Calcium-bindings of wild type and mutant troponin Cs of Caenorhabditis elegans

EF-hand proteins and the regulation of actin-myosin interaction in the eutardigrade Hypsibius klebelsbergi (tardigrada)

Many tardigrade species resist harsh environmental conditions by entering anhydrobiosis or cryobiosis. Desiccation as well as freeze resistance probably leads to changes of the ionic balance that includes the intracellular calcium concentration. In order to search for protein modifications affecting the calcium homoeostasis, we studied the regulatory system controlling actin-myosin interaction of the eutardigrade Hypsibius klebelsbergi and identified full-length cDNA clones for troponin C (TnC, 824 bp), calmodulin (CaM, 1,407 bp), essential myosin light chain (eMLC, 1,015 bp), and regulatory myosin light chain (rMLC, 984 bp) from a cDNA library. All four proteins belong to the EF-hand superfamily typified by a calcium coordinating helix-loop-helix motif. Further, we cloned and obtained recombinant TnC and both MLCs. CaM and TnC revealed four and two potential calcium-binding domains, respectively. Gel mobility shift assays demonstrated calcium-induced conformational transition of TnC. From both MLCs, only the rMLC showed one potential N-terminal EF-hand domain. Additionally, sequence properties suggest phosphorylation of this myosin light chain. Based on our results, we suggest a dual-regulated system at least in somatic muscles for tardigrades with a calcium-dependent tropomyosin-troponin complex bound to the actin filaments and a phosphorylation of the rMLC turning on and off both actin and myosin. Our results indicate no special modifications of the molecular structure and function of the EF-hand proteins in tardigrades. Phylogenetic trees of 131 TnCs, 96 rMLCs, and 62 eMLCs indicate affinities to Ecdysozoa, but also to some other taxa suggesting that our results reflect the complex evolution of these proteins rather than phylogenetic relationships.

In silico prediction and analysis of Caenorhabditis EF-hand containing proteins

Calcium (Ca⁺²) is a ubiquitous messenger in eukaryotes including Caenorhabditis. Ca⁺²-mediated signalling processes are usually carried out through well characterized proteins like calmodulin (CaM) and other Ca⁺² binding proteins (CaBP). These proteins interact with different targets and activate it by bringing conformational changes. Majority of the EF-hand proteins in Caenorhabditis contain Ca⁺² binding motifs. Here, we have performed homology modelling of CaM-like proteins using the crystal structure of Drosophila melanogaster CaM as a template. Molecular docking was applied to explore the binding mechanism of CaM-like proteins and IQ1 motif which is a ∼25 residues and conform to the consensus sequence (I, L, V)QXXXRXXXX(R,K) to serve as a binding site for different EF hand proteins. We made an attempt to identify all the EF-hand (a helix-loop-helix structure characterized by a 12 residues loop sequence involved in metal coordination) containing proteins and their Ca⁺² binding affinity in Caenorhabditis by analysing the complete genome sequence. Docking studies revealed that F165, F169, L29, E33, F44, L57, M61, M96, M97, M108, G65, V115, F93, N104, E144 of CaM-like protein is involved in the interaction with IQ1 motif. A maximum of 170 EF-hand proteins and 39 non-EF-hand proteins with Ca⁺²/metal binding motif were identified. Diverse proteins including enzyme, transcription, translation and large number of unknown proteins have one or more putative EF-hands. Phylogenetic analysis revealed seven major classes/groups that contain some families of proteins. Various domains that we identified in the EF-hand proteins (uncharacterized) would help in elucidating their functions. It is the first report of its kind where calcium binding loop sequences of EF-hand proteins were analyzed to decipher their calcium affinities. Variation in Ca⁺²-binding affinity of EF-hand CaBP could be further used to study the behaviour of these proteins. Our analyses postulated that Ca⁺² is likely to be key player in Caenorhabditis cell signalling.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Diversification and Independent Evolution of Troponin C Genes in Insects

Troponin C (TpnC), the calcium-binding subunit of the troponin regulatory complex in the muscle thin filament, is encoded by multiple genes in insects. To understand how TpnC genes have evolved, we characterized the gene number and structure in a number of insect species. The TpnC gene complement is five genes in Drosophilidae as previously reported for D. melanogaster. Gene structures are almost identical in D. pseudoobscura, D. suboboscura, and D. virilis. Developmental patterns of expression are also conserved in Drosophila subobscura and D. virilis. Similar, but not completely equivalent, TpnC gene repertoires have been identified in the Anopheles gambiae and Apis mellifera genomes. Insect TpnC sequences can be divided into three groups, allowing a systematic classification of newly identified genes. The pattern of expression of the Apis mellifera genes essentially agrees with the pattern in Drosophilidae, providing further functional support to the classification. A model for the evolution of the TpnC genes is proposed including the most likely pathway of insect TpnC diversification. Our results suggest that the rapid increase in number and sequence specialization of the adult Type III isoforms can be correlated with the evolution of the holometabolous mode of development and the acquisition of asynchronous indirect flight muscle function in insects. This evolutionarily specialization has probably been achieved independently in different insect orders.

Structural characterization of a recombinant flagellar calcium-binding protein from Trypanosoma cruzi

Calflagin are flagellar calcium-binding proteins belonging to the EF-hand super family described in several protozoa, including Trypanosoma cruzi. Evidences have shown that Ca(2+) may play an important regulatory role in trypanosomatid flagellar mobility. In these parasites, the response of the cell to variations of Ca(2+) levels is determined by a variety of calcium-modulated proteins. Starting from T. cruzi cDNA lambdagt11 library trypomastigote, a clone encoding a 29-kDa flagellar protein designated recombinant calflagin (rC29) was selected. rC29 is a calcium-acyl switch protein modified by the addition of myristate and palmitate at its amino terminal segment. In this work, unmyristoylated rC29 was expressed in Escherichia coli as an intein fusion protein and purified by affinity chromatography. Circular dichroism (CD) and fluorescence measurements showed conformational changes of rC29 due to Ca(2+) binding. The Ca(2+) binding constants were obtained by tryptophan intrinsic fluorescence spectroscopy. Fluorescence titration exhibited two classes of Ca(2+)-binding sites in the unmyristoylated rC29, which bind calcium with apparent association constant of K(a) of 3.3+/-0.5 (10(6)) and 1.9+/-0.2 (10(4)) M(-1). Experiment using 8-anilinonaphthalene-1-sulfonic acid (ANS) as hydrophobic probe showed that the Ca(2+)-loaded form of rC29 contains exposed hydrophobic surfaces, thus suggesting that rC29 is probably functioning as a calcium sensor.

Troponin C in different insect muscle types: identification of two isoforms in Lethocerus, Drosophila and Anopheles that are specific to asynchronous flight muscle in the adult insect

The indirect flight muscles (IFMs) of Lethocerus (giant water bug) and Drosophila (fruitfly) are asynchronous: oscillatory contractions are produced by periodic stretches in the presence of a Ca(2+) concentration that does not fully activate the muscle. The troponin complex on thin filaments regulates contraction in striated muscle. The complex in IFM has subunits that are specific to this muscle type, and stretch activation may act through troponin. Lethocerus and Drosophila have an unusual isoform of the Ca(2+)-binding subunit of troponin, troponin C (TnC), with a single Ca(2+)-binding site near the C-terminus (domain IV); this isoform is only in IFMs, together with a minor isoform with an additional Ca(2+)-binding site in the N-terminal region (domain II). Lethocerus has another TnC isoform in leg muscle which also has two Ca(2+)-binding sites. Ca(2+) binds more strongly to domain IV than to domain II in two-site isoforms. There are four isoforms in Drosophila and Anopheles (malarial mosquito), three of which are also in adult Lethocerus. A larval isoform has not been identified in Lethocerus. Different TnC isoforms are expressed in the embryonic, larval, pupal and adult stages of Drosophila; the expression of the two IFM isoforms is increased in the pupal stage. Immunoelectron microscopy shows the distribution of the major IFM isoform with one Ca(2+)-binding site is uniform along Lethocerus thin filaments. We suggest that initial activation of IFM is by Ca(2+) binding to troponin with the two-site TnC, and full activation is through the action of stretch on the complex with the one-site isoform.