Loss of calcium sensitivity of plasma gelsolin is associated with the presence of calcium ions during preparation

Topological assignment of the N-terminal extension of plasma gelsolin to the gelsolin surface

The actin-binding protein gelsolin is highly conserved in vertebrates and exists in two isoforms, a cytoplasmic and an extracellular variant, generated by alternative splicing. In mammals, these isoforms differ only by an N-terminal extension in plasma gelsolin, a short sequence of up to 25 amino acids. Cells and tissues may contain both variants, as plasma gelsolin is secreted by many cell types. The tertiary structure of equine plasma gelsolin has been elucidated, but without any information on the N-terminal extension. In this paper, we present topographical data on the N-terminal extension, derived using a biochemical and immunological approach. For this purpose, a monoclonal antibody was generated that exclusively recognizes cytoplasmic gelsolin but not the extracellular variant and thus allows isoform-specific immunodetection and quantification of cytoplasmic gelsolin in the presence of plasma gelsolin. Using limited proteolysis and pepscan analysis, we mapped the binding epitope and localized it within two regions in segment 1 of the cytoplasmic gelsolin sequence: Tyr34-Ile45 and Leu64-Ile78. In the tertiary structure of the cytoplasmic variant, these sequences are mutually adjacent and located in the proximity of the N-terminus. We therefore conclude that the binding site of the antibody is covered by the N-terminal extension in plasma gelsolin and thus sterically hinders antibody binding. Our results allow for a topological model of the N-terminal extension on the surface of the gelsolin molecule, which was unknown previously.

Calcium-Induced Conformational Changes in the C-Terminal Half of Gelsolin Stabilize Its Interaction with the Actin Monomer

The basic mechanism for the nucleating effect of gelsolin on actin polymerization is the formation of a complex of gelsolin with two actin monomers. Probably due to changes in the C-terminal part of gelsolin, a stable ternary complex is only formed at [Ca(2+)] >10(-5) M [Khaitlina, S., and Hinssen, H. (2002) FEBS Lett. 521, 14-18]. Therefore, we have studied the binding of actin monomer to the isolated C-terminal half of gelsolin (segments 4-6) over a wide range of calcium ion concentrations to correlate the conformational changes to the complex formation. With increasing [Ca(2+)], the apparent size of the C-terminal half as determined by gel filtration was reduced, indicating a transition into a more compact conformation. Moreover, Ca(2+) inhibited the cleavage by trypsin at Lys 634 within the loop connecting segments 5 and 6. Though the inhibitory effect was observed already at [Ca(2+)] of 10(-7) M, it was enhanced with increasing [Ca(2+)], attaining saturation only at >10(-4) M Ca(2+). This indicates that the initial conformational changes are followed by additional molecular transitions in the range of 10(-5)-10(-4) M [Ca(2+)]. Consistently, preformed complexes of actin with the C-terminal part of gelsolin became unstable upon lowering the calcium ion concentrations. These data provide experimental support for the role of the type 2 Ca-binding sites in gelsolin segment 5 proposed by structural studies [Choe et al. (2002) J. Mol. Biol. 324, 691]. We assume that the observed structural transitions contribute to the stable binding of the second actin monomer in the ternary gelsolin-actin complex.

Intracellular calcium response of Sf-9 insect cells exposed to intense fluid forces

Suspension cell cultures are exposed to periodic high intensity, short duration fluid forces by circulating them through a flow loop containing a capillary, which simulates what a cell experiences in a stirred bioreactor. Sf-9 insect cells exhibit an increase in intracellular calcium concentration, [Ca2+]i when exposed to these cyclic fluid forces. Flow through the capillary spans both the laminar and turbulent regime and the calcium response is not dependent on the transition to turbulence. The calcium response is a nearly linear function of the rate of energy dissipation per mass of fluid in the capillary. The source of the increased calcium ions in the cell cytosol is within the cell itself, indicating that the calcium response is a cellular response to fluid forces and not a matter of increased plasma membrane permeability to Ca2+ ions. Flow cytometry on hydrodynamically stimulated and unstimulated cells reveals that the increase in the intracellular calcium concentration averaged over the cell population is due to an increase in intracellular calcium concentration in only a small sub-population of the entire suspension.

The Ca(2+)-induced conformational change of gelsolin is located in the carboxyl-terminal half of the molecule

We have purified the two functionally distinct domains of gelsolin, a Ca(2+)-dependent actin binding protein, by proteolytic cleavage and characterized their size and shape in solution by dynamic light scattering. In the absence of calcium we obtained the same translational diffusion coefficient for both fragments which are of approximately equal molecular mass. The frictional ratio fo/fexp (1.33-1.39) is similar to the value as obtained for intact gelsolin (1.37) in aqueous solution (Patkowski, A., J. Seils, H. Hinssen, and T. Dorfmüller. 1990. Biopolymers. 30:427-435), indicating a similar molecular shape for the native protein as well as for the two subdomains. Upon addition of Ca2+ the translational diffusion coefficient of the carboxyl-terminal half decreased by almost 10%, while there was no change observed for the amino terminus. This result indicates that the ligand-induced conformational change as seen for intact gelsolin is probably located on the carboxyl-terminal domain of the protein. Since gelsolin has binding sites in both domains, and the isolated amino terminus binds and severs actin in a calcium-independent manner, our results suggests that the structural transition in the carboxyl-terminal part of intact gelsolin also affects the actin binding properties of the amino-terminal half.

Crystallization of human gelsolin

Human gelsolin has been crystallized by microdialysis techniques to give single crystals that diffract to 3.5 A resolution. The crystals belong to space group P42(1)2 and have cell dimensions a = 175.0 A, c = 151.6 A. They contain two gelsolin molecules in the asymmetric unit.

ASP-56, a new actin sequestering protein from pig platelets with homology to CAP, an adenylate cyclase-associated protein from yeast

A new 56 kDa actin-binding protein (ASP-56) was isolated from pig platelet lysate. In falling ball viscosimetry it caused a reduction in viscosity that could be attributed to a decrease in the concentration of polymeric actin. Fluorescence measurements with NBD-labelled actin showed reduction of polymeric actin, too. These results could be explained by sequestering of actin in a non-polymerizable 1:1 ASP-56/actin complex. Sequencing of about 20 tryptic peptides of ASP-56 and comparison with known sequences revealed about 60% homology to the adenylate cyclase-associated protein (CAP) from yeast.

Differential effects of gelsolins on tissue culture cells

Gelsolins, prepared from a number of different sources, showed similar severing activity on F-actin in vitro or on stress fibers of detergent-extracted cells but differed in their effects on actin in stress fibers of microinjected cells. When human gelsolin isolated from plasma was injected into cells in a Ca(++)-containing buffer, stress fibers were degraded, the cellular morphology was changed, and numerous actin patches appeared. These effects were particularly striking when the Ca(++)-insensitive N-terminal proteolytic fragment of this gelsolin was injected. By contrast, Ca(++)-sensitive gelsolins isolated from human platelets, pig stomach smooth muscle and pig plasma showed no comparable activity. Furthermore, the Ca(++)-independent N-terminal proteolytic fragments prepared from these gelsolins also had no effect despite their in vitro actin severing activity. Most striking was the finding that human plasma gelsolin expressed in E. coli did not degrade stress fibers, in contrast to the same protein isolated from plasma; nor was there any stress fiber disruption observed with the N-terminal half of human gelsolin expressed in Escherichia coli. The different behavior of these gelsolins in cells cannot be explained by sequence diversity between plasma and cytoplasmic forms, nor by variability in the Ca++ sensitivity of the preparations. It suggests the presence of factors, as yet unidentified, that may regulate gelsolin activity in the cytoplasm of living cells and discriminate between gelsolins of different origin. Such discrimination could be achieved as a result of post-translational modification of the gelsolin; only in this way can differences between apparently identical proteins isolated from human plasma and expressed in E. coli be reconciled.