Electron microscopy of alpha 2-macroglobulin with a thiol ester bound ligand

Determination of Rigidity of Protein Bound Au(144) Clusters by Electron Cryomicroscopy

A method for estimating the positional displacement of protein bound gold nanoparticles is presented and used to estimate the rigidity of linkage of Au(144) nanoparticles bound to a tetrameric model protein. We observe a distribution of displacement values where most Au(144) clusters are immobilized to within 3Å relative to the protein center of mass. The shape of the distribution suggests two physical processes of thermal motion and protein deformation. The application of this and similar rigid gold nanoparticle/protein conjugates in high resolution single particle electron cryo-microscopy is discussed.

Inhibition of intracellular proteolytic processing of soluble proproteins by an engineered alpha 2-macroglobulin containing a furin recognition sequence in the bait region

The bait region of the general protease inhibitor alpha 2-macroglobulin (alpha 2M) was mutated by introducing a recognition sequence of furin. This did not interfere with folding, S-ester formation or tetramerization of the mutant recombinant alpha 2M (r alpha 2M). Mutant r alpha 2M inhibited furin in vitro, by a similar mechanism to that used by plasma alpha 2M to inhibit high-molecular-mass proteases. The mutant alpha 2M was intracellularly active in COS-1 cells in inhibiting the endogenous processing of the soluble substrates for furin (von Willebrand factor, transforming growth factor beta1 and a soluble form of the envelope glycoprotein gp160 from HIV-1) but not the membrane-bound form of gp160. The intracellular activity of mutant alpha 2M strongly indicated that alpha 2M attains its native conformation, and thus that the unusual internal S-ester is formed, before alpha 2M passes through the cleavage compartment(s). Our results show for the first time that modulation of the bait region of alpha 2M allows the creation of an inhibitor against membrane-bound proteases. It can be expected that the use of alpha 2M-bait mutants will become important as a technique for the study of various proteolytic processes and for the identification of the proteases involved.

Design of a new protease inhibitor by the manipulation of the bait region of alpha 2-macroglobulin: inhibition of the tobacco etch virus protease by mutant alpha 2-macroglobulin

Human alpha 2-macroglobulin (alpha 2M) inhibits a broad spectrum of proteases by changing its conformation and physically confining the enzyme. The inhibitory spectrum of alpha 2M is defined by a stretch of 39 amino acids, the bait region, located near the middle of the alpha 2M monomers. To investigate whether a new inhibitory specificity can be introduced by the manipulation of the bait region, recombinant alpha 2M (r alpha 2M) was produced in which the primary cleavage site was replaced by a heptapeptide containing the cleavage specificity of tobacco etch virus (TEV) protease. This protease is not inhibited by wild-type alpha 2M. The r alpha 2M, produced in an episomal expression system, was fully functional and able to inhibit the tobacco etch virus protease according to its normal 'trap' mechanism. The manipulation of the bait region of alpha 2M thus allows the design of new, specific protease inhibitors.

Low resolution X-ray structure of human methylamine-treated alpha 2-macroglobulin

The structure of methylamine-treated human alpha 2-macroglobulin (alpha 2M-Ma), a 720-kDa tetrameric inactivated proteinase inhibitor from plasma, has been determined to a resolution of 10 A. Data were collected with synchrotron radiation at 120 K, and phases were calculated by multiple isomorphous replacement and solvent flattening. A novel feature of the structure of alpha 2-M is present in its proteinase-binding cavity, dividing it into two compartments. The potential sites for proteinase entrapment in these compartments are sterically restricted. The positions of the thiol groups appearing from the functional important thiol esters upon their cleavage have been determined. They are found at the walls of the compartments at the center of the structure. The overall structure of alpha 2M-MA is much more sphere-like than previously inferred from electron microscopy studies. However, several aspects of the structure are well described by recent three-dimensional reconstructions. Possible models for the monomer, the disulfide bridged dimer, and native alpha 2M are discussed.

Alpha 2-macroglobulin bait region variants. A role for the bait region in tetramer formation

To test the hypothesis that a large portion of the bait region of human alpha 2-macroglobulin (alpha 2M) can be removed without adversely affecting the protein's structural and functional properties, we expressed two human alpha 2M variants with truncated bait regions and examined whether these variants folded normally and functioned as proteinase inhibitors. Each variant contains sites that are normal bait region cleavage sites in wild-type alpha 2M, including the primary trypsin cleavage site. The truncated bait regions are shorter by 23 and 27 residues, respectively, and lack the C-terminal portion as well as different parts of the N-terminal section of the bait region. We found that such bait region truncation permitted normal folding of the monomers as well as formation of the thiol ester and dimerization by disulfide cross-linking, although the resulting species bound 6-(p-toluidino)-2-naphthalenesulfonic acid in a manner more like thiol ester-cleaved alpha 2M than native alpha 2M. The variants' thiol esters reacted with nucleophiles at rates identical to wild-type alpha 2M. Surprisingly, however, the truncations prevented the noncovalent association of the covalent 360-kDa dimers that normally gives tetrameric alpha 2M, decoupled bait region cleavage from thiol ester activation, and resulted in the inability of the two variants to "trap" proteinase. This was despite apparent cleavage of the bait region by proteinase, albeit at very much reduced rates relative to wild-type tetrameric alpha 2M. The kinetics of thiol ester cleavage-dependent protein conformational changes also changed from sigmoidal to exponential. These findings indicate that residues in the bait region appear to be necessary for noncovalent association of 360-kDa disulfide-linked dimers to give tetrameric alpha 2M and suggest a role for the bait region in normal alpha 2M in coupling bait region cleavage to the sequence of conformational changes that result in thiol ester activation and ultimately proteinase trapping.

Three-dimensional structures of the human alpha 2-macroglobulin-methylamine and chymotrypsin complexes

The three-dimensional structures of chymotrypsin- and methylamine-treated negatively stained human alpha 2-macroglobulin have been determined by weighted back projection from electron microscope data. Projections of the reconstructions show good concordance with two-dimensional averages of both stained and frozen-hydrated molecules. The reconstructions reveal that the H-shaped front projection of the molecule is related to the smaller ellipsoidal end view by a rotation of 90 degrees about the crossbar (minor axis) of the H. This finding is in agreement with tilt studies. The reconstruction of the alpha 2-macroglobulin-methylamine reveals an hour-glass shaped void which is filled by the two proteinase molecules in the reconstruction of alpha 2-macroglobulin-chymotrypsin. Protein plugs which appear to block the exterior entrances to the cavity may function to prevent access of proteins to the encapsulated proteinase and serve to block its escape. Extensive thresholding of each reconstruction leaves a "backbone" consisting of two side-by-side rod-like structures, suggesting that this is the arrangement of the two protomeric units which form the molecule. Both structures show some departure from the expected symmetry. The asymmetries are robust features of the reconstructions and may reflect structurally asymmetric features of the transformation from the native to the chymotrypsin-treated form of the molecule.

Ultrastructure of alpha 2-macroglobulins

New results concerning the ultrastructure of human alpha 2-macroglobulin (alpha 2M) molecules are presented in connection and comparison with the historical, the current and our own most recent, even unpublished results on the structure and function of alpha 2M and related proteins. The electron microscopic approach uses classical negative staining, combined with the new imaging mode "Electron Energy Loss Spectroscopy", which provides unusual contrast, resolution and readability of the electron micrographs. Immuno- and cryoelectron microscopy, as well as image processing has provided new data necessary to the building of tentative 3D models of the molecule. A model for the native tetrameric alpha 2M is described for the first time, and tries to explain and gather the various observations, sometimes contradictory, taken from different laboratories. A revised version for a model of the methylamine- and proteinase-transformed forms of alpha 2M is also shown. The probable positions of the bait regions and the thiol esters are given on both models. We confirm that alpha 2M is a twin trap capable of inactivating one or two proteinases by partial immobilization. Preliminary results on the production of crystals of alpha 2M-chymotrypsin complexes are also presented. A critical analysis of our models is presented in comparison with others. The technical limitations reached with some techniques and some possible extensions of future research in the field are also presented.