Actin-membrane interaction in fibroblasts: what proteins are involved in this association?

In this review we discuss some of the proteins for which a role in linking actin to the fibroblast plasma membrane has been suggested. We focus on the family of proteins related to erythrocyte spectrin, proteins that have generally been viewed as having an organization and a function in actin-membrane attachment similar to those of erythrocyte spectrin. Experiments in which we precipitated the nonerythrocyte spectrin within living fibroblasts have led us to question this supposed similarity of organization and function of the nonerythrocyte and erythrocyte spectrins. Intracellular precipitation of fibroblast spectrin does not affect the integrity of the major actin-containing structures, the stress fiber microfilament bundles. Unexpectedly, however, we found that the precipitation of spectrin results in a condensation and altered distribution of the vimentin class of intermediate filaments in most cells examined. Although fibroblast spectrin may have a role in the attachment of some of the cortical, submembranous actin, it is surprising how little the intracellular immunoprecipitation of the spectrin affects the cells. Several proteins have been found concentrated at the ends of stress fibers, where the actin filaments terminate at focal contacts. Two of these proteins, alpha-actinin and fimbrin, have properties that suggest that they are not involved in the attachment of the ends of the bundles to the membrane but are more probably involved in the organization and cross-linking of the filaments within the bundles. On the other hand, vinculin and talin are two proteins that interact with each other and may form part of a chain of attachments between the ends of the microfilament bundles and the focal contact membrane. Their role in this attachment, however, has not been established and further work is needed to examine their interaction with actin and to identify any other components with which they may interact, particularly in the plasma membrane.

An interaction between vinculin and talin

In cultured fibroblasts, microfilament bundles terminate at adhesion plaques (focal contacts), the specialized regions where the cells adhere most tightly to the underlying substrate. Vinculin is a protein concentrated in adhesion plaques and has been suggested as a possible link between the ends of the bundles of actin filaments and the plasma membrane. If vinculin is one protein in a chain of attachment between the bundles of microfilaments and the plasma membrane, it is important to identify other components which interact with vinculin. We have recently discovered a new protein in adhesion plaques which we refer to as talin. Here we show that talin binds to vinculin, which suggests that talin may be involved with vinculin in the attachment of microfilament bundles to the plasma membrane at the adhesion plaques.

Immunoprecipitation of nonerythrocyte spectrin within live cells following microinjection of specific antibodies: relation to cytoskeletal structures

The intracellular precipitation of nonerythrocyte spectrin has been achieved by the microinjection into cells of either a monoclonal antibody (IgM) directed against the alpha chain of nonerythrocyte spectrin or an affinity-purified polyclonal antibody raised against bovine brain spectrin (fodrin). This antibody-induced precipitation of spectrin was observed in fibroblastic and epithelial cell types, including embryonic bovine tracheal fibroblasts, a bovine kidney epithelial cell line (MDBK), Hela cells, gerbil fibroma cells, and fibroblast lines of human and mouse origins. The precipitation of the spectrin was specific and two proteins with a similar distribution to the nonerythrocyte spectrin were not induced to co-precipitate in the spectrin aggregates. Comparing the two types of antibody microinjected, the affinity-purified polyclonal antibody resulted in more compact aggregates of spectrin and these were frequently aligned with microfilament bundles. The rate at which the spectrin aggregates were cleared into presumptive lysosomes varied with different cell types: in some such as the bovine kidney epithelial cells, this appeared complete within 3 h after microinjection, whereas in some of the fibroblasts the spectrin aggregates were prominent in the cytoplasm at 24 and even 48 h after microinjection. Microfilament bundles appeared unaffected by the aggregation of spectrin. We conclude that the integrity of the actin microfilament bundles does not require nonerythrocyte spectrin and that most probably these structures are linked at their termini to the membrane through proteins other than nonerythrocyte spectrin. No effect of the intracellular spectrin precipitation was observed on cell shape, or on the distribution of coated vesicles or microtubules. The aggregation of the nonerythrocyte spectrin, however, did affect the distribution of the vimentin type of intermediate filaments in most of the cell types studied. These filaments became more distorted and condensed, but generally did not collapse around the nucleus as occurs following microtubule disruption induced by colchicine treatment. The clumped intermediate filaments were frequently seen to coincide with regions of aggregated spectrin. This aggregation of intermediate filaments was not induced by microinjection of irrelevant antibodies, nor was it induced by the monoclonal antibody against spectrin in cells with which it did not cross-react.(ABSTRACT TRUNCATED AT 400 WORDS)

Visualization of the exocytosis/endocytosis secretory cycle in cultured adrenal chromaffin cells

Cultured bovine adrenal medullary chromaffin cells were stimulated to secrete catecholamines by addition of veratridine or nicotine. The formation of an exocytotic pit exposes a major secretory granule membrane antigen, the enzyme dopamine beta-hydroxylase, to the external medium. By including antiserum to this enzyme in the medium, we were able to visualize sites of exocytosis by decoration of bound antibody using a fluorescent second antibody. Internalization of this antibody-antigen complex was then followed in chase experiments: approximately half the surface complex was internalized in 15-30 min. In other experiments, secretion was triggered in the absence of antiserum, and surface enzyme was revealed by binding antibodies at various times after secretion had been halted by an antagonist. Surface patches of antigen remained discrete from the bulk of the plasma membrane for at least 30 min, although a substantial proportion of the antigen was internalized within this time. Cell surface concanavalin A receptors were internalized at a roughly similar rate, suggesting that mechanisms may be similar. After internalization, chromaffin granule membranes fused to larger structures, possibly lysosomes, and were transported over a few hours to the perinuclear region of the cell.

Cytoskeletal proteins in cultured secretory cells from rat pituitary

Cultured cells from female rat pituitaries were examined by immunofluorescence microscopy for the presence of pituitary hormones and for cytoskeletal proteins. The cells attach well to glass coverslips and, when well-dispersed initially, develop networks of branching processes. Their cytoplasm appears granular and shows granular fluorescence when the cells are permeabilized and treated with antisera to pituitary hormones. The cultures contain fibroblasts which are morphologically distinct and much larger than the secretory cells. Consequently, they provide an internal control for cytoskeletal proteins in nonsecretory cells. Fibroblasts have relatively prominent stress fibres. By contrast, the secretory cells have a diffuse cytoplasmic distribution of all cytoskeletal proteins investigated. One cell type, the gonadotrope, which secretes luteinizing hormone and follicle-stimulating hormone, was examined in particular detail.

A new protein of adhesion plaques and ruffling membranes

A protein with a molecular weight on SDS polyacrylamide gels of 215,000 (referred to here as 215K) was purified from chicken gizzard smooth muscle. Antibodies against this protein localized it in fibroblasts to adhesion plaques (focal contacts), to regions underlying cell surface fibronectin, and to ruffling membranes. In the first two distributions it was similar to vinculin in cellular location, and this was confirmed by double-label immunofluorescence microscopy, but the concentration of 215K in membrane ruffles distinguished it from vinculin. There was no cross-reaction of the antibody against 215K with vinculin, and immunoprecipitation and antibody staining of SDS gels of whole cells revealed a single cross-reactive component with a molecular weight of 215,000. Immunoprecipitation from cultures labeled with [32P]phosphate revealed 215K to be a phosphoprotein. Transformation of rat or chicken fibroblasts by Rous sarcoma virus resulted in a reorganization of 215K, in some cases into complex intracellular structures. The localization of 215K where microfilament bundles terminate as well as in close relation to cell surface fibronectin and in membrane ruffles suggests that the protein has some function in the organization of actin filaments at or close to regions of actin-membrane attachment.

Talin: a cytoskeletal component concentrated in adhesion plaques and other sites of actin-membrane interaction

Talin is a recently identified cytoskeletal protein with a polypeptide molecular weight of 215,000 daltons. In cultured fibroblasts talin has been localized by immunofluorescence in adhesion plaques (focal contacts), in the ruffling membranes and leading lamellae of the cell periphery, and in fibrillar patterns that align with microfilament bundles and/or with cell surface fibronectin. These cellular locations suggest that the protein could function either in the attachment of microfilaments to the plasma membrane or in the organization of microfilaments close to membrane attachment sites. Cell transformation by viruses such as Rous sarcoma virus disrupts the normal organization of talin, and in most transformed cells talin appears distributed diffusely through the cytoplasm. In a few cells talin is detected in doughnut-shaped aggregates, as a ring surrounding a central core of actin. The significance of these structures is uncertain, but in some cells the individual structures will condense to form much larger aggregates with a striking appearance when viewed by immunofluorescence microscopy.

Binding of HeLa spectrin to a specific HeLa membrane fraction

From 30-40 g of Hela-S3 cells grown in suspension, 0.25-0.50 mg of spectrin has been purified by conventional biochemical procedures starting from a low ionic strength extraction at alkaline pH of crude Hela membranes. Hela spectrin consists in its native form of a tetramer alpha 2 beta 2 of two high molecular weight polypeptides (240,000 and 230,000 daltons). Three different populations of Hela membranes depleted of both spectrin and actin have been prepared on discontinuous sucrose gradients. Surprisingly, spectrin will reassociate with only the heavier membrane fraction. This reassociation is specific for Hela spectrin, since three other purified Hela proteins as well as human erythrocyte spectrin do not reassociate under the same conditions. This binding is not due to the presence of traces of actin still present in the membrane fraction since two Hela actin-binding proteins (filamin I and II) do not show any significant binding to this fraction. The nature of the membrane-binding site for Hela spectrin is discussed.

Proteins involved in the attachment of actin to the plasma membrane

Proteins that may be involved in two types of actin-membrane association are discussed. The first set includes alpha-actinin, vinculin, fimbrin and a new cytoskeletal protein that are all concentrated in adhesion plaques, those regions of cultured fibroblasts where bundles of actin microfilaments terminate and where the plasma membrane comes close to the underlying substrate. The properties of non-muscle alpha-actinin suggest that it functions to cross-link actin filaments and thereby stabilize microfilament bundles rather than functioning in their attachment to the membrane. Fimbrin also appears to be involved in bundling of filaments rather than in attachment. In contrast, vinculin binds to the ends of actin filaments in vitro and is probably the best candidate for a role in the attachment of actin to membranes at the adhesion plaque. The discovery of a new protein, 215k, of unknown function, in the adhesion plaque suggests that many more proteins remain to be identified in this region. Attachment of actin filaments to other regions of the plasma membrane is also considered and a protein is described that seems to be a spectrin in brain and other tissues. The brain protein resembles erythrocyte spectrin in its physical properties, in binding actin, in being associated with cell membranes and in cross-reacting immunologically. We suggest that the brain protein and erythrocyte spectrin both belong to a family of related proteins (the spectrins) which function in the attachment of actin to membranes in many different cell types.

Nonerythrocyte spectrins: actin-membrane attachment proteins occurring in many cell types

The properties of brain fodrin have been analyzed and compared with those of erythrocyte spectrin. Both proteins consist of high molecular weight polypeptide doublets on SDS polyacrylamide gels and in solution behave as very large asymmetric molecules. Both proteins show a characteristic increase in sedimentation coefficient in the presence of 20 mM KCl. Antibodies against the brain protein cross-react with erythrocyte spectrin and cross-react with similar high molecular weight doublet polypeptides in SDS polyacrylamide gels of other cell types and plasma membrane preparations. Both proteins bind actin. The brain protein and erythrocyte spectrin show specific and competitive binding to erythrocyte membranes and this binding is inhibited by antibodies against erythrocyte ankyrin. Several of these properties distinguish these proteins from the class of high molecular weight actin-binding proteins that includes filamin and macrophage actin-binding protein. We conclude that together with erythrocyte spectrin, the brain protein and equivalent, immunologically related proteins in other cell types belong to a single class of proteins with the common function of attachment of actin to plasma membranes. Based on the structural and functional similarities, the name spectrin would seem appropriate for this whole class of proteins.

Co-existence of vinculin and a vinculin-like protein of higher molecular weight in smooth muscle

Recently, a protein component of adhesion plaques with a molecular weight of 130,000 (named vinculin) has been purified from smooth muscle and non-muscle cells. As detected by immunological methods, the only vinculin-related polypeptides in fibroblasts are proteins of Mr = 130,000. However, we show here that smooth muscle contains, in addition to vinculin, an apparently distinct protein with a Mr = 152,000 that shares both structural and immunological features with vinculin. Amino acid analysis, peptide mapping, and antibody cross-reaction studies were used to elucidate these similarities. Mr = 152,000 protein seems to be restricted to muscle (mainly or exclusively to smooth muscle). The possibility that vinculin is derived from proteolytic processing of the Mr = 152,000 protein or that the proteins are related by some other type of post-translational modification appears unlikely (although this cannot be completely ruled out) since both proteins are made in a rabbit reticulocyte cell-free translation system when mRNA derived from smooth muscle is used as the template. Both proteins are capable of used as the template. Both proteins are capable of lowering the viscosity of F-actin solutions, although the activity of the Mr = 152,000 protein is stimulated by Ca2+ while the activity of smooth muscle vinculin is not.

Isolation and characterization of a calcium-sensitive alpha-actinin-like protein from human platelet cytoskeletons

Platelet cytoskeletons were isolated by extracting these highly contractile cells with a solution containing 1% Triton X-100 and 10 mM ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid as recently described (Rosenberg, S., Stracher, A., and Lucas, R. C. (1981) J. Cell Biol. 91, 201-211). The Triton-insoluble cytoskeleton consists mostly of actin, a high molecular weight actin-binding protein and a previously unidentified protein with an apparent molecular weight on sodium dodecyl sulfate gels of 105,000 (+/- 5,000). We describe the purification of this 105,000-dalton protein from the platelet cytoskeleton using ammonium sulfate fractionation and ion exchange chromatography. This 105,000-dalton protein was found to cross-react with antibodies to beef cardiac alpha-actinin. One-dimensional partial proteolysis maps showed similarity to, but not identity with, the major peptides of the platelet 105,000-dalton protein and skeletal muscle alpha-actinin. The platelet 105,000-dalton cytoskeletal protein binds to and causes the sedimentation of skeletal muscle F-actin under comparatively low centrifugal force. This process, however, is inhibited by calcium ions, unlike the binding of any of the muscle alpha-actinins described to date. Thus, it is likely that the 105,000-dalton protein is the platelet form of alpha-actinin, its different structure accounting for its different actin-binding behavior.

Characterization of antibodies to smooth muscle myosin kinase and their use in localizing myosin kinase in nonmuscle cells

Antibodies to myosin light chain kinase, purified from turkey gizzard smooth muscle, were developed in rabbits and purified by affinity chromatography on a myosin light chain kinase-Sepharose 4B column. The purified antibodies crossreact with purified smooth muscle myosin light chain kinase but not with a variety of contractile or cytoskeletal proteins. The antibodies inhibit the catalytic activity of smooth muscle myosin light chain kinase and there is an inverse relationship between the kinase activity and the amount of antibody present in an assay. Half-maximal inhibition of myosin kinase activity occurs at an antibody/myosin kinase molar ratio of 10:1. The affinity-purified antibodies to smooth muscle myosin kinase were used to study the location of myosin kinase in a variety of nonmuscle cells. Immunofluorescence studies indicate that myosin light chain kinase is localized on microfilament bundles (stress fibers) in cultured fibroblasts. The stress fiber staining pattern is abolished when the antibodies are incubated with purified smooth muscle myosin light chain kinase prior to staining cells, while the staining pattern is unaffected when the antibodies are incubated with actin, myosin, alpha-actinin, or tropomyosin prior to staining. Moreover, the stress fiber staining pattern is periodic in well-spread gerbil fibroma cells and experiments have demonstrated that myosin light chain kinase appears to have the same periodic distribution as myosin but an antiperiodic distribution relative to alpha-actinin. These data indicate that myosin light chain kinase and its substrate, myosin, are in close proximity and are consistent with the hypothesis that myosin light chain kinase regulates actin-myosin interactions in nonmuscle cells.

A rapid purification of alpha-actinin, filamin, and a 130,000-dalton protein from smooth muscle

Brief, low ionic strength extraction of chicken gizzard at 37 degrees C yields a solution containing a limited number of proteins including alpha-actinin, filamin, actin, desmin, and a 130,000-dalton polypeptide. The proteins are then fractionated by Mg2+- and (NH4)2SO4-induced precipitations and by ion exchange and gel filtration column chromatography to give rise to highly purified preparations of alpha-actinin, filamin, and a 130,000-dalton protein. The alpha-actinin and filamin isolated by this scheme are "native" based upon their S20,w values and their ability to bind to F-actin. These procedures, with minor modification, can be used for the purification of alpha-actinin from skeletal muscle and non-muscle tissues as well as for the purification of filamin from non-muscle tissue.

The association of alpha-actinin with the plasma membrane

The role of alpha-actinin in the attachment of actin to plasma membranes has been investigated. Specific antibody staining of SDS gels has indicated that alpha-actinin is a major component in isolated plasma membranes prepared from three different cell types by two different procedures. Using specific extraction conditions, most of the alpha-actinin can be selectively extracted from the membranes with relatively little parallel release of actin. This selective dissociation of alpha-actinin from the plasma membrane leads us to conclude that alpha-actinin is present in these membrane preparations, because it is bound to actin, and that alpha-actinin does not form a direct link between actin and the membrane.