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Electron microscopic images of negatively stained fibrinogen are predominantly asymmetric rods 450 A in length and about 60 A in width. The molecules appear to have considerable flexibility, and mass distribution along the major axis is not uniquely distinguished despite apparent beading in some particles. Scanning transmission electron microscopy of unstained fibrinogen again demonstrates that a majority of molecules are rodlike. The results differ from those obtained by negative staining in that a substantial fraction of images are trinodular with striking resemblance to those obtained by C. Electron microscopic studies involving the high-resolution technique of negative staining have resulted either in overcrowded images wherein one cannot see isolated molecules or in small viewing fields where it is difficult to be certain if a "particle" is fibrinogen or the result of electron microscopic artifacts. These studies have been interpreted in terms of a number of models for the fibrinogen molecule, ranging from a much more elongated multibeaded molecule (4, 7) to an isotropic pentagonal dodecahedron (5). We present evidence here that conventional negative staining methods, which are so successful in imaging well-behaved macromolecules, fail to provide negatively stained fields of fibrinogen that fulfill reasonable prerequisites for meaningful interpretation.Our major purpose here is to define precise conditions for the negative staining of fibrinogen wherein the following criteria are always met: (i) The great majority of images of "apparent particles" in any field are of fibrinogen molecules. (Ui) The number of identifiable fibrinogen molecules is defined by fibrinogen concentration and application time and not by the buffer, pH, or staining conditions. (Mii) Large-field views of fibrinogen contain predominantly well-separated molecules with clearly defined morphology and dimensions.Conditions required to achieve these goals are primarily related to attachment of fibrinogen to the substrate film. Once this problem was solved, images of the unstained molecule were also obtained by high-resolution scanning transmission electron microscopy (STEM).The data presented here may be combined with published work to present an overall interpretation of the general morphology of the fibrinogen molecule that is free of ambiguities. MATERIALS AND METHODSThe fibrinogen used in this work was a kind gift of Birger Blomback. For the neutral pH studies, bovine or human fraction I-4 (>95% clottable), lyophilized or frozen in 0.1 M Tris.HCl pH 7.4, was dialyzed against 0.02 M sodium phosphate buffer. For the low pH studies, the fibrinogen was dialyzed against 0.01 M acetic acid. Covalent crosslinking of fibrinogen was accomplished by adding 10% glutaraldehyde to a solution of bovine fibrinogen at 1.5 mg/ml (0.2 M sodium phosphate buffer, pH 7.0), to give a final glutaraldehyde concentration of 2%. The solution was allowed to react at room temperature for 90 min and was then extensively dialyzed at 40C against 0.02 M sodiu...

SUMMARY Uranyl sulphate is a negative stain of high quality for electron microscopy of macromolecules below their isoelectric point. The stain results in good contrast and high resolution as demonstrated by optical diffraction of periodic structure. Analysis of scanning transmission electron microscopic data reveals that uranyl sulphate is lower in background noise level than uranyl acetate and is dramatically more resistant toward granularization upon continued exposure to electron irradiation. Electron microscopic images of most macromolecules contrasted with uranyl sulphate were indistinguishable from those obtained with uranyl acetate. However, electron microscopic images of Reo virus contrasted with uranyl sulphate are always readily distinguished from those obtained with uranyl acetate.

DAB486IL-2 is a genetically engineered fusion protein consisting of a portion of diphtheria toxin fused to human IL-2. It is specifically cytotoxic for tumor cells which bear high-affinity IL-2 receptors (IL-2R). DAB389IL-2 is a similarly constructed hybrid protein which is smaller than DAB486IL-2 and is slightly more potent in vitro. We have developed a murine model of IL-2R-expressing malignancy to study the in vivo efficacy of these genetically engineered cytotoxins. Following intravenous administration of CP3 cells, C57BL/6 mice develop tumors which are lymphatic in distribution. When mice are injected i.v. with 10(6) CP3 cells, 90% of the animals show signs of observable tumor by day 10 to 20; death occurs in 50% of untreated animals by day 30. Intravenous treatment of mice with DAB486IL-2 (10 micrograms daily for 10 days), beginning 24 hr after administration of CP3 cells, increases mean survival time by approximately 50%. In comparative studies, DAB389IL-2 is more potent in vivo than DAB486IL-2, with approximately 90% of treated animals with no evidence of tumor at 60 days. The mechanism of action of tumor inhibition by DAB486IL-2 is specific, since treatment of animals which have IL-2R-negative EL4 tumors has not resulted in increased survival time. In addition, treatment of such tumors with DAglu53B486IL-2, a fusion protein which can bind to the IL-2R but is incapable of inhibiting protein synthesis, is ineffective.