Regulatory role of the second gelsolin-like domain of Caenorhabditis elegans gelsolin-like protein 1 (GSNL-1) in its calcium-dependent conformation and actin-regulatory activities

Transcriptomic and Proteomic Analysis of Marine Nematode Litoditis marina Acclimated to Different Salinities

Salinity is a critical abiotic factor for all living organisms. The ability to adapt to different salinity environments determines an organism’s survival and ecological niches. Litoditis marina is a euryhaline marine nematode widely distributed in coastal ecosystems all over the world, although numerous genes involved in its salinity response have been reported, the adaptive mechanisms underlying its euryhalinity remain unexplored. Here, we utilized worms which have been acclimated to either low-salinity or high-salinity conditions and evaluated their basal gene expression at both transcriptomic and proteomic levels. We found that several conserved regulators, including osmolytes biosynthesis genes, transthyretin-like family genes, V-type H⁺-transporting ATPase and potassium channel genes, were involved in both short-term salinity stress response and long-term acclimation processes. In addition, we identified genes related to cell volume regulation, such as actin regulatory genes, Rho family small GTPases and diverse ion transporters, which might contribute to hyposaline acclimation, while the glycerol biosynthesis genes gpdh-1 and gpdh-2 accompanied hypersaline acclimation in L. marina. This study paves the way for further in-depth exploration of the adaptive mechanisms underlying euryhalinity and may also contribute to the study of healthy ecosystems in the context of global climate change.

Gelsolin in Onychophora and Tardigrada with notes on its variability in the Ecdysozoa

Rearrangements of the filamentous actin network involve a broad range of actin binding proteins. Among these, the gelsolin proteins sever actin filaments, cap their fast growing end and nucleate actin assembly in a calcium-dependent manner. Here, we focus on the gelsolin of the onychophoran Peripatoides novaezealandiae and the eutardigrade Hypsibius dujardini. From the cDNA of P. novaezealandiae we obtained the complete coding sequence with an open reading frame of 2178bp. It encodes a protein of 726 amino acids with a calculated molecular mass of 82,610.9Da and a pI of 5.57. This sequence is comprised of six segments (S1-S6). However, analysis of data from TardiBase reveals that the gelsolin of the eutardigrade Hypsibius dujardini has only three segments (S1-S3). The coding sequence consist of 1119bp for 373 amino acids with a calculated molecular mass of 42,440.95Da and a pI of 6.17. The Peripatoides and Hypsibius gelsolin revealed both conserved binding motifs for G-actin, F-actin and phosphatidylinositol 4,5-bisphosphate (PIP₂), along with a full set of type-1 and type-2 Ca²⁺-binding sites which could result in the binding of eight and four calcium ions, respectively. Both gelsolin proteins lack a C-terminal latch-helix indicating a more rapid activation in the submicromolar Ca²⁺ range. We suggest that a gelsolin with three segments was present in the last common ancestor of the ecdysozoan clade Panarthropoda (Onychophora, Tardigrada, Arthropoda), primarily because the gelsolin of all non-Ecdysozoa studied so far (except Chordata) reveals this number of segments. Mapping of our molecular data onto a well-established phylogeny revealed that the number of gelsolin segments does not correlate with the phylogenetic lineage but rather with particular functional demands to alter the kinetics of actin polymerization.

Gelsolin-Like Domain 3 Plays Vital Roles in Regulating the Activities of the Lily Villin/Gelsolin/Fragmin Superfamily

The villin/gelsolin/fragmin superfamily is a major group of Ca2+-dependent actin-binding proteins (ABPs) involved in various cellular processes. Members of this superfamily typically possess three or six tandem gelsolin-like (G) domains, and each domain plays a distinct role in actin filament dynamics. Although the activities of most G domains have been characterized, the biochemical function of the G3 domain remains poorly understood. In this study, we carefully compared the detailed biochemical activities of ABP29 (a new member of this family that contains the G1-G2 domains of lily ABP135) and ABP135G1-G3 (which contains the G1-G3 domains of lily ABP135). In the presence of high Ca2+ levels in vitro (200 and 10 μM), ABP135G1-G3 exhibited greater actin severing and/or depolymerization and nucleating activities than ABP29, and these proteins had similar actin capping activities. However, in the presence of low levels of Ca2+ (41 nM), ABP135G1-G3 had a weaker capping activity than ABP29. In addition, ABP29 inhibited F-actin depolymerization, as shown by dilution-mediated depolymerization assay, differing from the typical superfamily proteins. In contrast, ABP135G1-G3 accelerated F-actin depolymerization. All of these results demonstrate that the G3 domain plays specific roles in regulating the activities of the lily villin/gelsolin/fragmin superfamily proteins.

Regulation of structure and function of sarcomeric actin filaments in striated muscle of the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has been used as a valuable system to study structure and function of striated muscle. The body wall muscle of C. elegans is obliquely striated muscle with highly organized sarcomeric assembly of actin, myosin, and other accessory proteins. Genetic and molecular biological studies in C. elegans have identified a number of genes encoding structural and regulatory components for the muscle contractile apparatuses, and many of them have counterparts in mammalian cardiac and skeletal muscles or striated muscles in other invertebrates. Applicability of genetics, cell biology, and biochemistry has made C. elegans an excellent system to study mechanisms of muscle contractility and assembly and maintenance of myofibrils. This review focuses on the regulatory mechanisms of structure and function of actin filaments in the C. elegans body wall muscle. Sarcomeric actin filaments in C. elegans muscle are associated with the troponin-tropomyosin system that regulates the actin-myosin interaction. Proteins that bind to the side and ends of actin filaments support ordered assembly of thin filaments. Furthermore, regulators of actin dynamics play important roles in initial assembly, growth, and maintenance of sarcomeres. The knowledge acquired in C. elegans can serve as bases to understand the basic mechanisms of muscle structure and function.