Wild-type strains of Dictyostelium discoideum can be transformed using a novel selection cassette driven by the promoter of the ribosomal V18 gene

Almost all methods for transformation of the social ameba Dictyostelium discoideum rely on axenic growth, that is, growth in a synthetic medium, for at least part of the procedure. Axenic growth requires several mutations. Here we describe a procedure that can be used to transform wild-type strains which are able to grow only on the natural food source, bacteria. The method relies on a new selection cassette driven by the V18 promoter, a promoter that we show is substantially more active during growth on bacteria than the actin-6 promoter, which is widely used for axenic transformation. The procedure gives transformation frequencies of about 10(-5) with both strains Ax2 (capable of axenic growth) and NC4 (capable of growth only on bacteria). Using this vector, we have obtained NC4 strains carrying several beta-galactosidase reporter cassettes. Our vector can also be used in axenic transformations.

SEC3 mutations are synthetically lethal with profilin mutations and cause defects in diploid-specific bud-site selection

Replacement of the wild-type yeast profilin gene (PFY1) with a mutated form (pfy1-111) that has codon 72 changed to encode glutamate rather than arginine results in defects similar to, but less severe than, those that result from complete deletion of the profilin gene. We have used a colony color-sectoring assay to identify mutations that cause pfy1-111, but not wild-type, cells to be inviable. These profilin synthetic lethal (psl) mutations result in various degrees of abnormal growth, morphology, and temperature sensitivity in PFY1 cells. We have examined psl1 strains in the most detail. Interestingly, these strains display a diploid-specific defect in bud-site selection; haploid strains bud normally, while homozygous diploid strains show a dramatic increase in random budding. We discovered that PSL1 is the late secretory gene, SEC3, and have found that mutations in several other late secretory genes are also synthetically lethal with pfy1-111. Our results are likely to reflect an interdependence between the actin cytoskeleton and secretory processes in directing cell polarity and growth. Moreover, they indicate that the secretory pathway is especially crucial for maintaining budding polarity in diploids.

Involvement of Rho and Rac small G proteins and Rho GDI in Ca2+-dependent exocytosis from PC12 cells

BACKGROUND

The Rho small G protein family, which includes the Rho, Rac and Cdc42 subfamilies, is implicated in various cell functions such as cell shape change, cell motility and cytokinesis, through the reorganization of actin filaments. Rho GDI is an inhibitory regulator of the Rho small G protein family and inhibits the Rho family dependent cell functions. Reorganization of actin filaments is also known to regulate Ca2+-dependent exocytosis.

RESULTS

We have examined here whether the Rho family members are also involved in Ca2+-dependent exocytosis. We have found, by the use of the human growth hormone (GH) co-expression assay system on PC12 cells, that overexpression of Rho GDI inhibits high K+-induced, Ca2+-dependent GH release. This inhibitory action of Rho GDI is restored by co-expression of a dominant active mutant of RhoA or Rac1, but not of a dominant active mutant of Cdc42. C3 transferase, known to ADP-ribosylate Rho and to inhibit its function, also inhibits this GH release. Overexpression of a dominant active mutant of RhoA or Rac1 alone shows only a small effect on GH release. Moreover, immunocytochemical studies show that the overexpression of Rho GDI prevents a partial disruption of the cortical actin network which accompanies exocytosis.

CONCLUSIONS

These results suggest that RhoA, Rac1 and Rho GDI are involved in Ca2+-dependent exocytosis at least partly through the reorganization of actin filaments, and that the activation of RhoA or Rac1 alone is not sufficient for this reaction.

Proposed modification of the Huxley-Simmons model for myosin head motion along an actin filament

A model is proposed for myosin head motion along an actin filament which accommodates recent experimental data. The model includes three attached states of a myosin head and is thus similar to the classical Huxley & Simmons (1971) model, but differs in that an explicit expression is given for the spatial distribution of potential energy wells for the myosin head. Our model also differs from the classical model, in that it assumes that the proportion of myosin heads attached to actin filament is constant and independent of shortening velocity, as suggested by X-ray diffraction data. Furthermore, it posits that the crossbridge is string-like rather than spring-like. This modified model fits well to the experimental data in the following respects. (1) The calculated tension dependence of muscle stiffness agrees with the observation by Ford et al. (1985 J. Physiol. 361, 131-150). (2) A myosin head under low load can move as far as 60 nm along an actin filament during one ATP hydrolysis cycle in muscle, in agreement with the results by Yanagida et al. (1985 Nature 316, 366-369) and others. (3) The model predicts that such movements consist of a series of elementary steps of 11 nm. (4) A single myosin head hardly moves after the first step of 11 nm under the condition of in vitro experiment carried out by Finer et al. (1994 Nature 368, 113-119), in agreement with their observation. (5) The calculated energy liberation rate reproduces the characteristics of Hill's equation. (6) The "double-hyperbolic force-velocity relation" reported by Edman (1988 J. Physiol. 404, 301-321) can be understood in terms of a potential barrier against movement of a potential well in which a myosin head is trapped.

[A comparative study of the properties of ouabain-sensitive phosphatase in rat erythrocyte ghosts and spectrin-free membranes]

The removal of the membrane skeleton proteins (MSP), chiefly spectrin and actin, from the rat erythrocyte ghosts was shown to result in a decrease of both the total Na, K-ATPase activity and a partial reaction of the enzyme, namely the phosphatasic one. Besides, modulating effects of the effectors promoting the enzyme conformational transitions (ATP and Mg2+) on the ouabain-sensitive K-phosphatase is changed. For instance, a pronounced activation of the K-phosphate in a high-potassium medium in the presence of 1 mM ATP disappeared and the degree of the enzymatic activity enhancement in response to increasing MgCl2 concentrations (from 1.5 to 6 mM) is decreased. The data obtained are discussed from the viewpoint of possible involvement of the erythrocyte MSP in the catalytic of Na, K-ATPase.

[Cancer chemotherapy and apoptosis]

Tumor cells undergo apoptotic cell death when treated with several chemotherapeutic agents. Since these agents, acting on different cellular targets, induce a similar pattern of cell death (apoptosis), it is suggested that a common signaling pathway of apoptosis could exist and that apoptosis resistance could cause a new form of multi-drug resistance in tumor cells. Although the mechanisms of apoptosis are not fully understood, the involvement of ICE/ced-3 family proteases in chemotherapy-induced apoptosis is strongly suggested. Identification of factors directly acting on these apoptosis pathway will offer new strategies in cancer chemotherapy.

In-vitro cell culture models of the nasal epithelium: a comparative histochemical investigation of their suitability for drug transport studies

PURPOSE

To evaluate different in-vitro cell culture models for their suitability to study drug transport through cell monolayers.

METHODS

Bovine turbinate cells (BT; ATCC CRL 1390), human nasal septum tumor cells (RPMI, 2650; ATCC CCL 30), and primary cell cultures of human nasal epithelium were characterized morphologically and histochemically by their lectin binding properties. The development of tight junctions in culture was monitored by actin staining and transepithelial electrical resistance measurements.

RESULTS

The binding pattern of thin-sections of excised human nasal respiratory epithelium was characterized using a pannel of fluorescently-labelled lectins. Mucus in goblet cells was stained by PNA, WGA and SBA, demonstrating the presence of terminal N-acetylglucosamine, N-acetylgalactosamine and galactose residues respectively in the mucus of human nasal cells. Ciliated cells revealed binding sites for N-acetylglucosamine, stained by WGA, whereas Con A, characteristic for mannose moieties, labelled the apical cytoplasm of epithelial cells. Binding sites for DBA were not present in this tissue. Comparing three different cell culture models: BT, RPMI 2650, and human nasal cells in primary culture using three lectins (PNA, WGA, Con A) as well as intracellular actin staining and transepithelial electrical resistance measurements we found, that only human nasal epithelial cells in primary culture showed differentiated epithelial cells, ciliated nasal cells and mucus producing goblet cells, which developed confluent cell monolayers with tight junctions.

CONCLUSIONS

Of the in-vitro cell culture models studied, only human nasal cells in primary culture appears to be suitable for drug transport studies.

Intra-axial endophytic primitive neuroectodermal tumors in the pons: clinical, radiological, and immunohistochemical aspects in four children

We present the clinical findings, radiological aspects, operative results, and histopathological features of four typical primitive neuroectodermal tumors (PNET) located in the pontine region in children. All the tumors had an endophytic intra-axial growth pattern. All the children had a short history of severe neurological deficits with involvement of the cranial nerves and pyramidal tract. MRI did not reveal any common feature of malignancy. Compared to our successful experience in operations of intra-axial endophytic brainstem tumors in a total of 32 children, the outcome was poor: all 4 children died within 13 months. We conclude that PNET occurring in the pons is not as rare as was believed, and, compared to PNET in other areas the prognosis is worse.

Regulation of the actin cytoskeleton, integrins and cell growth by the Rho family of small GTPases

The Rho family of small GTP binding proteins play a key part in regulating the actin cytoskeleton and cell adhesion through integrin receptors. In addition, these proteins regulate signal transduction pathways essential for normal cell growth. Many of the molecules that regulate Rho have oncogenic activity, suggesting that members of the Rho family may have an important role in tumour formation.

Actin-based vesicle dynamics and exocytosis during wound wall formation in characean internodal cells

Characean internodal cells readily form wound walls upon local membrane damage. In the present study we documented the dynamics of vesicles involved in wound wall secretion and compared them with actin organization in equivalent cells using immunofluorescence. Single exocytotic events (spreading of vesicle contents) could be visualized using image enhancement by video microscopy. In control unwounded cells vesicles moved unidirectionally along parallel actin bundles and rarely contacted the plasma membrane. The wound response started with (1) local inhibition of active cytoplasmic streaming (unidirectional movements) due to inactivation, depolymerization, or mechanical displacement of the subcortical actin bundles. Accordingly, vesicles performed only oscillating motions and moved slowly with the same velocity and direction as passive endoplasmic flow. (2) Several minutes after wounding, vesicles started to perform random saltatory movements with frequently changing velocities, punctuated by oscillating motion and periods of immobility (docking) at the plasma membrane. Vesicle trajectories correlated with a fine-meshed actin network at the wound site. (3) Several hours after wounding, vesicles moved again unidirectionally along regenerated subcortical actin bundles. Spreading of vesicles (vesicle contents) was observed during wound wall formation, i.e., during the period of saltatory movements when vesicles had access to the plasma membrane. Dependent on the type of wound wall being secreted, three variants could be distinguished: (1) slow and continuous spreading over a time period of several seconds up to 30 min near the plasma membrane, (2) fast spreading within 80 ms inside an already formed wound wall, and/or (3) fast spreading at the plasma membrane. We conclude from our study that wounding-induced changes in vesicle dynamics are due to transient reorganization of the actin cytoskeleton from parallel bundles to a fine-meshed network. Furthermore, our results indicate that spreading of vesicle contents varies considerably with time and may be delayed by vesicle docking and/or discharge.

Evolution of a dynamic cytoskeleton

Actin filaments and microtubules form the cytoskeleton of all eukaryotic cells, and they are responsible for organizing the cytoplasm and supporting motile processes. Both polymers are highly dynamic, and their polymerization dynamics are central to their organization. Though their evolutionary origins appear to be distinct, actin and tubulin have a similar mechanism for promoting polymerization dynamics in which the energy of nucleotide triphosphate hydrolysis during polymerization is used to weaken the bonds between subunits, thus promoting subsequent depolymerization. The evolutionary origins of actin and tubulin are unclear. It is likely that motile mechanisms driven by reversible polymerization, termed thermal ratchets, are older than those based on ATPase motor proteins. Such mechanisms are still important in modern eukaryotes, and may have powered early versions of the critical motile processes of phagocytosis and chromosome segregation in primitive cells. Thus evolution of dynamic cytoskeletal polymers may have been one of the earliest and most important steps leading to the evolution of eukaryotes. Plausible evolutionary pathways can be constructed leading from simple enzymes to dynamic cytoskeletal polymers.

Force-velocity relations of rat cardiac myosin isozymes sliding on algal cell actin cables in vitro

The difference in kinetic properties between two myosin isozymes (V1 and V3) in rat ventricular myocardium was studied by determining the steady-state force-velocity (P-V) relations in the ATP-dependent movement of V1 and V3-coated polystyrene beads on actin cables of giant algal cells mounted on a centrifuge microscope. The maximum unloaded velocity of bead movement was larger for V1 than for V3. The velocity of bead movement decreased with increasing external load applied by the centrifuge microscope, and eventually reached zero when the load was equal to the maximum isometric force (P0) generated by the myosin heads. The maximum isometric force P0 was less than 10 pN, and did not differ significantly between V1 and V3. The P-V curves consisted of a hyperbolic part in the low force range and a non-hyperbolic part in the high force range. The critical force above which the curve deviated from the hyperbola was much smaller for V1 than for V3. An analysis using a model with an extremely small number of myosin heads involved in the bead movement suggested a marked difference in kinetic properties between V1 and V3.

Immunohistochemical evaluation of the developing outflow tract in the rat: achieving aortic to mitral fibrous continuity

OBJECTIVES

To study the development of aortic to mitral fibrous continuity in the normal rat heart.

METHODS

The hearts and great vessels of normally developed rat embryos and fetuses aged between 13.25 and 19.75 days of gestation were studied in conjunction with those of newborns aged 2 and 7 days post-partum. Standard histological methods and monoclonal antibodies raised against alpha smooth muscle actin (clone 1A4) and ventricular beta myosin heavy chain were used to demonstrate the ventricular outlets, ventriculo-arterial junction, inner heart curve and aortic infundibulum from the early stages of aortopulmonary septation to attainment of their definitive morphology.

RESULTS

The two antibodies demonstrated temporal specificity (actin specificity increased post-partum; myosin specificity maximal during fetal period) in the labelling of their intended structures which correlated with their known developmental profile. Full-thickness fibrous continuity between aortic and mitral valves was not complete until 1 week after birth. After ventricular septation was complete, and thereafter towards the end of fetal life and beyond, separation was maintained by a muscular structure histologically identical to the vestigial netro-aortic root branch of the conduction tissue, a structure known to be derived from the primitive ventricular myocardium within the environs of the inner heart curve.

CONCLUSIONS

Ventricular septation (occurring relatively early) and the attainment of fibrous continuity (occurring relatively late in development) are two independent processes. Muscular tissue separating left-sided arterial and atrioventricular valves is not derived from the aortic infundibulum but from the inner heart curve. Persistence of this structure is a feature of normal rat heart development and needs to be recognised when working with rodent-based animal models of congenital heart disease aimed at studying the disruption of the development of the ventricular outflow tracts.

Effect of bacterial wall lipopolysaccharide (LPS) on morphology, motility, and cytoskeletal organization of microglia in cultures

We examined the effect of bacterial wall lipopolysaccharide (LPS), a strong inflammatory agent, on the morphology, cell motility, cytoskeletal organization, and phagocytic activity of microglia in tissue cultures initiated from neopallia of newborn C3H/OuJ mice. Normally, the microglia in our cultures are non-migratory and Mac-1 positive, have ameboid cell morphology, no polarity, many short processes that extend into lamellipodia in opposing directions, and undulating cell membrane projections. When 1-5 micrograms/ml LPS is added to such cultures, some cells acquire polarity by forming a large lamellipodium and begin to migrate. Two hours later migration ceases; the membrane undulations stop; and the cells become non-polar, assume a large, round, flat shape, and gradually develop many microspikes all over the cell body. Those cells that do not transform into large, round, flat cells enlarge and extend numerous lamellipodia in opposing directions. We found that the cytoskeleton of microglia is composed of actin, vimentin-containing intermediate filaments (IF) and microtubules (MT). Vimentin-containing IF and MT form dense networks that radiate into the cell periphery, whereas F-actin is diffusely arranged throughout the cytoplasm. The LPS-treated cells show changes in the organization of the main components of the cytoskeleton. F-actin is reorganized by the formation of bundles underneath and parallel to the cell membrane and other bundles projecting into the cores of the microspikes. The vimentin-containing IF dense network reorganizes into two condensed rings, with fine strands of IF extended between the two rings and the MT networks become less dense and extend throughout the cytoplasm. The LPS treatment potentiates the phagocytic activity of the microglia. However, approximately 30% of microglia lose the expression of MHC class II antigens.

Acute administration of alpha-methyl-para-tyrosine alters levels of norepinephrine transporter mRNA in the rat brainstem

This study investigated whether rat norepinephrine transporter (NET) mRNA levels would be altered by alpha-methyl-p-tyrosine (alpha-MPT), a tyrosine hydroxylase inhibitor. While NE levels decreased at 1 and 3 days but recovered at 7 days after alpha-MPT, NET mRNA levels decreased at 3 and 7 days but not at 1 day after alpha-MPT. The results indicate that acute treatment with alpha-MPT led to a delayed time response in its effects on NET mRNA and NE levels in the rat brain.

Three-dimensional organization of cytoskeletons in the vestibular sensory cells

The cytoskeletal organization of the guinea pig vestibular sensory cells was investigated employing the saponin perfusion method using scanning electron microscopy as well as immunohistochemical technique. The intermediate filaments were demonstrated in the sensory cells. They were usually seen surrounding the nucleus and extending through the cytoplasm which connected the nuclear membrane to the plasma membrane, cuticular plate or other cytoorganelles. These findings may suggest that the intermediate filaments provide mechanical support to the cell and its nucleus. Microtubules were found mainly in the supranuclear portion of the cells and run parallel to the main axis of the cell body, which is directly connected to the cuticular plate. These findings may suggest that the microtubules provide mechanical support to the cell and may be closely related to the sensory cell transduction system. The actin filaments were densely packed in the site of the cuticular plate. They also distributed throughout the cytoplasm, which appeared as a dense network in the periphery region. These findings support the idea that the actin filaments could be responsible for sensory cell transduction as well as to give mechanical strength to the surface of the cell and enable the cell to change its shape and move.

Caldesmon and low Mr isoform of tropomyosin are localized in neuronal growth cones

Neuronal growth cones move actively, accompanying changes in intracellular Ca2+ concentration. The movement of growth cones may partly depend on the actomyosin system, considering the presence of actin and myosin II. Yet, Ca(2+)-sensitive regulatory proteins for the actomyosin system have not been identified in growth cones. In the present study, caldesmon, an inhibitory protein on actin-myosin interaction, was detected in the growth cone fraction isolated from embryonic rat brain, using immunoblotting with the antibody to chicken gizzard caldesmon. Morphological evidence of caldesmon in growth cones of cultured rat neurons was obtained using the indirect immunofluorescence method. Since inhibition of caldesmon on actin-myosin interaction can be overcome by calmodulin and Ca2+, caldesmon may be involved in the Ca(2+)-dependent regulation in growth cone motility. Tropomyosin is another member of the actomyosin system whose function may be regulated by caldesmon in smooth and nonmuscle cells. A low Mr isoform of tropomyosin was distributed in the growth cone fraction. Using specific antibodies against tropomyosin isoforms, we further clarified morphologically that the low Mr isoform was localized in growth cones, but not the high Mr isoform. High Mr isoforms of tropomyosin were present in nonneuronal cells. Actin filaments in growth cones may be unstable, since low Mr tropomyosin binds to actin filaments with a lower affinity than high Mr isoforms. The instability of actin filaments may be suitable for the rapid movement and shape changes of growth cones.

Immunohistochemical and ultrastructural evidence of a modified microvasculature in the giant cell granuloma of the jaws

A panel of five immunohistochemical markers, MB1, leukocyte common antigen, S-100 protein, smooth muscle specific actin, and factor VIII related antigen, were used to study 20 giant cell lesions. These included eight central giant cell granulomas, nine peripheral giant cell granulomas, and three giant cell tumors of bone. The multinucleated giant cells stained positively with MB1, the mononuclear round cells were positive to leukocyte common antigen and the spindle cells were unreactive to all the markers chosen in all the lesions. The most interesting finding was the staining pattern of the blood vessels to factor VIII related antigen in the giant cell granuloma. The blood vessels on the periphery of the lesions were strongly positive for this antibody. However, reaction product was not evident deeper in the lesion within the aggregations of giant cells. Two other endothelial cell markers, Ulex europaeus 1 lectin and QBend 10 were used to study 10 giant cell lesions and a similar pattern of staining was observed. Transmission electron microscopy was subsequently used to study the ultrastructure of the microvasculature of three peripheral giant cell granulomas, and the findings indicated that the reasons for the differential staining may lie in the differences in the structure of the microcirculation.

Stabilization of neurofilament transcripts during postnatal development

Neurofilament (NF) mRNAs in primary sensory neurons are long-lived transcripts that undergo transcription-dependent destabilization when placed in primary culture [32]. Destabilization of NF transcripts implies that the transcripts are stabilized in high-expressing neurons and that stabilization may coordinate and increase levels of NF expression. The present study examines the stabilities of the three NF subunit mRNAs in postnatal cultures of dorsal root ganglia (DRG) to determine whether increased stability of NF mRNAs could be responsible for the coordinate postnatal upregulation of the three NF subunits [29]. The studies show that the light (NF-L), mid-sized (NF-M) and heavy (NF-H) NF mRNAs are lost at 8 and 16 h in primary cultures from postnatal day 2 (P2) rats, but much less so in cultures from postnatal day 16 (P16) and day 30 (P30) rats. Losses of each NF mRNAs in P2 cultures occurs simultaneously in the presence or absence of actinomycin. The findings support the view that stabilization of NF transcripts contribute to the high and coordinate level NF expression and that components of the stabilizing process are acquired during postnatal development.

Cytoskeletal modulation of the response to mechanical stimulation in human vascular endothelial cells

Possible interactions of cytoskeletal elements with mechanically induced membrane currents and Ca2+ signals were studied in human endothelial cells by using a combined patch-clamp and Fura II technique. For mechanical stimulation, cells were exposed to hypotonic solution (HTS). The concomitant cell swelling activates a Cl- current, releases Ca2+ from intracellular stores and activates Ca2+ influx. To interfere with the cytoskeleton, cells were loaded either with the F-actin-stabilizing agent phalloidin (10 mumol/l), or the F-actin-depolymerizing substance cytochalasin B (50 mumol/l). These were administered either in the bath or the pipette solutions. The tubulin structure of the endothelial cells was modulated by taxol (50 mumol/l), which supports polymerization of tubulin, or by the depolymerizing agent colcemid (10 mumol/l) both applied to the bath. Immunofluorescence experiments show that under the chosen experimental conditions the cytoskeletal modifiers employed disintegrate the F-actin and microtubuli cytoskeleton. Neither of these cytoskeletal modifiers influenced the HTS-induced Cl- current. Ca2+ release was not affected by cytochalasin B, taxol or colcemid, but was suppressed if the cells were loaded with phalloidin. Depletion of intracellular Ca2+ stores by thapsigargin renders the intracellular [Ca2+] sensitive to the extracellular [Ca2+], which is indicative of a Ca2+ entry pathway activated by store depletion. Neither cytochalasin B nor phalloidin affected this Ca2+ entry. We conclude that F-actin turnover or depolymerization is necessary for Ca2+ release by mechanical activation. The tubulin network is not involved. The Ca2+ release- activated Ca2+ entry is not modulated by the F-actin cytoskeleton.

Effect of anisotonic conditions on the transport of hydrophilic model compounds across monolayers of human colonic cell lines

The effect of anisotonic solutions on the enhancement of the transport of hydrophilic model compounds across monolayers of Caco-2 and HT-29.cl19A intestinal epithelial cells was studied. In filter-grown monolayers of the highly differentiated villus-like Caco-2 cell line, a profound and dose-dependent drop in the transepithelial electrical resistance was found after apical treatment with a 30 or a 50% hypotonic solution (200 and 150 mOsmol, respectively). This drop was not observed after basolateral and two-sided application of a 50% hypotonic solution. During apical hypotonic treatment a 12- and 8-fold increase also was observed in transepithelial transport of two hydrophilic model compounds, i.e., fluorescein-Na and fluorescein-isothiocyanate-labeled dextran, MW 4000, respectively. Through confocal laser scanning microscopy, it was revealed that this enhanced transport was predominantly via the paracellular route. Moreover, morphological changes in the cell layers indicating cell swelling were observed after apical hypotonic, but not after basolateral or bilateral treatment, probably resulting from an incomplete regulatory volume decrease response. This swelling, and slight lateral retraction of the cells, allowed the hydrophilic compounds to pass between the cells. The effects of hypotonic challenge also were studied in monolayers of the more crypt cell-like HT-29.cl19A cell line. After apical hypotonic shock, these cells showed no effect on transepithelial electrical resistance, whereas an increase was observed after basolateral and bilateral treatment. Hypotonic shock failed to increase the transport of the hydrophilic model compounds in this cell line.(ABSTRACT TRUNCATED AT 250 WORDS)

Crowding-induced organization of cytoskeletal elements: II. Dissolution of spontaneously formed filament bundles by capping proteins

Through calculations of molecular packing constraints in crowded solutions, we have previously shown that dispersions of filament forming proteins and soluble proteins can be unstable at physiological concentrations, such that tight bundles of filaments are formed spontaneously, in the absence of any accessory binding proteins. Here we consider the modulation of this phenomenon by capping proteins. The theory predicts that, by shortening the average filament length, capping alleviates the packing problem. As a result, the dispersed isotropic solution is stable over an expanded range of compositions.