A gelsolin-related actin-severing protein with fully reversible actin-binding properties from the tail muscle of crayfish, Astacus leptodactylus

Gelsolin from mussel's catch muscle

Proteins of the gelsolin family are Ca2+-dependent, multifunctional, actin-binding proteins containing three (S1-S3, about 40 kDa) or six (S1-S6, about 80 kDa) highly conserved repeats in the amino acid sequence. The pattern of interaction of these proteins with actin is complex: they can sever actin filaments; promote polymer nucleation after binding to two actin monomers; and cap the growing barbed end of actin filaments. In the present study, an actin polymerizing factor (46 kDa) from the adductor muscle of a bivalve mollusc has been discovered and identified for the first time. This protein has turned out to belong to the gelsolin family of actin regulatory proteins. The expression of gelsolin-like proteins in the tissues of bivalves was predicted after analyzing their proteome, but this is the first study where an actually expressed protein has been found. A primary determination of its physicochemical properties such as molecular weight, charge, resistance to urea, influence on actin polymerization by viscosity, and light scattering is carried out and the molecular structure analyzed.

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.

Evidence for gelsolin as a corneal crystallin in zebrafish

We have shown that gelsolin is one of the most prevalent water-soluble proteins in the transparent cornea of zebrafish. There are also significant amounts of actin. In contrast to actin, gelsolin is barely detectable in other eye tissues (iris, lens, and remaining eye) of the zebrafish. Gelsolin cDNA hybridized intensely in Northern blots to RNA from the cornea but not from the lens, brain, or headless body. The deduced zebrafish gelsolin is approximately 60% identical to mammalian cytosolic gelsolin and has the characteristic six segmental repeats as well as the binding sites for actin, calcium, and phosphatidylinositides. In situ hybridization tests showed that gelsolin mRNA is concentrated in the zebrafish corneal epithelium. The zebrafish corneal epithelium stains very weakly with rhodamine-phalloidin, indicating little F-actin in the cytoplasm. In contrast, the mouse corneal epithelium contains relatively little gelsolin and stains intensely with rhodamine-phalloidin, as does the zebrafish extraocular muscle. We propose, by analogy with the diverse crystallins of the eye lens and with the putative enzyme-crystallins (aldehyde dehydrogenase class 3 and other enzymes) of the mammalian cornea, that gelsolin and actin-gelsolin complexes act as water-soluble crystallins in the zebrafish cornea and contribute to its optical properties.

Purification and functional characterization of an 85-kDa gelsolin from the ascidians Microcosmus sulcatus and Phallusia mammilata

From the pharyngeal baskets of the ascidians Microcosmus sulcatus and Phallusia mammilata we have purified an 85-kDa protein that is characterized as a member of the gelsolin family. These proteins from both species show the same behaviour in functional assays. The ascidian gelsolin binds two actin monomers in a highly cooperative manner. This complex formation is Ca(2+)-dependent, but not completely reversible, as on removal of Ca2+ one actin monomer dissociates leaving a 1:1 complex between gelsolin and G-actin. The properties of F-actin severing and G-actin nucleation depend on the presence of free Ca2+ in a micromolar range, with half maximum activation at about 3 x 10(-6) M. The protein becomes inactivated when Ca2+ concentrations of 0.5 mM are exceeded. Fragmentation of F-actin by the ascidian gelsolin is comparably fast to that of vertebrate gelsolin. A steady state of actin fragmentation is reached within 2-4 s. Promotion of G-actin nucleation is also comparable to that of vertebrate gelsolin. Regarding functional aspects, the ascidian gelsolin is more closely related to vertebrate gelsolin than to an arthropod gelsolin from crayfish tail muscle.

C-terminally deleted fragments of 40-kDa earthworm actin modulator still show gelsolin activities

C- and N-terminally truncated fragments of earthworm gelsolin were constructed, cloned and expressed in Escherichia coli. G-actin-binding properties of these fragments and their influences on the polymeric state of actin were investigated. A construct lacking a large part of the third segment [E(1-295)] supports actin nucleation similar to the complete protein and shows reduced actin fragmentation property, but is no longer Ca2+-sensitive in its activity. The first and the second segments (E1 and E2) each contain one actin-binding site. In contrast to human gelsolin, E1 in combination with a short N-terminal region of E2 is not sufficient for the F-actin-severing activity of the protein.

A gelsolin-related protein from lobster muscle: cloning, sequence analysis and expression

The tail muscle of the lobster Homarus americanus contains an actin-binding protein with an apparent molecular mass of 105 kDa determined by SDS/PAGE and gelsolin-like properties. We isolated this protein and peptide sequences were obtained after limited proteolysis with chymotrypsin. A tail-muscle-specific cDNA library was constructed in a lambda expression vector and a full-length clone was obtained by screening with a polyclonal anti-(crustacean gelsolin) antibody. The cDNA insert of approx. 3.2 kb length was sequenced. The cDNA contained an open reading frame of 2.265 kb, and the deduced amino acid sequence of 754 residues (83,469 Da) identified the protein as a cytoplasmic member of the gelsolin/villin protein family. Comparison of the lobster gelsolin amino acid sequence with other members of this protein family revealed the characteristic 6-fold repeated segmental structure as well as the three conserved sequence motifs typical of each segment [Way and Weeds (1988) J. Mol. Biol. 203, 1127-1133]. Strong homologies were found with Drosophila gelsolin, human gelsolin, villin core, Dictyostelium severin and Physarum fragmin. In addition, the gelsolin-like protein from lobster muscle revealed motifs that were clearly similar to the actin-bundling region of human villin headpiece although it did not itself contain a distinct headpiece domain. The recombinant lobster gelsolin-like protein, expressed in Escherichia coli as a fusion protein, was purified from inclusion bodies and renatured as a functional protein. There were no significant differences in the biological activity tested between the recombinant and the native protein isolated from lobster muscle.