Phosphorylation sites at the C-terminus of the platelet-derived growth factor receptor bind phospholipase C gamma 1

We have identified two tyrosine phosphorylation sites, Tyr 1009 and Tyr 1021, in the C-terminal noncatalytic region of the human platelet-derived growth factor (PDGF) receptor beta subunit. Mutant receptors with phenylalanine substitutions at either or both of these tyrosines were expressed in dog epithelial cells. Mutation of Tyr 1021 markedly reduced the PDGF-stimulated binding of phospholipase C (PLC) gamma 1 but had no effect on binding of the GTPase activator protein of Ras or of phosphatidylinositol 3 kinase. Mutation of Tyr 1009 reduced binding of PLC gamma 1 less severely. Mutation of Tyr 1021, or both Tyr 1009 and Tyr 1021, also reduced the PDGF-dependent binding of a transiently expressed fusion protein containing the two Src-homology 2 domains from PLC gamma 1. Mutation of Tyr 1021, or both Tyr 1009 and Tyr 1021, greatly reduced PDGF-stimulated tyrosine phosphorylation of PLC gamma 1 but did not prevent the tyrosine phosphorylation of other cell proteins, including mitogen-activated protein kinase. We conclude that Tyr 1021, and possibly Tyr 1009, is a binding site for PLC gamma 1.

Localization of CapZ during myofibrillogenesis in cultured chicken muscle

Actin filaments undergo dramatic changes in their organization during myofibrillogenesis. In mature skeletal muscle, both CapZ and the barbed end of the actin filaments are located at Z-discs. In vitro, CapZ binds the barbed end of actin filaments and prevents actin subunit addition and loss; CapZ also nucleates actin polymerization in vitro. Taken together, these properties suggest that CapZ may function to organize actin filaments during myofibrillogenesis. We report here that the amount of CapZ in myofibrils from adult chicken pectoral muscle is sufficient to "cap" each actin filament of the sacromere. Double immunofluorescence microscopy of skeletal muscle cells in culture was used to determine the spatial and temporal distributions of CapZ relative to actin, alpha-actinin, titin, and myosin during myofibrillogenesis. Of particular interest was the assembly of CapZ at nascent Z-discs in relation to the organization of actin filaments in nascent myofibrils. In myoblasts and young myotubes, CapZ was diffusely distributed in the cytoplasm. As myotubes matured, CapZ was initially observed in a uniform distribution along non-striated actin filaments called stress fiber-like structures (SFLS). CapZ was observed in a periodic pattern characteristic of mature Z-discs along the SFLS prior to the appearance of a striated staining pattern for actin. In older myotubes, when actin was observed in a pattern characteristic of I-bands, CapZ was distributed in a periodic pattern characteristic of mature Z-discs. The finding that CapZ was assembled at nascent Z-discs before actin was observed in a striated pattern is consistent with the hypothesis that CapZ directs the location and polarity of actin filaments during I-band formation in skeletal muscle cells. The assembly of CapZ at nascent Z-disc structures also was observed relative to the assembly of sarcomeric alpha-actinin, titin, and thick filaments. Titin and myosin were observed in structures having the organization of mature sarcomeres prior to the appearance of CapZ at nascent Z-discs. The distribution of CapZ and sarcomeric alpha-actinin in young myotubes was not coincident; in older myotubes, both CapZ and alpha-actinin were co-localized at Z-discs. In cardiac myocytes, CapZ was detected at Z-discs and was distributed in a punctate pattern throughout the cytoplasm. CapZ also was co-localized with A-CAM and vinculin at cell-cell junctions formed by the myocytes.

Phosphorylation of Src mutants at Tyr 527 in fibroblasts does not correlate with in vitro phosphorylation by CSK

In normal fibroblasts, the product of the cellular src gene, p60c-src or Src, is repressed by phosphorylation at its C-terminal tyrosine residue, Tyr 527. Mutations in Src that prevent phosphorylation cause enzymatic activation and malignant transformation. The tyrosine kinases that phosphorylate Src at Tyr 527 in vivo have not been identified, but a tyrosine kinase known as CSK is an excellent candidate. CSK has the unusual ability to phosphorylate Src in vitro only at Tyr 527. To examine whether CSK has the appropriate sequence specificy to explain the phosphorylation of Src at Tyr 527 in fibroblasts, we have made use of a set of C-terminal substitution mutants of Src. These mutants were previously characterized for their levels of Tyr 527 phosphorylation when expressed in Rat2 fibroblasts. The ability of CSK to phosphorylate selected mutants has now been tested, using both in vitro phosphorylation assays and co-expression of CSK with the Src mutants in a heterologous organism, Saccharomyces cerevisiae. We also tested whether the mutant Src molecules could autophosphorylate at Try 527, by examining the phosphorylation state of catalytically active forms expressed in the absence of CSK in yeast cells. The results show that CSK has strict sequence specificity for the normal Src sequence, although it can also phosphorylate the Lck sequence. The other mutant Src molecules tested were not phophorylated by CSK, even though some of these mutants are highly phosphorylated at Tyr 527 in Rat 2 cells. All the mutants that are phosphorylated at Tyr 527 in Rat2 cells are also able to autophosphorylate at Tyr 527. The results suggest that CSK, autophosphorylation, and phosphorylation by kinases other than CSK, may all contribution to repressing Src catalytic activity in fibroblasts.

Overexpressed Drosophila src 64B is phosphorylated at its carboxy-terminal tyrosine, but is not catalytically repressed, in cultured Drosophila cells

Little is known about the regulation of non-receptor tyrosine kinases in invertebrates. We have studied the relationship between the phosphorylation state of the Drosophila src 64B (Dsrc) gene product, p62D, and its tyrosine kinase activity in Drosophila Schneider 2 cells, using wild-type and mutated Dsrc constructs that were overexpressed by transient transfection. Phosphopeptide mapping showed that the putative regulatory C-terminal tyrosine (Tyr-547) of p62D was phosphorylated in vivo. In contrast to vertebrate src family kinases overexpressed in fibroblasts, wild-type p62D overexpressed in Schneider 2 cells was phosphorylated at additional tyrosines outside of the C-terminus. These tyrosines corresponded to the major in vitro autophosphorylation sites. Overexpression of wild-type p62D or several catalytically active p62D mutants significantly increased the phosphorylation of numerous Schneider cell proteins on tyrosine, while expression of catalytically inactive mutants of p62D had no such effect. Thus, in contrast to the repression of src family kinase activity in fibroblasts, p62D is catalytically active when overexpressed in Drosophila cells, perhaps because of substoichiometric C-terminal tyrosine phosphorylation. These results raise the possibility that fly development will be sensitive to ectopic expression of p62D.

Effects of in vitro amiodarone exposure on alveolar macrophage inflammatory mediator production

Administration of amiodarone, although often lifesaving, is associated with pulmonary side effects. Patients with amiodarone pulmonary toxicity can present with either a chronic disorder that suggests pulmonary fibrosis or a more acute process. Mechanisms of acute pulmonary injury resulting from amiodarone are unclear. Previous studies have demonstrated that the drug is preferentially concentrated in alveolar macrophages. In the present study, the authors examined whether in vitro exposure to amiodarone resulted in alteration of rat alveolar macrophage superoxide, leukotriene B4, or fibronectin release. In addition, the authors assessed whether macrophages were ultrastructurally altered by in vitro amiodarone exposure. Twenty four hour exposure to therapeutic tissue concentrations of amiodarone resulted in enhancement of phorbol myristate acetate-stimulated macrophage superoxide release. In addition, 48 hours exposure to amiodarone caused a dose-dependent inhibition of spontaneous fibronectin release by macrophages. Macrophages exposed to 48 hours of 10 micrograms/ml amiodarone were ultrastructurally abnormal, containing lamellar inclusions and demonstrating a large degree of vacuolization. The authors concluded that alveolar macrophages are very sensitive to therapeutic tissue concentrations of amiodarone. Alteration of macrophage mediator release by amiodarone may be one mechanism for lung damage induced by the drug.

Different effects of dopaminergic and anticholinergic therapies on cognitive and motor function in Parkinson's disease. A follow-up study of untreated patients

The cognitive performance of a group of 82 newly diagnosed patients with Parkinson's disease who had never been treated was reassessed approximately 4 mths after randomization to one of three monotherapies (levodopa, bromocriptine or anticholinergic drugs). Dopaminergic and anticholinergic treatments both led to improvement in motor control but their effects upon cognitive performance dissociated. Anticholinergic drugs produced impairment in processes underlying the immediate registration of information whilst dopaminergic therapy produced improvement on a task dependent on working memory and cognitive sequencing. Other cognitive measures showed no change on treatment. The deficits that were affected by cholinergic and dopaminergic modulation are those that were most compromised in the early, untreated state in Parkinson's disease. The data support the notion that cognitive impairment in Parkinson's disease is multifactorial in origin: short-term memory processes are served by both dopaminergic and cholinergic subcortico-frontal systems but much of the cognitive impairment of Parkinson's disease is independent of this subcortical neurochemical pathology and may be due to early neuronal dysfunction within the cerebral cortex.

Effects of null mutations and overexpression of capping protein on morphogenesis, actin distribution and polarized secretion in yeast

CAP1, the gene encoding the alpha subunit of Saccharomyces cerevisiae capping protein, was cloned using a probe prepared by PCR with primers based on the amino acid sequence of purified alpha subunit peptides. The sequence is similar to that of capping protein alpha subunits of other species but not to that of the S. cerevisiae capping protein beta subunit or any other protein. Null mutants of capping protein, prepared by deletion of the coding region of CAP1 and CAP2 separately or together, are viable and have a similar phenotype. Deletion of the gene for one subunit leads to a loss of protein for the other subunit. The null mutant has a severe deficit of actin cables and an increased number of actin spots in the mother. Cells are round and relatively large. These features are heterogeneous within a population of cells and vary with genetic background. Overexpression of CAP1 and CAP2 also causes loss of actin cables and cell enlargement, as well as the additional traits of aberrant morphogenesis and cell wall thickening. Capping protein null strains and overexpression strains exhibited normal polarized secretion during bud growth as demonstrated by labeling with fluoresceinated Con A. Projection formation and chitin deposition in response to mating pheromone, mating efficiency, and bud site selection were also normal in capping protein null strains. In addition, bulk secretion of invertase was unimpaired. These data indicate that actin cables are not required for polarized secretion in S. cerevisiae.

Developmentally regulated expression of a mitogen-activated protein kinase in Xenopus laevis

Mitogen-activated protein (MAP) kinases are activated in somatic cells in response to many extracellular stimuli and in oocytes during meiotic maturation. We have examined the tissue specificity of expression of a MAP kinase (Xp42) in adult and larval Xenopus laevis. MAP kinase RNA and protein were abundant in the nervous system and lymphoid tissues and were readily detected in most other organs. A remarkably high level of RNA was detected in ovary. Fractionation of oocytes showed that MAP kinase RNA is expressed at the highest level in small oocytes, suggesting that it is a maternal RNA that is stored for early embryogenesis. The levels of MAP kinase RNA and protein did not change from the time of fertilization through to late blastula. The results are consistent with functions for MAP kinases in signal transduction in embryonic as well as adult cells.

Membrane-associated proteases process Plasmodium falciparum merozoite surface antigen-1 (MSA1) to fragment gp41

The Plasmodium falciparum merozoite surface antigen-1 (MSA1) undergoes stage-specific processing; this processing appears isolate-specific during cleavage to fragment gp41. Recombinant substrates were prepared from the two allelic forms of MSA1; the MAD20 substrate was cleaved at four sites in the molecule whilst the K1 form was cleaved once. However both parasite isolates, although expressing different allelic forms of MSA1, possess the same repertoire of MSA1-specific proteases. The cleavage site in native gp41 is conserved between P. falciparum isolates. The specificity of substrate cleavage was determined by N-terminal sequencing of cleaved substrate fragments; two cleavage sites, identical to native MAD20 processed fragments, were not conserved between alleles. An additional non-conserved site was cleaved by an erythrocyte protease. The MSA1-specific proteases were membrane-associated but soluble forms were purified by anion-exchange chromatography. The gp41-specific protease activity was inhibited by serine, thiol and metalloprotease inhibitors whilst the two other MSA1-specific proteases were serine proteases (as was the erythrocyte protease).

The cost of keeping one's hands clean

After reading 'A Practical Gloving and Handwashing Regimen for Dental Practice' (Field E A, Jedynakiewicz N M, King C M. A Practical Gloving and Handwashing Regimen for Dental Practice. Br Dent J 1992; 172: 111-113), I undertook to do some simple research into the economics of glove wearing for the general dental practitioner based on the regimens mentioned in the article. The conclusion which I reached is that one not only needs to take into account the cost of the gloves but also the valuable surgery time taken in the washing of one's hands and gloves. The most cost-effective method of hand and glove washing was found to be the use of a single non-sterile glove with Hibisol.

Cognitive components of reaction time in Parkinson's disease

Studies of reaction time in Parkinson's disease (PD) have suggested a selective deficit in simple reaction time (SRT), compared with choice reaction time (CRT). This finding has been interpreted as a deficit in motor preprogramming but could involve other factors, such as attentional focussing and stimulus predictability. Moreover, not all studies show the same selective deficit, possibly because of differences in patient selection and treatment effects. The neurochemical basis of RT deficits in PD remains unclear. Accordingly, the contribution of cognitive factors to impaired RT was assessed in a large group of PD patients, including early untreated cases, and performance was examined in relation to clinical variables and the effect of treatment in longitudinal study. Motor output was constant in both SRT and CRT tasks. In the SRT task, all stimuli required a response; in the CRT task, subjects were required to respond to only one of the two possible stimuli. Attentional focussing on SRT was examined by variation of the interval between cue and stimulus; effects of stimulus uncertainty were evaluated from a comparison of SRT and CRT; temporal predictability of the stimulus was examined from a comparison of conditions in which the interval between warning signal and imperative stimulus was constant or variable. The PD patients showed similar deficits in SRT and CRT, but normal effects of cue-stimulus interval and temporal predictability. Reaction time correlated with measures of global cognitive capacity and frontal-lobe function, as well as motor disability. Treatment had no effect on SRT or CRT, despite clinical benefit. These findings indicate that RT deficits in PD are not due to impaired attentional focussing or stimulus predictability but are compatible with a deficit in higher-order processes concerned with the orientation of both cognitive and motor responses to a stimulus. These processes are not substantially dopamine-dependent but may be served by non-dopaminergic neurotransmission.

Effect of patient motion on tomographic myocardial perfusion imaging

We evaluated the effect of patient motion on inducing false-positive tomographic 201Tl myocardial perfusion studies. The effects of the angle of camera rotation at which movement occurs, the direction of movement and the distance of movement were studied. Movement was stimulated by shifting the raw data from normal motion-free 201Tl tomographic myocardial perfusion studies. The visual detectability of motion artifact was evaluated with receiver-operating characteristic curve analysis. The clinical importance of patient movement was determined by measuring the incidence of quantitative bull's-eye abnormalities induced by motion. Visual artifacts were more detectable and quantitative abnormalities more frequent as the distance of movement increased. Artifacts from 3.25 mm of movement were not visually detectable. Artifacts from 6.5 mm of movement were visually detectable, but were infrequently clinically important. Movement of 13 mm or greater frequently caused quantitative abnormalities. Quantitative abnormalities from axial movement were more frequent than artifacts from lateral movement. Quantitative abnormalities were more frequent when the movement occurred at the beginning or end. We conclude that when patients move during 201Tl tomographic myocardial perfusion imaging, the incidence and character of false-positive results depend on the angle of camera rotation at which the movement occurs, the direction of the movement and distance of the movement.

Phosphorylation and regulatory effects of the carboxy terminus of a Drosophila src homolog

The kinase activities of the vertebrate src family members are repressed by phosphorylation of a tyrosine residue in the carboxy-terminal 'tail' of these molecules. To explore whether the tail of an invertebrate src homolog might also serve a regulatory function, we examined the ability of the carboxy terminus of a Drosophila src homolog (p62D), which contains a tyrosine homologous to those in the vertebrate src family members, to regulate the following molecules in mammalian fibroblasts: (1) a chimeric protein, p60CD, containing the amino terminus and catalytic domains of chicken p60c-src joined to the C-terminus of p62D; and (2) full-length p62D itself. By a variety of criteria p60CD appears to be a partially, rather than fully, repressed form of p60c-src. Phosphopeptide mapping indicates that partial repression correlates with partial phosphorylation of the tyrosine in the p62D tail of the chimera. Phosphorylation of the tail may also regulate full-length p62D. Expression of p62D in fibroblasts does not affect cell morphology or the overall abundance of tyrosine-phosphorylated proteins. The molecule is phosphorylated at its C-terminal tyrosine (Tyr-547), but not at its in vitro autophosphorylation sites, suggesting that it is catalytically repressed in fibroblasts. Expression of a truncated p62D mutant lacking Tyr-547 is associated with a clear alteration in cellular morphology and a two- to threefold increase in cellular phosphotyrosine levels. These results suggest that phosphorylation of the C-terminal tyrosine of the tail of an invertebrate src-like kinase can repress the activity of adjacent catalytic domains.

Purification and characterization of mitogen-activated protein kinase activator(s) from epidermal growth factor-stimulated A431 cells

Two peaks of mitogen-activated protein (MAP) kinase activator activity are resolved upon ion exchange chromatography of cytosolic extracts from epidermal growth factor-stimulated A431 cells. Two forms of the activator (1 and 2) have been purified from these peaks, using chromatography on Q-Sepharose, heparin-agarose, hydroxylapatite, ATP-agarose, Sephacryl S-300, Mono S, and Mono Q. The two preparations each contained one major protein band with an apparent molecular mass of 46 or 45 kDa, respectively, on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Evidence identifying the MAP kinase activators as the 46- and 45-kDa proteins is presented. Using inactive mutants of MAP kinase as potential substrates, it was found that each preparation of MAP kinase activator catalyzes phosphorylation of the regulatory residues, threonine 188 and tyrosine 190, of Xenopus MAP kinase. These results support the concept that the MAP kinase activators are protein kinases. These MAP kinase kinases demonstrate an apparent high degree of specificity toward the native conformation of MAP kinase, although slow autophosphorylation on serine, threonine, and tyrosine residues and phosphorylation of myelin basic protein on serine and threonine residues is detected as well.

Localization of capping protein in chicken epithelial cells by immunofluorescence and biochemical fractionation

We have localized capping protein in epithelial cells of several chicken tissues using affinity-purified polyclonal antibodies and immunofluorescence. Capping protein has a distribution in each tissue coincident with proteins of the cell-cell junctional complex, which includes the zonula adherens, zonula occludens, and desmosome. "En face" views of the epithelial cells showed capping protein distributed in a polygonal pattern coincident with cell boundaries in intestinal epithelium, sensory epithelium of the cochlea, and the pigmented epithelium of the retina and at regions of cell-cell contact between chick embryo kidney cells in culture. "Edge-on" views obtained by confocal microscopy of intact single intestinal epithelial cells and of retinal pigmented epithelium showed that capping protein is located in the apical region of the epithelial cells coincident with the junctional complexes. These images do not resolve the individual types of junctions of the junctional complex. Immunolabeling of microvilli or stereocilia was faint or not detectable. Capping protein was also detected in the cytoplasm of intact intestinal epithelial cells and in nuclei of cells in the pigmented retina and in the kidney cell cultures, but not in nuclei of cells of the intestinal epithelium or sensory epithelium. Biochemical fractionation of isolated intestinal epithelial cells shows capping protein in the brush border fraction, which contains the junctional complexes, and in the soluble fraction. These results are consistent with the results of the immunolabeling experiments. Highly purified microvilli of the brush borders also contained capping protein; this result was unexpected based on the low intensity of immunofluorescence staining of microvilli and stereocilia. The microvilli were not contaminated with junctional complexes, as defined by the absence of several markers for cell junctions. The cause and significance of this discrepancy is not certain at this time. Since capping protein binds the barbed end of actin filaments in vitro, we hypothesize that capping protein is bound to the barbed ends of actin filaments associated with one or more of the junctions of the junctional complex.

A component analysis of the generation and release of isometric force in Parkinson's disease

Paradigms of isometric force control allow study of the generation and release of movement in the absence of complications due to disordered visuomotor coordination. The onset and release of isometric force in Parkinson's disease (PD) was studied, using computerised determinants of latency of response and rate of force generation and release. Components of isometric force control were related to measures of cognitive, affective and clinical motor disability. The effects of treatment were determined by longitudinal study of de novo patients. Patients with PD showed impairment in latency and rate of force change for movement release as well as onset. Rate of force change correlated with depression, clinical motor disability and memory quotient but latency showed no correlation with any of these measures. Treatment improved rate of force release, in concert with clinical motor disability, but not latency. These results suggest dissociations between latency and rate of force change that may be linked to different neurochemical deficits. Further, they demonstrate akinetic deficits in force release that argue against the "neural energy hypothesis" of akinesia.

Molecular signal integration. Interplay between serine, threonine, and tyrosine phosphorylation

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Purification, characterization, and immunofluorescence localization of Saccharomyces cerevisiae capping protein

Capping protein binds the barbed ends of actin filaments and nucleates actin filament assembly in vitro. We purified capping protein from Saccharomyces cervisiae. One of the two subunits is the product of the CAP2 gene, which we previously identified as the gene encoding the beta subunit of capping protein based on its sequence similarity to capping protein beta subunits in chicken and Dictyostelium (Amatruda, J. F., J. F. Cannon, K. Tatchell, C. Hug, and J. A. Cooper. 1990. Nature (Lond.) 344:352-354). Yeast capping protein has activity in critical concentration and low-shear viscometry assays consistent with barbed-end capping activity. Like chicken capping protein, yeast capping protein is inhibited by PIP2. By immunofluorescence microscopy yeast capping protein colocalizes with cortical actin spots at the site of bud emergence and at the tips of growing buds and shmoos. In contrast, capping protein does not colocalize with actin cables or with actin rings at the site of cytokinesis.

GTPase-activating protein and phosphatidylinositol 3-kinase bind to distinct regions of the platelet-derived growth factor receptor beta subunit

In response to binding of platelet-derived growth factor (PDGF), the PDGF receptor (PDGFR) beta subunit is phosphorylated on tyrosine residues and associates with numerous signal transduction enzymes, including the GTPase-activating protein of ras (GAP) and phosphatidylinositol 3-kinase (PI3K). Previous studies have shown that association of PI3K requires phosphorylation of tyrosine 751 (Y751) in the kinase insert and that this region of receptor forms at least a portion of the binding site for PI3K. In this study, the in vitro binding of GAP to the PDGFR was investigated. Like PI3K, GAP associates only with receptors that have been permitted to autophosphorylate, and GAP itself does not require tyrosine phosphate in order to stably associate with the phosphorylated PDGFR. To define which tyrosine residues are required for GAP binding, a panel of PDGFR phosphorylation site mutants was tested. Mutation of Y771 reduced the amount of GAP that associates to an undetectable level. In contrast, the F771 (phenylalanine at 771) mutant bound wild-type levels of PI3K, whereas the F740 and F751 mutants bound 3 and 23%, respectively, of the wild-type levels of PI3K but wild-type levels of GAP. The F740/F751 double mutant associated with wild-type levels of GAP, but no detectable PI3K activity, while the F740/F751/F771 triple mutant could not bind either GAP or PI3K. The in vitro and in vivo associations of GAP and PI3K activity to these PDGFR mutants were indistinguishable. The distinct tyrosine residue requirements suggest that GAP and PI3K bind different regions of the PDGFR. This possibility was also supported by the observation that the antibody to the PDGFR kinase insert Y751 region that blocks association of PI3K had only a minor effect on the in vitro binding of GAP. In addition, highly purified PI3K and GAP associated in the absence of other cellular proteins and neither cooperated nor competed with each other's binding to the PDGFR. Taken together, these studies indicate that GAP and PI3K bind directly to the PDGFR and have discrete binding sites that include portions of the kinase insert domain.

Strenuous exercise, plasma fibrinogen, and factor VII activity

OBJECTIVE

To assess the effect of physical activity on plasma fibrinogen and factor VII activity and thus on the risk of ischaemic heart disease.

DESIGN

Cross sectional survey.

SETTING

Ten group practices in the Medical Research Council's General Practice Research Framework.

PATIENTS

3967 men aged 45-69 attending screening clinics for the thrombosis prevention trial.

METHODS

Structured interview to elicit the intensity and frequency of physical exercise during past month. Measurement of fibrinogen, factor VII activity, cholesterol concentration, blood pressure, and other indices of ischaemic heart disease risk.

RESULTS

Strenuous exercise was associated with significantly lower fibrinogen concentrations than mild exercise, implying a difference of about 15% in the risk of ischaemic heart disease. Strenuous exercise was also associated with lower cholesterol concentrations. More frequent strenuous exercise was associated with lower factor VII activity.

CONCLUSIONS

With the recognition of plasma fibrinogen as a strong index of ischaemic heart disease risk the results of this and other studies suggest a pathway through which the protective effect of strenuous exercise may partly be mediated and they provide doctors and patients with a valuable incentive towards prevention, particularly in those whose risk of ischaemic heart disease is substantially due to raised fibrinogen concentrations.

Mapping of the region predominantly recognized by antibodies to the Plasmodium falciparum merozoite surface antigen MSA 1

The locations of the epitopes of a panel of mouse monoclonal antibodies directed against the Plasmodium falciparum merozoite surface antigen MSA 1 were mapped by using naturally occurring processed fragments, by chemical cleavage of the protein and by comparison of the isolate-specificity of binding with known sequence variation. By these criteria, the most antigenic region occurs in the cysteine-rich, invariant 19-kDa carboxyl terminal domain with 12/19 monoclonal antibodies (mAbs) binding to this region. One of these mAbs recognized an epitope near the C-terminal putative glycosylphosphatidylinositol anchor site. This was the only mAb which significantly inhibited parasite growth in vitro. The other mAbs recognized conformational epitopes involving the cysteine residues located throughout this fragment. This study has identified further naturally occurring processing sites and a consensus processing site sequence is now emerging.

Phosphorylation sites in the PDGF receptor with different specificities for binding GAP and PI3 kinase in vivo

Tyrosine residues have been identified in the human platelet-derived growth factor (PDGF) receptor beta-subunit whose phosphorylation is stimulated by PDGF. These sites are also in vitro autophosphorylation sites. There are a total of three phosphorylation sites in the kinase insert region, tyrosines 740, 751 and 771. Mutagenesis studies show that Tyr740 and 751 are involved in the PDGF-stimulated binding of phosphatidylinositol (PI) 3 kinase, and Tyr771 is required for efficient binding of GAP, the GTPase activator of Ras. The requirement for Tyr751 is only detected at low PDGF receptor levels, suggesting that it increases the affinity of binding of PI3 kinase but is not absolutely required. Small deletions in the kinase insert only 10 residues from Tyr740 and Tyr771 do not significantly reduce binding of PI3 kinase or GAP, indicating that distant sequences are probably unimportant for recognition. The data suggest that the receptor signals to different pathways via different phosphorylated tyrosines, and that certain proteins, such as PI3 kinase, can recognize two phosphorylated tyrosines in a single receptor.

Identification and characterization of an actin-binding site of CapZ

A mAb (1E5) that binds the COOH-terminal region of the beta subunit of chicken CapZ inhibits the ability of CapZ to bind the barbed ends of actin filaments and nucleate actin polymerization. CapZ prepared as fusion proteins in bacteria or nonfusion proteins by in vitro translation has activity similar to that of CapZ purified from muscle. Deletion of the COOH-terminus of the beta subunit of CapZ leads to a loss of CapZ's ability to bind the barbed ends of actin filaments. A peptide corresponding to the COOH-terminal region of CapZ beta, expressed as a fusion protein, binds actin monomers. The mAb 1E5 also inhibits the binding of this peptide to actin. These results suggest that the COOH-terminal region of the beta subunit of CapZ is an actin- binding site. The primary structure of this region is not similar to that of potential actin-binding sites identified in other proteins. In addition, the primary structure of this region is not conserved across species.