The cytoskeletal protein vinculin contains transformation-sensitive, covalently bound lipid

Differential lipid binding of vinculin isoforms promotes quasi-equivalent dimerization

The main cause of death globally remains debilitating heart conditions, such as dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM), which are often due to mutations of specific components of adhesion complexes. Vinculin regulates these complexes and plays essential roles in intercalated discs that are necessary for muscle cell function and coordinated movement and in the development and function of the heart. Humans bearing familial or sporadic mutations in vinculin suffer from chronic, progressively debilitating DCM that ultimately leads to cardiac failure and death, whereas autosomal dominant mutations in vinculin can also provoke HCM, causing acute cardiac failure. The DCM/HCM-associated mutants of vinculin occur in the 68-residue insert unique to the muscle-specific, alternatively spliced isoform of vinculin, termed metavinculin (MV). Contrary to studies that suggested that phosphoinositol-4,5-bisphosphate (PIP2) only induces vinculin homodimers, which are asymmetric, we show that phospholipid binding results in a domain-swapped symmetric MV dimer via a quasi-equivalent interface compared with vinculin involving R975. Although one of the two PIP2 binding sites is preserved, the symmetric MV dimer that bridges two PIP2 molecules differs from the asymmetric vinculin dimer that bridges only one PIP2 Unlike vinculin, wild-type MV and the DCM/HCM-associated R975W mutant bind PIP2 in their inactive conformations, and R975W MV fails to dimerize. Mutating selective vinculin residues to their corresponding MV residues, or vice versa, switches the isoform's dimeric constellation and lipid binding site. Collectively, our data suggest that MV homodimerization modulates microfilament attachment at muscular adhesion sites and furthers our understanding of MV-mediated cardiac remodeling.

Protein-lipid interactions: correlation of a predictive algorithm for lipid-binding sites with three-dimensional structural data

Background

Over the past decade our laboratory has focused on understanding how soluble cytoskeleton-associated proteins interact with membranes and other lipid aggregates. Many protein domains mediating specific cell membrane interactions appear by fluorescence microscopy and other precision techniques to be partially inserted into the lipid bilayer. It is unclear whether these protein-lipid-interactions are dependent on shared protein motifs or unique regional physiochemistry, or are due to more global characteristics of the protein.

Results

We have developed a novel computational program that predicts a protein's lipid-binding site(s) from primary sequence data. Hydrophobic labeling, Fourier transform infrared spectroscopy (FTIR), film balance, T-jump, CD spectroscopy and calorimetry experiments confirm that the interfaces predicted for several key cytoskeletal proteins (alpha-actinin, Arp2, CapZ, talin and vinculin) partially insert into lipid aggregates. The validity of these predictions is supported by an analysis of the available three-dimensional structural data. The lipid interfaces predicted by our algorithm generally contain energetically favorable secondary structures (e.g., an amphipathic alpha-helix flanked by a flexible hinge or loop region), are solvent-exposed in the intact protein, and possess favorable local or global electrostatic properties.

Conclusion

At present, there are few reliable methods to determine the region of a protein that mediates biologically important interactions with lipids or lipid aggregates. Our matrix-based algorithm predicts lipid interaction sites that are consistent with the available biochemical and structural data. To determine whether these sites are indeed correctly identified, and whether use of the algorithm can be safely extended to other classes of proteins, will require further mapping of these sites, including genetic manipulation and/or targeted crystallography.

Evolutionary Relationships and Protein Domain Architecture in an Expanded Calpain Superfamily in Kinetoplastid Parasites

Employing whole-genome analysis we have characterized a large family of genes coding for calpain-related proteins in three kinetoplastid parasites. We have defined a total of 18 calpain-like sequences in Trypanosoma brucei, 27 in Leishmania major, and 24 in Trypanosoma cruzi. Sequence characterization revealed a well-conserved protease domain in most proteins, although residues critical for catalytic activity were frequently altered. Many of the proteins contain a novel N-terminal sequence motif unique to kinetoplastids. Furthermore, 24 of the sequences contain N-terminal fatty acid acylation motifs indicating association of these proteins with intracellular membranes. This extended family of proteins also includes a group of sequences that completely lack a protease domain but is specifically related to other kinetoplastid calpain-related proteins by a highly conserved N-terminal domain and by genomic organization. All sequences lack the C-terminal calmodulin-related calcium-binding domain typical of most mammalian calpains. Our analysis emphasizes the highly modular structure of calpains and calpain-like proteins, suggesting that they are involved in diverse cellular functions. The discovery of this surprisingly large family of calpain-like proteins in lower eukaryotes that combines novel and conserved sequence modules contributes to our understanding of the evolution of this abundant protein family.

Mechanobiology of force transduction in dermal tissue

BACKGROUND/AIMS

The influence of mechanical forces on skin has been examined since 1861 when Langer first reported the existence of lines of tension in cadaver skin. Internal tension in the dermis is not only passively transferred to the epidermis but also gives rise to active cell-extracellular matrix and cell-cell mechanical interactions that may be an important part of the homeostatic processes that are involved in normal skin metabolism. The purpose of this review is to analyse how internal and external mechanical loads are applied at the macromolecular and cellular levels in the epidermis and dermis.

METHODS

A review of the literature suggests that internal and external forces applied to dermal cells appear to be involved in mechanochemical transduction processes involving both cell-cell and cell-extra-cellular matrix (ECM) interactions. Internal forces present in dermis are the result of passive tension that is incorporated into the collagen fiber network during development. Active tension generated by fibroblasts involves specific interactions between cell membrane integrins and macromolecules found in the ECM, especially collagen fibrils. Forces appear to be transduced at the cell-ECM interface via re-arrangement of cytoskeletal elements, activation of stretch-induced changes in ion channels, cell contraction at adherens junctions, activation of cell membrane-associated secondary messenger pathways and through growth factor-like activities that influence cellular proliferation and protein synthesis.

CONCLUSIONS

Internal and external mechanical loading appears to affect skin biology through mechanochemical transduction processes. Further studies are needed to understand how mechanical forces, energy storage and conversion of mechanical energy into changes in chemical potential of small and large macromolecules may occur and influence the metabolism of dermal cells.

CAP5.5, a life-cycle-regulated, cytoskeleton-associated protein is a member of a novel family of calpain-related proteins in Trypanosoma brucei

The cell shape of African trypanosomes is determined by the presence of an extensive subpellicular microtubule cytoskeleton. Other possible functions of the cytoskeleton, such as providing a potential framework for signalling proteins transducing information from the intracellular and extracellular environment, have not yet been investigated in trypanosomes. In this study, we have identified a novel cytoskeleton-associated protein in Trypanosoma brucei. CAP5.5 is the first member of a new family of proteins in trypanosomes, characterised by their similarity to the catalytic region of calpain-type proteases. CAP5.5 is only expressed in procyclic, but not in bloodstream, trypanosomes. Furthermore, CAP5.5 has been shown to be both myristoylated and palmitoylated, suggesting a stable interaction with the cell membrane. A bioinformatics analysis of the trypanosome genome revealed a diverse family of calpain-related proteins with primary structures similar to CAP5.5, but of varying length. We suggest a nomenclature for this new family of proteins in T. brucei.

Molecular basis for tissue expansion: clinical implications for the surgeon

A wide variety of tissue expansion techniques have been used for breast reconstruction, craniofacial surgery, and burn care in plastic reconstructive surgery. However, the basic mechanism by which skin and surrounding tissue respond to mechanical expansion remains unclear. Recent studies have revealed the biomechanical aspects of cells subjected to strain and various factors involved in the stretch-induced signal transduction pathway. In this regard, we have reported previously that mechanical force increases keratinocyte growth and protein synthesis and alters cell morphology. The mechanism by which strain causes an enhancement of cellular growth appears to be a network of several integrated cascades, implicating growth factors, cytoskeleton, and the protein kinase family. Recently, additional evidence has accumulated that mechanical strain stimulates signal transduction pathways that could trigger a series of cascades eventually leading to a new skin production. For example, we have evidence suggesting a key role for protein kinase C (PKC) in mechanosignaling as PKC is activated and translocated in keratinocytes subjected to strain in an isoform-specific manner. In this report, molecular mechanisms leading to enhancement of skin surface area are reviewed, and possible future applications are discussed.

Interaction of cytoskeletal proteins with membrane lipids

Rapid and significant progress has been made in understanding lipid/protein interactions involving cytoskeletal components and the plasma membrane. Covalent and noncovalent lipid modifications of cytoskeletal proteins mediate their interaction with lipid bilayers. The application of biophysical techniques such as differential scanning colorimetry, neutron reflection, electron spin resonance, CD spectroscopy, nuclear magnetic resonance, and hydrophobic photolabeling, allow various folding stages of proteins during electrostatic adsorption and hydrophobic insertion into lipid bilayers to be analyzed. Reconstitution of proteins into planar lipid films and liposomes help to understand the architecture of biological interfaces. During signaling events at plasma membrane interfaces, lipids are important for the regulation of catalytic protein functions. Protein/lipid interactions occur selectively and with a high degree of specificity and thus have to be considered as physiologically relevant processes with gaining impact on cell functions.

Posttranslational modification of proteins with fatty acids in platelets

Direct modification of proteins by fatty acid can occur as cotranslational N-myristoylation of an N-terminal glycine residue or as posttranslational thioesterification of cysteine residue(s). Platelets provide an excellent model system for studying the posttranslational type of modification in the absence of active protein synthesis and in the absence of protein synthesis-related protein modifications with lipids. Using this model system it was shown that thioesterification of proteins with fatty acid is less specific for palmitate than it was thought earlier and that other saturated, mono- and even polyunsaturated long chain fatty acids can also participate. The chain length and the extent of unsaturation of the protein-linked fatty acid moiety can, very likely, modulate hydrophobic protein-membrane lipid and protein-protein interactions. CD9, HLA class I glycoprotein, glycoproteins Ib, IX and IV, P-selectin and alpha subunits of G proteins have been demonstrated unequivocally as S-fatty acid acylated platelet proteins.

Progesterone acts through protein kinase C to remodel the cytoplasm as the amphibian oocyte becomes the fertilization-competent egg

The fertilization-competent Xenopus egg undergoes a contraction of its cortex towards the apex of the pigmented animal hemisphere within 10 min of fertilization. Evidence suggests that protein kinase C (PKC) is involved in the assembly of this contractile network and we show that PKC is rapidly activated as a result of exposure of oocytes to progesterone. Xenopus oocytes contain at least five different isotypes of PKC. Three actin-binding proteins (i.e. vinculin, talin and ankyrin) appear to play an early role in the assembly of the contractile network and one of the proteins (vinculin) becomes phosphorylated shortly after progesterone treatment as the contractile network is assembling. Our results indicated that progesterone acts through a phospholipase to activate PKC and that PKC participates in the remodeling of the cytoplasmic compartment as the oocyte becomes the egg.

Talin requires beta-integrin, but not vinculin, for its assembly into focal adhesion-like structures in the nematode Caenorhabditis elegans

In cultured cells, the 230-kDa protein talin is found at discrete plasma membrane foci known as focal adhesions, sites that anchor the intracellular actin cytoskeleton to the extracellular matrix. The regulated assembly of focal adhesions influences the direction of cell migrations or the reorientation of cell shapes. Biochemical studies of talin have shown that it binds to the proteins integrin, vinculin, and actin in vitro. To understand the function of talin in vivo and to correlate its in vitro and in vivo biochemical properties, various genetic approaches have been adopted. With the intention of using genetics in the study of talin, we identified a homologue to mouse talin in a genetic model system, the nematode Caenorhabditis elegans. C. elegans talin is 39% identical and 59% similar to mouse talin. In wild-type adult C. elegans, talin colocalizes with integrin, vinculin, and alpha-actinin in the focal adhesion-like structures found in the body-wall muscle. By examining the organization of talin in two different C. elegans mutant strains that do not make either beta-integrin or vinculin, we were able to determine that talin does not require vinculin for its initial organization at the membrane, but that it depends critically on the presence of integrin for its initial assembly at membrane foci.

Novel inhibitory action of tunicamycin homologues suggests a role for dynamic protein fatty acylation in growth cone-mediated neurite extension

In neuronal growth cones, the advancing tips of elongating axons and dendrites, specific protein substrates appear to undergo cycles of posttranslational modification by covalent attachment and removal of long-chain fatty acids. We show here that ongoing fatty acylation can be inhibited selectively by long-chain homologues of the antibiotic tunicamycin, a known inhibitor of N-linked glycosylation. Tunicamycin directly inhibits transfer of palmitate to protein in a cell-free system, indicating that tunicamycin inhibition of protein palmitoylation reflects an action of the drug separate from its previously established effects on glycosylation. Tunicamycin treatment of differentiated PC12 cells or dissociated rat sensory neurons, under conditions in which protein palmitoylation is inhibited, produces a prompt cessation of neurite elongation and induces a collapse of neuronal growth cones. These growth cone responses are rapidly reversed by washout of the antibiotic, even in the absence of protein synthesis, or by addition of serum. Two additional lines of evidence suggest that the effects of tunicamycin on growth cones arise from its ability to inhibit protein long-chain acylation, rather than its previously established effects on protein glycosylation and synthesis. (a) The abilities of different tunicamycin homologues to induce growth cone collapse very systematically with the length of the fatty acyl side-chain of tunicamycin, in a manner predicted and observed for the inhibition of protein palmitoylation. Homologues with fatty acyl moieties shorter than palmitic acid (16 hydrocarbons), including potent inhibitors of glycosylation, are poor inhibitors of growth cone function. (b) The tunicamycin-induced impairment of growth cone function can be reversed by the addition of excess exogenous fatty acid, which reverses the inhibition of protein palmitoylation but has no effect on the inhibition of protein glycosylation. These results suggest an important role for dynamic protein acylation in growth cone-mediated extension of neuronal processes.

Dystrophin as a mechanochemical transducer in skeletal muscle

This review is primarily concerned with two key issues in research on dystrophin: (1) how the protein interacts with the plasma membrane of skeletal muscle fibres and (2) how an absence of dystrophin gives rise to Duchenne muscular dystrophy. In relation to the first point, we suggest that the post-translational acylation of dystrophin may contribute to its interaction with the plasma membrane. Regarding the second point, it is generally considered that an absence of dystrophin makes the plasma membrane susceptible to damage by contraction/relaxation cycles. In this connection, we propose that the progressive nature of Duchenne dystrophy, and the phenotypic characteristics of mdx mice, are more consistent with dystrophin functioning as a mechanical transducer that transmits growth stimuli from the enlarging skeleton to the muscle. On the basis of this hypothesis, dystrophin-deficient muscles would be unable to grow at the same rate as the skeleton.

Antagonism of [3H]fatty acid incorporation into vimentin by sodium pyruvate: pitfalls of protein acylation

In the course of studying possible fatty acid acylation of vimentin by cultured bovine lens epithelial cells, several potential pitfalls of protein-fatty acid acylation were recognized. Even exhaustive delipidation of vimentin with organic solvents failed to remove all noncovalently associated [3H]palmitate and [3H]myristate. Hydroxylamine treatment of vimentin, separated by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE), failed to remove either palmitate or myristate derived radiolabel. Hydroxylamine treatment did remove palmitate label from a group of lower molecular weight proteins. The myristate radiolabel associated with vimentin recovered after SDS-PAGE and subjected to acid hydrolysis was shown due to incorporated [3H]amino acids, mainly glutamic acid, generated from the fatty acid. Adding excess sodium pyruvate to labeling media has been used by others to reduce the metabolic conversion of fatty acids to amino acids; however, no direct evidence in support of this antagonism was presented. We observed that inclusion of sodium pyruvate at between 5 and 20 mM in the labeling medium produced a dramatic decrease in incorporation of myristic acid radiolabel into vimentin. However, inclusion of even 20 mM pyruvate did not completely antagonize the metabolic conversion of fatty acid label to amino acids. Furthermore, the sodium pyruvate antagonism could be totally obscured if the exposure of X-ray film by fluorography was even slightly prolonged. The results illustrate the danger in assuming that solvent extraction totally delipidates proteins and that adding sodium pyruvate to labeling media prevents the transfer of fatty acid label to amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Nicotinic receptor-associated 43K protein and progressive stabilization of the postsynaptic membrane

An extrinsic membrane protein of apparent molecular mass 43 kDa is specifically localized in postsynaptic membranes closely associated with the nicotinic acetylcholine receptor (AChR). Since its discovery in 1977, biochemical and morphological studies have combined to provide relatively clear pictures of 43K protein structure and subcellular compartmentalization. Nevertheless, despite these advances, the precise function of this synapse-specific protein remains unclear. Data gathered in recent years indicate that the postsynaptic apparatus develops through the incremental agglomeration of receptor microaggregates; evidence derived from a number of sources points to a role for 43K protein in certain underlying reactions. In this paper, I review 43K protein structural and anatomical data and analyze evidence for its role in the organization and maintenance of the postsynaptic membrane. Finally, I offer a model presenting a view of the role of 43K protein in the ontogeny of the motor endplate.

Cytoskeletal assembly and vinculin-cytoskeleton interaction in different phases of the activation of bovine platelets

Vinculin is an Mr 130 kDa protein that has been implicated in membrane-cytoskeleton interaction in various cell types. It has been demonstrated that vinculin is not a cytoskeletal component in resting platelets, but part of it becomes associated with the cytoskeleton during thrombin-induced activation. In this study, using a quantitative immunoblotting technique, the relation of vinculin to the cytoskeleton in different phases of activation of bovine platelets was explored, and the process of incorporation of vinculin into the cytoskeleton was related to that of cytoskeletal assembly. The assembly of cytoskeleton proceeded at a significantly faster rate than the association of vinculin with it, which shows that the latter process is not due to passive trapping of vinculin into the Triton-insoluble residue, but certain biochemical changes had to occur before such an interaction became possible. When the formation of pseudopodia was prevented by cytochalasin B, but neither aggregation nor the release reaction induced by thrombin were inhibited, the recovery of vinculin in the Triton-insoluble residue even increased. In both time- and thrombin-concentration-dependent studies, poor correlation was found between vinculin-cytoskeleton association and the extent of aggregation. Activation with phorbol-myristate-acetate, which is a strong stimulus for aggregation but produces only a slight release in the granular content, resulted in the association of only a negligible amount of vinculin with the cytoskeletal fraction. The incorporation of vinculin into the cytoskeletal fraction of thrombin activated platelets started with the release reaction but still proceeded, and the greatest part of the reaction occurred after secretion had gone to completion.(ABSTRACT TRUNCATED AT 250 WORDS)

Intercellular spread of Shigella flexneri through a monolayer mediated by membranous protrusions and associated with reorganization of the cytoskeletal protein vinculin

The spread of Shigella flexneri in a monolayer of infected Henle and HeLa cells was studied by using immunofluorescence and electron microscopy. Infected cells produced numerous bacterium-containing membranous protrusions up to 18 microns in length that penetrated adjacent cells and were subsequently phagocytosed. Fluorescence staining of actin and vinculin in infected cells with phalloidin and monoclonal antibody to vinculin, respectively, demonstrated that the protrusions containing the bacteria consisted of these cytoskeletal proteins. Actin accumulated predominantly at the poles of bacteria distal to the tip of protrusions and appeared as trails extending back towards the host cell cytoplasm. Vinculin, however, was distributed uniformly around the bacteria and throughout the protrusion. A profound rearrangement of vinculin occurred in Henle and HeLa cells following infection with shigellae: whereas in uninfected cells it was distributed mainly around the cell periphery, in infected cells it concentrated mainly around clusters of bacteria in the cytoplasm. This suggests a possible involvement of the vinculin cytoskeletal protein in the intercellular spread of shigellae during an infection.

Chorea-acanthocytosis: abnormal composition of covalently bound fatty acids of erythrocyte membrane proteins

Phospholipid class, peak profile of each phospholipid class, loosely bound fatty acids, covalently (tightly) bound fatty acids of the erythrocyte membranes, and plasma fatty acids were investigated using high-performance liquid chromatography in six patients with chorea-acanthocytosis and 14 age- and sex-matched normal control subjects. Additionally, six patients with Huntington's disease were included as disease control subjects in the study of covalently bound fatty acids. Study of covalently (tightly) bound fatty acids in erythrocyte membrane proteins after alkaline hydrolysis, hitherto undescribed in chorea-acanthocytosis, revealed that palmitic acid (C16:0) was significantly increased and stearic acid (C18:0) was decreased in the patients with chorea-acanthocytosis. Analyses for total covalently bound fatty acids disclosed that palmitic and docosahexaenoic (C22:6) acids were increased and stearic acid was decreased in chorea-acanthocytosis. Phospholipid class (phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, and phosphatidylserine) and peak profile of each phospholipid class from the erythrocyte membranes did not differ between the patients with chorea-acanthocytosis and the control subjects. Of the loosely bound fatty acids, linoleic acid (C18:2) was significantly decreased in those with chorea-acanthocytosis, which seemed to be nonspecific.

Myristoylation of heparan sulfate proteoglycan and proteins occurs post-translationally in human colon carcinoma cells

We have recently shown that the heparan sulfate proteoglycan of human colon carcinoma cells is acylated with both myristate and palmitate, two long-chain saturated fatty acids. In this study we show that cycloheximide did not significantly inhibit the incorporation of myristic acid into either proteoglycan or total protein pool. This lack of inhibition occurred under a condition in which protein synthesis was inhibited greater than 90%. Cycloheximide, on the other hand, did not affect the incorporation of [3H]myristic acid into fatty acid nor the intracellular interconversion of myristate to palmitate. Characterization of fatty acyl moiety in the proteoglycan and protein by reverse-phase HPLC revealed that approximately 60% of the covalently bound fatty acids was myristate and the remaining 40% was palmitate. These results indicate that in human colon carcinoma cells myristoylation of heparan sulfate proteoglycan and proteins occurs post-translationally, presumably in the Golgi complex.

Vinculin is a permanent component of the membrane skeleton and is incorporated into the (re)organising cytoskeleton upon platelet activation

Vinculin, a 130-kDa protein discovered in chicken gizzard smooth-muscle cells and subsequently also described in platelets, is believed to be involved in membrane-cytoskeleton interactions. In this study we investigated vinculin distribution in human blood platelets. Two skeletal fractions and a remaining cytosolic fraction were prepared with a recently described Triton X-100 lysis buffer causing minimal post-lysis breakdown by proteolysis. The presence of vinculin was demonstrated in the membrane skeleton and cytosol of resting and thrombin-activated human platelets. Upon thrombin stimulation vinculin also appeared in the cytoskeleton. this cytoskeletal incorporation was completed during the early stages of platelet aggregation and secretion, when the uptake of myosin, actin-binding protein and talin was still not maximal. We conclude therefore, that vinculin may play an important role in the structural (re)organisation of the human platelet cytoskeleton upon platelet activation.

A 39-kD plasma membrane protein (IP39) is an anchor for the unusual membrane skeleton of Euglena gracilis

The major integral plasma membrane protein (IP39) of Euglena gracilis was radiolabeled, peptide mapped, and dissected with proteases to identify cytoplasmic domains that bind and anchor proteins of the cell surface. When plasma membranes were radioiodinated and extracted with octyl glucoside, 98% of the extracted label was found in IP39 or the 68- and 110-kD oligomers of IP39. The octyl glucoside extracts were incubated with unlabeled cell surface proteins immobilized on nitrocellulose (overlays). Radiolabel from the membrane extract bound one (80 kD) of the two (80 and 86 kD) major membrane skeletal protein bands. Resolubilization of the bound label yielded a radiolabeled polypeptide identical in Mr to IP39. Intact plasma membranes were also digested with papain before or after radioiodination, thereby producing a cytoplasmically truncated IP39. The octyl glucoside extract of truncated IP39 no longer bound to the 80-kD membrane skeletal protein in the nitrocellulose overlays. EM of intact or trypsin digested plasma membranes incubated with membrane skeletal proteins under stringent conditions similar to those used in the nitrocellulose overlays revealed a partially reformed membrane skeletal layer. Little evidence of a membrane skeletal layer was found, however, when plasma membranes were predigested with papain before reassociation. A candidate 80-kD binding domain of IP39 has been tentatively identified as a peptide fragment that was present after trypsin digestion of plasma membranes, but was absent after papain digestion in two-dimensional peptide maps of IP39. Together, these data suggest that the unique peripheral membrane skeleton of Euglena binds to the plasma membrane through noncovalent interactions between the major 80-kD membrane skeletal protein and a small, papain sensitive cytoplasmic domain of IP39. Other (62, 51, and 25 kD) quantitatively minor peripheral proteins also interact with IP39 on the nitrocellulose overlays, and the possible significance of this binding is discussed.

Mechanisms of polar lobe formation in fertilized eggs of molluscs

Polar lobe formation in the marine mudsnail, Ilyanassa obsoleta, involves formation of a microtubule-, microfilament-dependent furrow that constricts at two rates, then stops and relaxes. Some artificial seawater mixtures allow relatively normal development, facilitate insertion of microelectrode tips, and prevent artifactual bleb formation during such punctures. Membrane events may affect formation of polar lobe constrictions: (1) Brief treatment with digitonin prevents constrictions, but not cytokinesis per se, and the suppression of constrictions is permanent. Tomatine (but not tomatidine) and filipin act similarly, although filipin often also stops cytokinesis as well. (2) Responses to digitonin, tomatine, and filipin occur with little change in membrane potential. (3) Adhesion to substrata in response to brief treatment at low pH prevents both constrictions and cytokinesis. (4) Adhesion to substrata via polylysine allows both constrictions and cytokinesis, but embryos are smaller in volume and develop abnormally. Formation of lobe constrictions may be sensitive to perturbations of the plasma membrane.

Interaction in situ of the cytoskeletal protein vinculin with bilayers studied by introducing a photoactivatable fatty acid into living chicken embryo fibroblasts

The cytoskeletal protein vinculin, a putative actin--plasma-membrane linker, has been shown by hydrophobic photo-labeling to interact in vitro directly with bilayers of acidic phospholipids [Niggli et al. (1986) J. Biol. Chem. 261, 6912-6918]. In order to demonstrate that such an interaction occurs also in intact cells, chicken embryo fibroblasts were incubated for 2 h with a 3H-labeled photoactivatable fatty acid, 11-(4-[3-(trifluoromethyl)-diazirinyl]phenyl)-[2-3H]undecanoic acid. This resulted in biosynthetic incorporation into cellular lipids of a fraction of the fatty acid added. Following photolysis, vinculin was immunoprecipitated from different subcellular fractions using a specific polyclonal anti-vinculin antibody. The protein was recovered from both the cytosolic and the crude membrane fraction. Vinculin from both fractions incorporated label, but the membrane-associated population was at least eight times more strongly photolabeled than the cytosolic protein. Moreover, photolysis increased only labeling of the membrane-bound but not of the cytosolic protein. These results suggest that the direct interaction of vinculin with the hydrophobic core of the phospholipid layer observed in vitro may also be relevant in intact cells, and may be involved in its function as a linker protein.

N-myristyl-Lys-Arg-Thr-Leu-Arg: a novel protein kinase C inhibitor

In view of the critical role that the Ca2+- and phospholipid-dependent enzyme protein kinase C (PKC) plays in mediating proliferative responses to a number of growth factors, hormones, and tumor promoters, it is thought that selective PKC inhibitors may provide a new class of antiproliferative drugs. Established PKC inhibitors include three major classes of agents: agents that compete with the substrate ATP, agents that compete with the protein substrate, and agents that both compete with ATP and interact with the cofactor phosphatidylserine (PS). In this report, we have characterized the interactions between PKC and N-myristyl-Lys-Arg-Thr-Leu-Arg, a myristylated analogue of a synthetic peptide substrate of PKC. We determined that the myristylated peptide was a novel PKC inhibitor that interacted with PS as well as competed with the protein substrate of PKC. The inhibitory activity of the peptide was conferred by myristylation. We found that the myristylated peptide antagonized Ca2+- and PS-activated PKC with an IC50 of 75 microns, whereas the nonmyristylated peptide lacked this inhibitory activity. A fully active, Ca2+- and PS-independent catalytic fragment of PKC can be generated by limited proteolysis. Although the myristylated peptide was a very poor PKC substrate, this peptide inhibited the catalytic fragment of PKC by apparent competition with the phosphoacceptor substrate histone IIIS with an IC50 of 200 microM, whereas the nonmyristylated peptide showed no inhibitory activity against the catalytic fragment. Thus, the myristylated peptide may serve as a model for the development of selective PKC inhibitors, because its inhibitory mechanism exploits the substrate specificity of PKC, as well as the novel regulation of the enzyme. Furthermore, since endogenous PKC substrates include acylated proteins, the observations that we report here concerning a myristylated synthetic peptide suggest that acylation of proteins may be important in the regulation of PKC activity in vivo.

Purification and characterization of an 85 kDa talin-binding fragment of vinculin

Vinculin and talin are adhesion plaque proteins which have been shown to interact with each other in vitro. In order to begin to investigate where the talin-binding domain is in vinculin, vinculin was digested with Staphylococcus aureus V8 protease to generate two major fragments of 85 and 30 kDa, and these fragments were purified. Nitrocellulose overlays with 125I-talin and the 125I-85 kDa vinculin fragment and sucrose density gradient centrifugation demonstrated that the talin-binding domain was localized to the 85 kDa vinculin fragment. Quantification of 125I-talin binding in the overlays showed that four times more talin bound to the 85 kDa fragment as compared to intact vinculin. Competitive immunoprecipitation experiments demonstrated that unlabeled 85 kDa fragment was about three-fold more effective at competing for 125I-85 kDa binding to talin than was unlabeled vinculin. These results suggest that the 30 kDa fragment inhibits the vinculin-talin interaction even though the talin-binding domain is localized in the 85 kDa fragment.

Vinculin

Vinculin is clearly a key element in the transmembrane assemblages that link cells to each other or to the substrate. However, despite all the studies that have been done on the protein, we still do not know its function within these assemblages. The bulk of the biochemical and cell biological evidence suggests that, in some unknown way, its presence in the junctions may be involved in the stable association of actin with the membrane, yet vinculin by itself does not appear to interact with actin. In the future, identification of additional junctional molecules that interconnect actin and vinculin may resolve this dilemma. Alternatively, studies with vinculin that is phosphorylated or acylated may yield clues to its function. Perhaps the complexity of the protein composition of microfilament-containing junctions suggests that protein assemblages rather than individual proteins provide novel functions. As new proteins belonging to these junctions are discovered, it will be important to assess their interaction with already known components such as vinculin and to ask if the protein combination has a particular function.

An amphitropic cAMP-binding protein in yeast mitochondria. 1. Synergistic control of the intramitochondrial location by calcium and phospholipid

A cAMP-binding protein is found to be integrated into the inner mitochondrial membrane of the yeast Saccharomyces cerevisiae under normal conditions. It resists solubilization by high salt and chaotropic agents. The protein is, however, converted to a soluble form which then resides in the intermembrane space, when isolated mitochondria are incubated with low concentrations of calcium. Phospholipids or diacylglycerol (or analogues) dramatically increases the efficiency of receptor release from the inner membrane, whereas these compounds alone are ineffective. Also, cAMP does not effect or enhance liberation from the membrane of the cAMP-binding protein. Photoaffinity labeling with 8-N3-[32P]cAMP followed by mitochondrial subfractionation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis does not reveal differences in the apparent molecular weight between the membrane-bound and the soluble form of the cAMP receptor. The two forms differ, however, in their partitioning behavior in Triton X-114 as well as in their protease resistance, indicating that the release from the membrane is accompanied by a change in lipophilicity and conformation of the receptor protein. Evidence is presented that a change of the intramitochondrial location of the yeast cAMP-binding protein also occurs in vivo and leads to the activation of a mitochondrial cAMP-dependent protein kinase. The cAMP-binding protein is the first example of a mitochondrial protein with amphitropic character; i.e., it has the property to occur in two different locations, as a membrane-embedded and a soluble form.

Partial deduced sequence of the 110-kD-calmodulin complex of the avian intestinal microvillus shows that this mechanoenzyme is a member of the myosin I family

The actin bundle within each microvillus of the intestinal brush border is laterally tethered to the membrane by bridges composed of the protein complex, 110-kD-calmodulin. Previous studies have shown that avian 110-kD-calmodulin shares many properties with myosins including mechanochemical activity. In the present study, a cDNA molecule encoding 1,000 amino acids of the 110-kD protein has been sequenced, providing direct evidence that this protein is a vertebrate homologue of the tail-less, single-headed myosin I first described in amoeboid cells. The primary structure of the 110-kD protein (or brush border myosin I heavy chain) consists of two domains, an amino-terminal "head" domain and a 35-kD carboxy-terminal "tail" domain. The head domain is homologous to the S1 domain of other known myosins, with highest homology observed between that of Acanthamoeba myosin IB and the S1 domain of the protein encoded by bovine myosin I heavy chain gene (MIHC; Hoshimaru, M., and S. Nakanishi. 1987. J. Biol. Chem. 262:14625-14632). The carboxy-terminal domain shows no significant homology with any other known myosins except that of the bovine MIHC. This demonstrates that the bovine MIHC gene most probably encodes the heavy chain of bovine brush border myosin I (BBMI). A bacterially expressed fusion protein encoded by the brush border 110-kD cDNA binds calmodulin. Proteolytic removal of the carboxy-terminal domain of the fusion protein results in loss of calmodulin binding activity, a result consistent with previous studies on the domain structure of the 110-kD protein. No hydrophobic sequence is present in the molecule indicating that chicken BBMI heavy chain is probably not an integral membrane protein. Northern blot analysis of various chicken tissue indicates that BBMI heavy chain is preferentially expressed in the intestine.

Cell-specific fatty acylation of proteins in cultured cells of neuronal and glial origin

Distinct sets of cellular proteins were labeled with [3H]myristic and [3H]palmitic acids in primary (rat neurons and astroglia) and continuous (murine N1E-115 neuroblastoma and rat C6 glioma) cell cultures derived from the nervous system. Both soluble and membrane proteins were modified by myristate in a hydroxylamine-stable (amide) linkage, while palmitoylated proteins were ester-linked and almost exclusively membrane bound. Chain elongation of both labeled fatty acids prior to acylation was observed, but no protein amide-linked [3H]myristate originating from [3H]palmitate was detected. Fatty acylation profiles differed considerably among most of the cell lines, except for rat astroglial and glioma cells in which myristoylated proteins appeared to be almost identical based on SDS gel electrophoresis. An unidentified 47 kDa myristoylated protein was labeled to a significantly greater extent in astroglial than in glioma cells; the expression of this protein could be related to transformation or development in cells of glial origin.

Acylation of viral and eukaryotic proteins

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Posttranslational membrane attachment and dynamic fatty acylation of a neuronal growth cone protein, GAP-43

Growth cones, the motile apparatus at the ends of elongating axons, are sites of extensive and dynamic membrane-cytoskeletal interaction and insertion of new membrane into the growing axon. One of the most abundant proteins in growth cone membranes is a protein designated GAP-43, whose synthesis increases dramatically in most neurons during periods of axon development or regeneration. We have begun to explore the role of GAP-43 in growth cone membrane functions by asking how the protein interacts with those membranes. Membrane-washing experiments indicate that mature GAP-43 is tightly bound to growth cone membranes, and partitioning of Triton X-114-solubilized GAP-43 between detergent-enriched and detergent-depleted phases indicates considerable hydrophobicity. The hydrophobic behavior of the protein is modulated by divalent cations, particularly zinc and calcium. In vivo labeling of GAP-43 in neonatal rat brain with [35S]methionine shows that GAP-43 is initially synthesized as a soluble protein that becomes attached to membranes posttranslationally. In tissue culture, both rat cerebral cortex cells and neuron-like PC12 cells actively incorporate [3H]palmitic acid into GAP-43. Isolated growth cones detached from their cell bodies also incorporate labeled fatty acid into GAP-43, suggesting active turnover of the fatty acid moieties on the mature protein. Hydrolysis of ester-like bonds with neutral hydroxylamine removes the bound fatty acid and exposes new thiol groups on GAP-43, suggesting that fatty acid is attached to the protein's only two cysteine residues, located in a short hydrophobic domain at the amino terminus. Modulation of the protein's hydrophobic behavior by divalent cations suggests that other domains, containing large numbers of negatively charged residues, might also contribute to GAP-43-membrane interactions. Our observations suggest a dynamic and reversible interaction of GAP-43 with growth cone membranes.

Activation of the neutrophil respiratory burst by chemoattractants: regulation of the N-formyl peptide receptor in the plasma membrane

The N-formyl peptide receptor mediates a number of host defensive responses of human neutrophils that result in chemotaxis, secretion of hydrolytic enzymes, and superoxide generation. Inappropriate activation or defective regulation of these responses can result in pathogenic states responsible for inflammatory disease. The receptor is a 50 to 70-kD, integral plasma membrane glycoprotein with intracellular and surface localization. Its abundance in the membrane is regulated by membrane flow and recycling processes. Cytoskeletal interactions are believed to control its organization in the plane of the membrane and interaction with other proteins. The receptor's most important interaction is with guanyl nucleotide binding proteins that serve as signal transduction partners ultimately leading to activation of effector responses. Because the interaction of the receptor with G proteins is necessary for transduction, control of this interaction may be at the root of understanding the molecular control of responses in these cells. This review briefly summarizes some of the molecular properties, dynamics, and interactions of this receptor system in human neutrophils and discusses how these characteristics may pertain to the activation and control of superoxide generation.

cDNA-derived sequence of chicken embryo vinculin

We report the complete primary structure of chicken embryo vinculin. The amino acid sequence was derived from the nucleotide sequence of five overlapping cDNA clones isolated from a lambda gt11 phage library. Chicken embryo vinculin contains 1066 amino acids, has a calculated Mr of 116,990, a calculated pI of 5.9, and a hydropathy index of -4.22. A search of the National Biomedical Research Foundation protein sequence data base found no proteins with significant homology to vinculin. A striking feature of the linear sequence is a proline-rich region extending between residues 837 and 879. This region contains 45% proline and 19% aspartic plus glutamic acids; it is also the longest hydrophilic stretch in the molecule. The proline-rich region separates an amino-terminal domain with a calculated pI of 5.4 from a carboxyl-terminal domain with a calculated pI of 9.7. This feature suggests a structural basis for the specific interaction of vinculin with acidic phospholipids and a mechanism for the shuttling of vinculin between cytoplasm and membrane-associated junctional plaque.

Acetylcholine receptor-associated 43K protein contains covalently bound myristate

Torpedo electroplaque and vertebrate neuromuscular junctions contain high levels of a nonactin, 43,000-Mr peripheral membrane protein referred to as the 43K protein. 43K protein is associated with the cytoplasmic face of postsynaptic membranes at areas of high acetylcholine receptor density and has been implicated in the establishment and/or maintenance of these receptor clusters. Cloning of cDNAs encoding Torpedo 43K protein revealed that its amino terminus contains a consensus sequence sufficient for the covalent attachment of the rare fatty acid myristate. To examine whether 43K protein is, in fact, myristoylated, mouse muscle BC3H1 cells were metabolically labeled with either [35S]cysteine or [3H]myristate and immunoprecipitated with a monospecific antiserum raised against isolated Torpedo 43K protein. In cells incubated with either precursor, a single labeled species was specifically recovered that comigrated on SDS-PAGE with 43K protein purified from Torpedo electric organ. Approximately 95% of the 3H labeled material released from [3H]myristate-43K protein by acid methanolysis was extractable in organic solvents and eluted from a C18 reverse-phase HPLC column exclusively at the position of the methyl myristate internal standard. Thus, 43K protein contains authentic myristic acid rather than an amino or fatty acid metabolite of [3H]myristate. Myristate appears to be added to 43K protein cotranslationally and cannot be released from it by prolonged incubation in SDS, 2-mercaptoethanol, or hydroxylamine (pH 7.0 or 10.0), characteristics consistent with amino terminal myristoylation. Covalently linked myristate may be responsible for the high affinity of purified 43K protein for lipid bilayers despite the absence of a notably hydrophobic amino acid sequence.

Platelet vinculin: a substrate of activated factor XIII

In addition to plasma, Factor XIII of blood coagulation (FXIII) is also present in the cytosol of platelets, monocytes and macrophages. However, its intracellular function has not yet been revealed. Activated Factor XIII (FXIIIa) is a transglutaminase (protein-glutamine: amine gamma-glutamyltransferase, EC 2.3.2.13) of highly restricted substrate specificity with only a few known protein substrates. In this report, we showed that FXIIIa can link dansylcadaverine, radiolabelled histamine and putrescine to vinculin. Quantitative determinations revealed that in the vinculin molecule a single glutamine residue can serve as acyl donor for the incorporation of small-molecular-weight amines. Vinculin could not be crosslinked to another vinculin molecule. It could be covalently bound, however, to fibrinogen, which indicates that the acyl donor glutamine residue can be engaged in an epsilon-(gamma-glutamyl)lysyl crosslink formation. Since it has been shown that platelet actin and myosin, two main components of cytoskeleton, are also substrates for FXIIIa, and that vinculin is associated to the cytoskeleton during platelet activation, the involvement of FXIII in the stabilization of cytoskeleton at certain phases of cellular function is a likely possibility.