A 90 kDa fragment of filamin A promotes Casodex-induced growth inhibition in Casodex-resistant androgen receptor positive C4-2 prostate cancer cells

Prostate tumors are initially dependent on androgens for growth, but the majority of patients treated with anti-androgen therapy progress to androgen-independence characterized by resistance to such treatment. This study investigates a novel role for filamin A (FlnA), a 280 kDa cytoskeletal protein (consisting of an actin-binding domain (ABD) followed by 24 sequential repeats), in androgen-independent (AI) growth. Full-length FlnA is cleaved to 170 kDa (ABD+FlnA1-15) and 110 kDa fragments (FlnA16-24); the latter is further cleaved to a 90 kDa fragment (repeats 16-23) capable of nuclear translocation and androgen receptor (AR) binding. Here, we demonstrate that in androgen-dependent LNCaP prostate cancer cells, the cleaved 90 kDa fragment is localized to the nucleus, whereas in its AI subline C4-2, FlnA failed to cleave and remained cytoplasmic. Transfection of FlnA16-24 cDNA in C4-2 cells restored expression and nuclear localization of 90 kDa FlnA. Unlike LNCaP, C4-2 cells proliferate in androgen-reduced medium and in the presence of the AR-antagonist Casodex. They also exhibit increased Akt phosphorylation compared to LNCaP, which may contribute to their AI phenotype. Nuclear expression of 90 kDa FlnA in C4-2 cells decreased Akt phosphorylation, prevented proliferation in androgen-reduced medium and restored Casodex sensitivity. This effect was inhibited by constitutive activation of Akt indicating that FlnA restored Casodex sensitivity in C4-2 cells by decreasing Akt phosphorylation. In addition, FlnA-specific siRNA which depleted FlnA levels, but not control siRNA, induced resistance to Casodex in LNCaP cells. Our results demonstrate that expression of nuclear FlnA is necessary for androgen dependence in these cells.

Effects of profilin and thymosin beta4 on the critical concentration of actin demonstrated in vitro and in cell extracts with a novel direct assay

The free actin concentration at steady state, Ac, is a variable that determines how actin regulatory proteins influence the extent of actin polymerization. We describe a novel method employing fluorescence anisotropy to directly measure Ac in any sample after the addition of a trace amount of labeled thymosin beta4 or thymosin beta4 peptide. Using this assay, we confirm earlier theoretical work on the helical polymerization of actin and confirm the effects of actin filament-stabilizing drugs and capping proteins on Ac, thereby validating the assay. We also confirm a controversial prior observation that profilin lowers the critical concentration of Mg2+-actin. A general mechanism is proposed to explain this effect, and the first quantitative dose-response curve for the effect of profilin on Ac facilitates its evaluation. This mechanism also predicts the effect of profilin on critical concentration in the presence of the limited amount of capping protein, which is the condition often found in cells, and the effect of profilin on critical concentration in cell extracts is demonstrated for the first time. Additionally, nonlinear effects of thymosin beta4 on the steady state amount of F-actin are explained by the observed changes in Ac. This assay has potential in vivo applications that complement those demonstrated in vitro.

Dominant negative mutants of filamin A block cell surface expression of the D2 dopamine receptor

Protein interaction screens have revealed an interaction between the D2 dopamine receptor and the actin cross-linking protein filamin A. However, the physiological significance of this interaction has not been explained. To better understand the role of filamin A in D2 receptor-mediated signaling, we examined the effect of disrupting filamin A/D2 receptor interaction. Overexpression of a truncated form of filamin A (repeat units 18-19 containing the D2, but not the actin, binding domain) caused a marked reduction in both the number and half-life of cell surface D2 receptors. These results suggest that disruption of the linkage between D2 receptors and the actin cytoskeleton destabilizes plasma membrane-associated D2 receptors. Several missense mutations within repeat unit 19 of filamin A were identified that abrogate filamin A/D2 receptor interaction. Introduction of mutant and wild-type filamin A into filamin A-deficient M2 cells demonstrated that wild-type filamin A, but not the filamin A-binding mutants, was able to promote cell-surface expression of D2 receptors. Together, these studies provide evidence that filamin A/D2 receptor interaction is required for the proper targeting or stabilization of D2 dopamine receptors at the plasma membrane.

Comparison of filamin A-induced cross-linking and Arp2/3 complex-mediated branching on the mechanics of actin filaments

We compared the effects of human filamin A (FLNa) and the activated human Arp2/3 complex on mechanical properties of actin filaments. As little as 1 FLNa to 800 polymerizing actin monomers induces a sharp concentration-dependent increase in the apparent viscosity of 24 microm actin, a parameter classically defined as a gel point. The activated Arp2/3 complex, at concentrations up to 1:25 actins had no detectable actin gelation activity, even in the presence of phalloidin, to stabilize actin filaments against debranching. Increasing the activated Arp2/3 complex to actin ratio raises the FLNa concentration required to induce actin gelation, an effect ascribable to Arp2/3-mediated actin nucleation resulting in actin filament length diminution. Time lapse video microscopy of microparticles attached to actin filaments or photoactivation of fluorescence revealed actin filament immobilization by FLNa in contrast to diffusion of Arp2/3-branched actin filaments. The experimental results support theories predicting that polymer branching absent cross-linking does not lead to polymer gelation and are consistent with the observation that cells deficient in actin filament cross-linking activity have unstable surfaces. They suggest complementary roles for actin branching and cross-linking in cellular actin mechanics in vivo.

Androgen receptor nuclear translocation is facilitated by the f-actin cross-linking protein filamin

The human androgen receptor (hAR) is a ligand-dependent transcription factor responsible for the development of the male phenotype. The mechanism whereby nuclear translocation of the hAR is induced by its natural ligand 5alpha-dihydrotestosterone is a phenomenon not fully understood. The two-hybrid interaction trap assay has been used to isolate proteins that interact with the hAR in an attempt to identify molecules involved in hAR transactivation and movement. We have identified the actin-binding protein filamin, a 280-kDa component of the cytoskeleton, as an hAR interacting protein. This interaction is ligand independent but is enhanced in its presence. The functional significance of this interaction was analyzed using a cell line deficient in filamin via transient expression of a green fluorescent protein-hAR chimera. In filamin-deficient cells this revealed that hAR remained cytoplasmic even after prolonged exposure to synthetic ligand. Nuclear shuttling was restored when this cell line regained wild-type expression of filamin. These data suggest a novel role for filamin, implicating it as an important molecule in AR movement from the cytoplasm to the nucleus.

The bi-directional regulation of filamin on the ATPase activity of smooth muscle myosin

OBJECTIVE

The aim of this study is to investigate the functional relationship between filamin, a known actin binding protein, and myosin and the effects of filamin on the interaction between myosin and actin.

METHODS

Ultra-centrifugation method was used to investigate the binding of filamin to both phosphorylated and unphosphorylated myosins. Mg-ATPase activities of both phosphorylated and unphosphorylated myosins in the presence and absence of actin were measured to observe the effects resulted from filamin-actin and filamin-myosin interactions.

RESULTS

It was found that filamin is also a myosin binding protein. Filamin inhibited the actin activated Mg-ATPase activity of phosphorylated myosin and stimulated Mg-ATPase of phosphorylated myosin in the absence of actin; in addition, filamin stimulated Mg-ATPase activity of unphosphorylated myosin in both the presence or absence of actin.

CONCLUSION

The result suggest that the effects of filamin on the myosin Mg-ATPase activities are bi-directional, i.e., stimulatory via binding to myosin and inhibitory via binding to actin.

Actin filament organization is required for proper cAMP-dependent activation of CFTR

Previous studies have indicated a role of the actin cytoskeleton in the regulation of the cystic fibrosis transmembrane conductance regulator (CFTR) ion channel. However, the exact molecular nature of this regulation is still largely unknown. In this report human epithelial CFTR was expressed in human melanoma cells genetically devoid of the filamin homologue actin-cross-linking protein ABP-280 [ABP(-)]. cAMP stimulation of ABP(-) cells or cells genetically rescued with ABP-280 cDNA [ABP(+)] was without effect on whole cell Cl(-) currents. In ABP(-) cells expressing CFTR, cAMP was also without effect on Cl(-) conductance. In contrast, cAMP induced a 10-fold increase in the diphenylamine-2-carboxylate (DPC)-sensitive whole cell Cl(-) currents of ABP(+)/CFTR(+) cells. Further, in cells expressing both CFTR and a truncated form of ABP-280 unable to cross-link actin filaments, cAMP was also without effect on CFTR activation. Dialysis of ABP-280 or filamin through the patch pipette, however, resulted in a DPC-inhibitable increase in the whole cell currents of ABP(-)/CFTR(+) cells. At the single-channel level, protein kinase A plus ATP activated single Cl(-) channels only in excised patches from ABP(+)/CFTR(+) cells. Furthermore, filamin alone also induced Cl(-) channel activity in excised patches of ABP(-)/CFTR(+) cells. The present data indicate that an organized actin cytoskeleton is required for cAMP-dependent activation of CFTR.

A novel anti-inflammatory peptide inhibits endothelial cell cytoskeletal rearrangement, nitric oxide synthase translocation, and paracellular permeability increases

The endothelial cell (EC) membrane-cytoskeletal interface in part maintains plasma membrane integrity and promotes cell-cell apposition. Nonmuscle filamin (ABP-280), an actin crosslinking protein, promotes orthogonal branching of F-actin and is the major protein that links the peripheral actin network to the plasma membrane through its C-terminal glycoprotein binding site. In response to bradykinin, filamin translocates from the cell periphery to the cytosol within 1 min. A synthetic peptide, corresponding to filamin's C-terminal calcium/calmodulin-dependent protein kinase II phosphorylation site (CaM peptide), prevents calcium-activated filamin translocation in permeabilized bovine pulmonary artery EC. The myristoylated permeable form of this peptide inhibits bradykinin-induced filamin translocation and F-actin rearrangement in cultured intact ECs. In addition, bradykinin-induced paracellular gap formation is significantly attenuated by CaM peptide, which suggests that the presence of a filamin-based peripheral F-actin network is essential for maintaining EC barrier function. Moreover, CaM peptide reduces wound-induced EC migration rate by 40%, which indicates that F-actin rearrangement is required for efficient cell motility. The CaM peptide affects other bradykinin-induced inflammatory responses. EC nitric oxide synthase (eNOS) translocates from the cell membrane to the nuclear fraction within 1-2 min of bradykinin treatment. Pretreatment with CaM peptide inhibits eNOS translocation. However, the peptide has no effect on bradykinin-induced von Willebrand Factor release. In summary, the CaM peptide exhibits several anti-inflammatory properties that include maintaining EC junctional stability and inhibiting eNOS translocation.

Contrasting effects of betaxolol and propranolol on Ca(2+)-activated contractions in skinned fibers from canine coronary arteries and ventricular muscles

The effects of some beta-adrenoceptor antagonists (atenolol, betaxolol, bunitrolol, labetalol, pindolol, and propranolol) on Ca2+ (pCa 5.8)-activated tension development in chemically skinned fibers from canine coronary artery and right ventricular trabeculae were studied. In skinned coronary arteries, Ca(2+)-activated tension development was decreased by betaxolol and propranolol at concentration of more than 10(-5) and 10(-4) M, respectively. The pCa-tension relationships were shifted to the right and down by betaxolol. In contrast, in skinned cardiac muscle Ca(2+)-activated tension development was increased by betaxolol and propranolol at the same range of concentrations as in coronary arteries, with no change in maximum tension. The pCa-tension relation was shifted to the left by betaxolol. Other beta-adrenoceptor antagonists (atenolol, bunitrolol, labetalol, and pindolol) had no effect on Ca(2+)-induced contraction in either muscle type. These results indicate that among beta-adrenoceptor antagonists, only betaxolol and propranolol can directly modulate the Ca2+ sensitivity of myofilaments and have opposite effects on the contractile system in canine cardiac and vascular smooth muscle.

cAMP-independent regulation of CFTR by the actin cytoskeleton

Protein kinase A (PKA)-activation of epithelial Na+ channels requires actin filaments. Mouse mammary adenocarcinoma cells expressing the human cystic fibrosis transmembrane conductance regulator (CFTR) or mock transfectants were used to determine whether CFTR is also modulated by the actin cytoskeleton. The actin filament disrupter cytochalasin D (CD; approximately 5 micrograms/ml) readily activated whole cell currents in CFTR but not in mock-transfected (MOCK) cells. Addition of actin to the cytosolic side of quiescent excised inside-out patches of CFTR but not MOCK cells also activated CFTR. The actin-activated Cl- channels (symmetrical Cl-) had a linear conductance of 9.3 pS and were inhibited by diphenylamine-2-carboxylate and monoclonal antibodies raised against CFTR. Channel activity was also blocked by addition of the actin-binding proteins deoxyribonuclease I and filamin. Incubation of CFTR cells with CD (approximately 15 micrograms/ml) for > 6 h prevented CFTR activation by the addition of either 8-bromoadenosine 3',5'-cyclic monophosphate plus forskolin under whole cell conditions or PKA under excised inside-out conditions. However, CFTR activation was restored by subsequent addition of actin. The data indicate that CFTR is regulated by actin filaments whose effect may, in turn, be associated with the PKA-dependent pathway.

Binding of divalent cation and nucleotide to G-actin in the presence of profilin

The effect of profilin, a G-actin binding protein, on the mechanism of exchange of the tightly bound metal ion and nucleotide on G-actin, has been investigated. 1) In low ionic strength buffer, profilin increases the rates of Ca2+ and Mg2+ dissociation from G-actin 250- and 50-fold, respectively. On the profilin-actin complex as well as on G-actin alone, nucleotide exchange is dependent on the concentration of divalent metal ion and is kinetically limited, at low concentration of metal ion, by the dissociation of the metal ion. 2) Under physiological ionic conditions, nucleotide exchange on G-actin is 1 order of magnitude faster than at low ionic strength. The rate of MgATP dissociation is increased by profilin from 0.05 s-1 to 2 s-1, the rate of MgADP dissociation is increased from 0.2 s-1 to 24 s-1. The dependences of the exchange rates on profilin concentration are consistent with a high affinity (5 x 10(6) to 10(7) M-1) of profilin for ATP-G-actin, and a 20-fold lower affinity for ADP-G-actin. Profilin binding to actin lowers the affinity of metal-nucleotide by about 1 order of magnitude. These results restrain the possible roles of profilin in actin assembly in vivo.

Filamin and gelsolin influence Ca(2+)-sensitivity of smooth muscle thin filaments

Sheep aorta thin filaments were prepared by ultracentrifugation of an ATP-containing extract in the presence of different concentrations of ethanediol. Thin filaments prepared without ethanediol contained small quantities of tropomyosin (0.027 Tm:actin) and caldesmon (0.017 CD:actin) and activated the MgATPase of skeletal myosin independently of Ca2+. Ultracentrifugation in the presence of 10-20% ethanediol resulted in preparation of thin filaments with increased content of tropomyosin (0.17 Tm:actin) and caldesmon (0.04 CD:actin). These thin filaments possessed high Ca(2+)-sensitivity in activation of skeletal muscle myosin ATPase. Besides actin, tropomyosin and caldesmon, thin filaments contained gelsolin and filamin. Gelsolin content (0.007 gelsolin:actin) was independent of the presence of ethanediol. The filamin content decreased from 0.015 to 0.007 mol:mol actin when the ethanediol concentration was increased from 0 to 20%, and was negatively correlated with the Ca2+ sensitivity of thin filaments. In a reconstituted system, pure filamin or gelsolin affected caldesmon regulation of actomyosin ATPase. Gelsolin (0.01:actin) reduced the inhibition of actomyosin ATPase caused by caldesmon and increased the potency of Ca(2+)-calmodulin in reversing this inhibition. Filamin (0.007:actin) also decreased the inhibitory action of caldesmon on actin-activated myosin ATPase and also potentiated the reversal of this inhibition by calmodulin. We conclude that minor components of smooth muscle thin filaments (gelsolin and filamin) significantly modify caldesmon mediated regulation of actomyosin ATPase. We suggest a tropomyosin-mediated mechanism by which filamin or gelsolin could exert similar effects.

Modulation of the interaction between G-actin and thymosin beta 4 by the ATP/ADP ratio: possible implication in the regulation of actin dynamics

The interaction of G-actin with thymosin beta 4 (T beta 4), the major G-actin-sequestering protein in motile and proliferating cells, has been analyzed in vitro. T beta 4 is found to have a 50-fold higher affinity for MgATP-actin than for MgADP-actin. These results imply that in resting platelets and neutrophils, actin is sequestered by T beta 4 as MgATP-G-actin. Kinetic experiments and theoretical calculations demonstrate that this ATP/ADP dependence of T beta 4 affinity for G-actin can generate a mechanism of desequestration of G-actin by ADP, in the presence of physiological concentrations of T beta 4 (approximately 0.1 mM). The desequestration of G-actin by ADP is kinetically enhanced by profilin, which accelerates the dissociation of ATP from G-actin. Whether a local drop in the ATP/ADP ratio can allow local, transient desequestration and polymerization of actin either close to the plasma membrane, following platelet or neutrophil stimulation, or behind the Listeria bacterium in the host cell, while the surrounding cytoplasm contains sequestered ATP-G-actin, is an open issue raised by the present work.

Effects of pH and inorganic phosphate on force production in alpha-toxin-permeabilized isolated rat uterine smooth muscle

1. Strips of longitudinal smooth muscle isolated from rat uterus were permeabilized using crude alpha-toxin from the bacterium Staphylococcus aureus. This treatment rendered the surface membrane permeable to small molecular weight substances. Simultaneous measurements of tension and calcium concentration ([Ca2+]) (using indo-1 fluorescence) were used to investigate the effects of pH and inorganic phosphate concentration ([Pi]) on Ca(2+)-activated force generated by the contractile proteins. 2. Raising the [Pi] from 1 to 11 mM at a pH of 7.2 depressed both maximal and submaximal Ca(2+)-activated force. This effect of Pi was concentration dependent having the majority of its effect by 6 mM. 3. Further experiments at a submaximal [Ca2+] showed that Ca(2+)-activated force was enhanced by raising [Pi] from 6 to 11 mM suggesting that Pi increased the Ca2+ sensitivity of tension production. Based on these results, calculations indicate that the apparent affinity constant of Ca2+ for the contractile proteins increased from 4 x 10(6) M-1 to 6 x 10(6) M-1 on raising [Pi] from 1 to 11 mM. 4. Lowering pH from 7.2 to 6.7 at a [Pi] of 1 mM potentiated Ca(2+)-activated force with a small depression in the apparent Ca2+ sensitivity of tension production. This effect of pH on maximum (100 microM Ca2+) and submaximum (0.3 microM Ca2+) Ca(2+)-activated force was observed over a range of acidic pHs (7.0-6.7). 5. Increasing pH from 7.2 to 7.7 at a [Pi] of 1 mM depressed Ca(2+)-activated force with no effect on Ca2+ sensitivity of tension production. 6. Spontaneous contractions in intact rat myometrium are abolished under hypoxic conditions. Under these same conditions intracellular [Pi] rises and pH falls. The results of this study suggest that taken individually neither the effect of a rise in [Pi] nor a fall in pH on Ca(2+)-activated force generated by the contractile proteins can account for the effect of hypoxia on spontaneous contractions.

Effects of profilin and profilactin on actin structure and function in living cells

Previous studies have yielded conflicting results concerning the physiological role of profilin, a 12-15-kD actin- and phosphoinositide-binding protein, as a regulator of actin polymerization. We have addressed this question by directly microinjecting mammalian profilins, prepared either from an E. coli expression system or from bovine brain, into living normal rat kidney (NRK) cells. The microinjection causes a dose-dependent decrease in F-actin content, as indicated by staining with fluorescent phalloidin, and a dramatic reduction of actin and alpha-actinin along stress fibers. In addition, it has a strong inhibitory effect toward the extension of lamellipodia. However, the injection of profilin causes no detectable perturbation to the cell-substrate focal contact and no apparent depolymerization of filaments in either the nonlamellipodial circumferential band or the contractile ring of dividing cells. Furthermore, cytokinesis of injected cells occurs normally as in control cells. In contrast to pure profilin, high-affinity profilin-actin complexes from brain induce an increase in total cellular F-actin content and an enhanced ruffling activity, suggesting that the complex may dissociate readily in the cell and that there may be multiple states of profilin that differ in their ability to bind or release actin molecules. Our results indicate that profilin and profilactin can function as effective regulators for at least a subset of actin filaments in living cells.

Actin filaments regulate epithelial Na+ channel activity

The functional role of the cytoskeleton in the control of ion channel activity is unknown. In the present study, immunocolocalization of Na+ channels with specific antibodies and fluorescein isothiocyanate-phalloidin to stain the cortical cytoskeleton indicates that actin is always present in close proximity to apical Na+ channels in A6 cells. The patch-clamp technique was used to assess the effect of cortical actin networks on apical Na+ channels in these A6 epithelial cells. The actin filament disrupter, cytochalasin D (5 micrograms/ml), induced Na+ channel activity in cell-attached patches within 5 min of addition. Cytochalasin D also induced and/or increased Na+ channel activity in 90% of excised patches tested within 2 min. Addition of short actin filaments (greater than 5 microM) to excised patches also induced channel activity. This effect was enhanced by addition of ATP and/or cytochalasin D. The effect of actin on Na+ channel activity was reversed by addition of the G actin-binding protein DNase I or completely prevented by treatment of the excised patches with this enzyme. Addition of the actin-binding protein, filamin, reversibly inhibited both spontaneous and actin-induced Na+ channels. Thus actin filament networks, achieved by either depolymerizing endogenous actin filaments by treatment with cytochalasin D, the addition of exogenous short actin filaments plus ATP, or actin plus cytochalasin D, regulate apical Na+ channel activity. This conclusion was supported by the observation that the addition of short actin filaments in the form of actin-gelsolin complexes in molar ratios less than 8:1 was also effective in activating Na+ channels. We have thus demonstrated a functional role for the cortical actin network in the regulation of epithelial Na+ channels that may complement a structural role for membrane protein targetting and assembly.

Calpain abolishes the effect of filamin on the actomyosin system in platelets

Platelet filamin was shown to cross-link F-actin and inhibit actomyosin ATPase activity. Filamin was also shown to be degraded by calpain (calcium-activated neutral proteinase; CANP) when the platelet was activated. The consequences of the proteolysis of filamin on the actomyosin system have been investigated. When degraded by calpain in the presence of Ca2+, filamin loses its ability to cross-link F-actin. Under the same conditions, its inhibitory effects on the superprecipitation and ATPase activity of actomyosin are abolished. The result suggests that the degradation of filamin is favorable for contraction of the activated platelets.

Filamin inhibits actomyosin ATPase activity in platelet

Filamin, an actin cross-linker protein, has been shown to exist in platelet. The role of this protein in the platelet has remained unclear. In this report, we show that filamin inhibits the actin-activated Mg2+ -ATPase activity of platelet myosin. The activation caused by platelet actin is inhibited by 50% at the molar ratio of filamin to actin of 1/50. Platelet tropomyosin, which we showed to enhance the ATPase activity, does not abolish the effect of filamin. The results support the view that filamin stabilizes the actin network in the resting platelet.

Dual effect of filamin on actomyosin ATPase activity

Filamin binds to F-actin and influences the myosin-actin interaction. At relatively low concentrations, filamin activates actomyosin Mg2+-ATPase, whereas higher concentrations of filamin exert an inhibitory effect. Activation of ATPase activity occurs under conditions where a loose meshwork of actin filaments is present and inhibition is associated with the appearance of closely apposed bundles of actin filaments. Maximum activation (about fourfold) of actomyosin ATPase activity by filamin occurs between 30 and 65 mM KCl, at pH 6.5, and at temperatures not less than 30 degrees C. ATPase activation requires higher concentrations of filamin in the presence than in the absence of tropomyosin. Filamin does not activate Mg2+-ATPase activity of acto-subfragment-1 and has only a slight effect on the Mg2+-ATPase of acto-heavy meromyosin, but it inhibits the activity of both these systems under conditions similar to those that inhibit actomyosin ATPase activity.

Potentiation of actomyosin ATPase activity by filamin

It was found that thin filaments from chicken gizzard muscle activate skeletal muscle myosin Mg2+-ATPase to a greater extent than does the complex of chicken gizzard actin and tropomyosin. The protein factor responsible for this additional activation has been now identified as the high Mr actin binding protein, filamin.

Actin from Thyone sperm assembles on only one end of an actin filament: a behavior regulated by profilin

Thyone sperm were demembranated with Triton X-100 and, after washing, extracted with 30 mM Tris at pH 8.0 and 1 mM MgCl2. After the insoluble contaminants were removed by centrifugation, the sperm extract was warmed to 22 degrees C. Actin filaments rapidly assembled and aggregated into bundles when KCl was added to the extract. When we added preformed actin filaments, i.e., the acrosomal filament bundles of Limulus sperm, to the extract, the actin monomers rapidly assembled on these filaments. What was unexpected was that assembly took place on only one end of the bundle--the end corresponding to the preferred end for monomer addition. We showed that the absence of growth on the nonpreferred end was not due to the presence of a capper because exogenously added actin readily assembled on both ends. We also analyzed the sperm extract by SDS gel electrophoresis. Two major proteins were present in a 1:1 molar ratio: actin and a 12,500-dalton protein whose apparent isoelectric point was 8.4. The 12,500-dalton protein was purified by DEAE chromatography. We concluded that it is profilin because of its size, isoelectric point, molar ratio to actin, inability to bind to DEAE, and its effect on actin assembly. When profilin was added to actin in the presence of Limulus bundles, addition of monomers on the nonpreferred end of the bundle was inhibited, even though actin by itself assembled on both ends. Using the Limulus bundles as nuclei, we determined the critical concentration for assembly off each end of the filament and estimated the Kd for the profilin-actin complex (approximately 10 microM). We present a model to explain how profilin may regulate the extension of the Thyone acrosomal process in vivo: The profilin-actin complex can add to only the preferred end of the filament bundle. Once the actin monomer is bound to the filament, the profilin is released, and is available to bind to additional actin monomers. This mechanism accounts for the rapid rate of filament elongation in the acrosomal process in vivo.

Interaction of rat muscle AMP deaminase with myosin. II. Modification of the kinetic and regulatory properties of rat muscle AMP deaminase by myosin

The problems of whether the kinetic and regulatory properties of AMP deaminase were modified by formation of a deaminase-myosin complex were investigated with an enzyme preparation from rat skeletal muscle. Results showed that AMP deaminase was activated by binding to myosin. Myosin-bound AMP deaminase showed a sigmoidal activity curve with respect to AMP concentration in the absence of ATP and ADP, but a hyperbolic curve in their presence. Addition of ATP and ADP doubled the V value, but did not affect the Km value. Myosin-bound AMP deaminase also gave a sigmoidal curve in the presence of alkali metal ions, whereas free AMP deaminase gave a hyperbolic curve. GTP abolished the activating effects of both myosin and ATP.