gCap39, a calcium ion- and polyphosphoinositide-regulated actin capping protein

Inositol phospholipid-induced suppression of F-actin-gelating activity of smooth muscle filamin

Filamin, a high molecular weight actin-binding protein, cross-links actin filaments and produces a gel composed of F-actin. The effects of polyphosphoinositides on the gelating activity of smooth muscle filamin were examined by measuring the low shear viscosity of the F-actin solutions containing filamin incubated with phosphatidylinositol (PI), phosphatidylinositol 4-monophosphate (PIP), or phosphatidylinositol 4,5-bisphosphate (PIP2). Micelles of these inositol phospholipids bound to filamin inhibited the ability to form a gel of F-actin. The inhibiting activity of each phospholipid was in the following order, PIP2 greater than PIP greater than PI. The F-actin binding assay of filamin revealed that the inhibition of F-actin-gelation resulted in the loss of the F-actin-binding activity of filamin. Thus, polyphosphoinositides may play important roles in regulating the gelating activity of filamin.

Are the conserved sequences in segment 1 of gelsolin important for binding actin?

The minimal region required for actin binding in the smallest of the three domains of gelsolin (termed Segment 1 or S1) was previously defined by deletion mutagenesis as residues 37-126. Further analysis of NH2-terminal deletions here redefines the minimal functional core as residues 41-126. Amino acid substitutions within this core further elucidate the nature of the interaction of segment 1 with actin. Of 26 point mutants analyzed, 14 reduced the affinity for actin. The charged residues His 119, Arg 120, Glu 121, and Gln 123 appear to be involved in direct interaction with actin. Substitutions of Leu 108, Leu 112, and Val 117 by polar groups all affect the structural stability of segment 1 and thereby reduce binding affinity. In addition replacement of Glu 126 by aspartic acid modifies the physical properties of segment 1 and weakens binding. We have further shown that changing charged residues within the highly conserved pentapeptide sequence LDDYL (residues 108-112) has no effect on actin binding. This sequence, found in a number of different actin binding proteins, does not therefore constitute part of the interaction site. Similarly, substitution of the two acidic residues by basic ones within the DESG motif of segment 1 (residues 96-99, but also found near the COOH terminus of actin) does not impair binding. These results show the dangers of predicting functional sites on the basis of conserved sequences.

Structural relationships of actin-binding proteins

The sequences of a large number of actin-binding proteins have been compared. These findings, together with the results of protein-chemical analysis, peptide synthesis and site-directed and deletion mutagenesis, have led to the assignment of actin-binding sites. Within these segments, small actin-binding motifs have been delineated. Most actin-binding proteins interact with actin subdomain-1 but our analyses reveal neither primary nor secondary structure homology among these proteins, suggesting that actin binding does not follow simple structural principles.

Cap100, a novel phosphatidylinositol 4,5-bisphosphate-regulated protein that caps actin filaments but does not nucleate actin assembly

The fast and transient polymerization of actin in nonmuscle cells after stimulation with chemoattractants requires strong nucleation activities but also components that inhibit this process in resting cells. In this paper, we describe the purification and characterization of a new actin-binding protein from Dictyostelium discoideum that exhibited strong F-actin capping activity but did not nucleate actin assembly independently of the Ca2+ concentration. These properties led at physiological salt conditions to an inhibition of actin polymerization at a molar ratio of capping protein to actin below 1:1,000. The protein is a monomer, with a molecular mass of approximately 100 kDa, and is present in growing and in developing amoebae. Based on its F-actin capping function and its apparent molecular weight, we designated this monomeric protein cap100. As shown by dilution-induced depolymerization and by elongation assays, cap100 capped the barbed ends of actin filaments and did not sever F-actin. In agreement with its capping activity, cap100 increased the critical concentration for actin polymerization. In excitation or emission scans of pyrene-labeled G-actin, the fluorescence was increased in the presence of cap100. This suggests a G-actin binding activity for cap100. The capping activity could be completely inhibited by phosphatidylinositol 4,5-bisphosphate (PIP2), and bound cap100 could be removed by PIP2. The inhibition by phosphatidylinositol and the Ca(2+)-independent down-regulation of spontaneous actin polymerization indicate that cap100 plays a role in balancing the G- and F-actin pools of a resting cell. In the cytoplasm, the equilibrium would be shifted towards G-actin, but, below the membrane where F-actin is required, this activity would be inhibited by PIP2.

Direct observation of actin filament severing by gelsolin and binding by gCap39 and CapZ

Dynamic behavior of actin filaments in cells is the basis of many different cellular activities. Remodeling of the actin filament network involves polymerization and depolymerization of the filaments. Proteins that regulate these behaviors include proteins that sever and/or cap actin filaments. This report presents direct observation of severing of fluorescently-labeled actin filaments. Coverslips coated with gelsolin, a multi-domain, calcium-dependent capping and severing protein, bound rhodamine-phalloidin-saturated filaments along their length in the presence of EGTA. Upon addition of calcium, attached filaments bent as they broke. Actophorin, a low molecular weight, monomer sequestering, calcium-independent severing protein did not sever phalloidin-saturated filaments. Both gCap 39, a gelsolin-like, calcium-dependent capping protein that does not sever filaments, and CapZ, a heterodimeric, non-calcium-dependent capping protein, bound the filaments by one end to the coverslip. Visualization of individual filaments also revealed severing activity present in mixtures of actin-binding proteins isolated by filamentous actin affinity chromatography from early Drosophila embryos. This activity was different from either gelsolin or actophorin because it was not inhibited by phalloidin, but was calcium independent. The results of these studies provide new information about the molecular mechanisms of severing and capping by well-characterized proteins as well as definition of a novel type of severing activity.

Inhibition of tumor cell invasion by verapamil

Verapamil, a calcium channel antagonist, inhibits murine B16 melanoma and colon adenocarcinoma C26 tumor metastasis by altering platelet aggregation [Tsuruo, T., et al. (1985) Cancer Chemother. Pharmacol., 14:30-33]. However, the role of calcium homeostasis in regulating several biochemical pathways implicated in other steps of the metastatic cascade suggests that calcium channel antagonists could also inhibit metastasis by other mechanisms. In this report, non-toxic doses of verapamil reversibly decreased human A375M and C8161 melanoma cell invasion and metastasis in a dose-dependent manner. Verapamil reduced cellular invasion and metastases by up to 96% (range 78-96%). Concomitantly, verapamil disrupts microtubule and microfilament organization and inhibits unidirectional cell migration but does not affect cellular adhesion to endothelial monolayers or reconstituted basement membranes. In addition, tumor cells treated with verapamil have a decrease in mRNA of type IV collagenase, a proteinase important in tumor cell degradation of basement membranes. Collectively, these data offer additional evidence regarding the mechanisms of action of verapamil as an anti-metastatic agent.

Effects of streptomycin on development of the apical structures of hair cells in the chick basilar papilla

A single dose of streptomycin (166 mg/kg egg weight) was given to chick embryos 7-15 days after the beginning of incubation. Embryos were fixed 4-12 days later, and the hair cells examined by scanning and transmission electron microscopy. The highest proportion of abnormalities was produced by injections on or before the 11th day of incubation, later injections affecting a substantially smaller proportion of embryos. This suggests the possibility of a critical period for streptomycin ototoxicity in the chick. In addition to the normal signs of cellular degeneration, the most striking abnormality was a massive expansion in the apical surface, sometimes by a factor of 20, in area. The organisation of stereocilia was also commonly affected. The stereocilia could be broken up into multiple small separate bundles, and often there was a wide separation between the kinocilium and the stereocilia. Stereocilia tended to be reduced in number, fused, and either of abnormally large, small, or irregular diameters. Structures with the appearance of stereociliar cores often lay horizontally within the surface of the cuticular plate, sometimes running for 15 microns or more. Sometimes abnormal 'stereocilia' were expressed around the extreme margins of the cuticular plate. In addition, adjacent hair cells could show very different developmental stages, as though the development of some cells had been arrested. With all these changes, the short hair cells in the centre of a papillar cross-section tended to be the most affected, with the tall hair cells and the short hair cells on the extreme inferior (i.e. abneural) edge being least affected. It is suggested that the streptomycin alters the balance of the different aspects of development of the hair cells. It might therefore be possible to use ototoxicity as a way of analysing hair-cell development.

Phosphoinositide kinase, diacylglycerol kinase, and phospholipase C activities associated to the cytoskeleton: effect of epidermal growth factor

In this paper we demonstrate that cytoskeletons isolated from A431 cells have associated with them high activities of several kinases involved in inositol lipid metabolism, such as phosphatidylinositol kinase, phosphatidylinositol phosphate kinase, and diacylglycerol kinase. In addition also phospholipase C activity was detected on isolated cytoskeletons. Controlled extraction of the cytoskeletons followed by in vitro polymerization of actin demonstrated an association of the kinases to the actin filament system consisting of actin and a number of actin-binding proteins. The cytoskeleton-associated lipid kinase activities were significantly increased upon treatment of intact cells with EGF. These data suggest that the association of the phosphoinositide kinases, diacylglycerol kinase, phospholipase C, and also the EGF receptor to the cytoskeleton may play a role in the efficient signal transduction induced by EGF, by providing a matrix for the various components involved in signal transduction.

The role of solation-contraction coupling in regulating stress fiber dynamics in nonmuscle cells

Serum-deprived Swiss 3T3 fibroblasts constitutively form stress fibers at their edges. These fibers move centripetally towards the perinuclear region where they disassemble. Serum stimulation causes shortening of fibers in a manner suggesting active actin-myosin-based contraction (Giuliano, K.A. and D.L. Taylor. 1990. Cell Motil. and Cytoskeleton. 16:14-21). To elucidated the role of actin-based gel structure in these movements, we examined the effects of disrupting actin organization with cytochalasin. Serum-deprived fibroblasts were microinjected with rhodamine analogs of actin or myosin II and fiber dynamics were monitored with a multimode light microscope workstation using video-enhanced contrast and fluorescence modes. When cells were perfused with greater than or equal to 3 microM cytochalasin B or 0.5 microM cytochalasin D, formation and transport of stress fibers were reversibly inhibited, and rapid and immediate shortening of existing fibers was induced. Quantification of actin and myosin II fluorescence associated with individual shortening fibers demonstrated that fluorescence per length of fiber increased for both components, suggesting sliding filament contraction. However, there was also a net loss of both actin and myosin II from fibers as they shortened, indicating a self-destructive process. Loss of material from fibers coupled with increased overlap of actin and myosin II remaining in the fibers suggested that contraction could be induced not only by increasing the force exerted by contractile motors, but also by decreasing gel structure through partial solation. Finally, cytochalasin accelerated contraction of actin-myosin-based gels reconstituted from purified proteins in the absence of myosin-based regulation, further supporting solation-contraction coupling as a possible mechanism for modulating cytoplasmic contractility (Taylor, D.L. and M. Fechheimer. 1982. Philos. Trans. R. Soc. Lond. B. Biol. Sci. 299:185-197).

The role of phospholipases and phospholipid-derived signals in cell activation

The complexity of receptor-regulated breakdown and modification of phospholipids continues to grow. New developments extend our concepts of signalling enzymes and possible messengers.

Actin-binding proteins

Much new information on the sequence, structure, and function of filament crosslinking, capping, and severing proteins is now known. Other significant findings include identification of a new abundant monomer-sequestering protein in platelets, and evidence that many actin-binding proteins interact with phosphoinositides and that this interaction may have metabolic consequences.