calmodulins, whereas calmodulin-2, localized in cell bodies, is 15,300. Both proteins, purified, are calmodulins by several criteria, including Ca2+-dependent stimulation of calmodulin-dependent cyclic nucleotide phosphodiesterase and affinity for antibodies to vertebrate calmodulin. The finding of two calmodulins is unusual. Since the only known difference is apparent molecular weight, one calmodulin could be derived from the other, except that both calmodulins are synthesized in a wheat germ, cell-free system directed by RNA from differentiating Naegleria. Translatable mRNAs encoding calmodulins 1 and 2, not detected in amebas, appear and subsequently disappear concurrently during the 100-min differentiation of Naegleria from amebas to flagellates. Furthermore, these mRNAs increase and then decrease in abundance concurrently with those for flagellar tubulins, which suggests the possibility that the expression of the unrelated genes for calmodulin and tubulin may be under coordinate control during differentiation.

Trifluoperazine and W-7 inhibit mating in Chlamydomonas at an early stage of gametic interaction

Gametic mating by Chlamydomonas reinhardi is inhibited in a dose-dependent and reversible manner by the calmodulin antagonists trifluoperazine (TFP) and W-7, but not by W-5, an analog of W-7 having lower affinity for calmodulin. Quantitation of the sequential steps of mating showed that TFP and W-7 both allow normal levels of flagellar agglutination but prevent all subsequent steps. Gametes agglutinate aberrantly and do not form mating pairs. Further, both of these drugs prevent the translocation of latex beads along the flagellar surface. Our observations suggest that calmodulin may play an integral role in the translocation of flagellar adhesion sites during the tip-locking stage of the Chlamydomonas mating reaction. Flagellar surface motility may be crucial to the transduction of signals during mating and may share regulatory mechanisms with other forms of surface motility.

Phosphorylation in isolated Chlamydomonas axonemes: a phosphoprotein may mediate the Ca2+-dependent photophobic response

An in vitro system was devised for studying phosphorylation of Chlamydomonas reinhardtii axonemal proteins. Many of the polypeptides phosphorylated in this system could be identified as previously described axonemal components that are phosphorylated in vivo. The in vitro system apparently preserved the activities of diverse axonemal kinases without greatly altering the substrate specificity of the enzymes. The in vitro system was used to study the effect of calcium concentration on axonemal protein phosphorylation. Calcium has previously been demonstrated to initiate the axonemal reversal reaction of the photophobic response; the in vitro system made it possible to investigate the possibility that this calcium effect is mediated by protein phosphorylation. Calcium specifically altered the phosphorylation of only two axonemal proteins; the phosphorylation of an otherwise unidentified 85,000 Mr protein was repressed by calcium concentrations greater than or equal to 10(-6) M, while the phosphorylation of the previously identified 95,000 Mr protein b4 was stimulated by calcium at concentrations greater than 10(-6) M. Protein b4 is one of six polypeptides that are deficient in the mbo mutants, strains that do not exhibit a photophobic reversal reaction. Therefore, this calcium-stimulated phosphorylation may be involved in initiating the photophobic response. Neither calmodulin nor the C-kinase could be implicated in b4 phosphorylation. The calcium-dependent activation of the b4 kinase was not affected by several drugs that bind to and inhibit calmodulin, or by the addition of exogenous calmodulin. Activators and inhibitors of the calcium-phospholipid-dependent C kinase also had no effect on b4 phosphorylation.