Induction of broadly neutralizing antibodies (bnAbs) is a major HIV vaccine goal. Germline-targeting immunogens aim to initiate bnAb induction by activating bnAb germline precursor B cells. Critical unmet challenges are to determine whether bnAb precursor naïve B cells bind germline-targeting immunogens and occur at sufficient frequency in humans for reliable vaccine responses. We employed deep mutational scanning and multi-target optimization to develop a germline-targeting immunogen (eOD-GT8) for diverse VRC01-class bnAbs. We then used the immunogen to isolate VRC01-class precursor naïve B cells from HIV-uninfected donors. Frequencies of true VRC01-class precursors, their structures, and their eOD-GT8 affinities support this immunogen as a candidate human vaccine prime. These methods could be applied to germline targeting for other classes of HIV bnAbs and for Abs to other pathogens.
Cystathionine β-synthase (CBS) controls the flux of sulfur from methionine to cysteine, a precursor of glutathione, taurine, and H 2 S. CBS condenses serine and homocysteine to cystathionine with the help of three cofactors, heme, pyridoxal-5′-phosphate, and S-adenosyl-L-methionine. Inherited deficiency of CBS activity causes homocystinuria, the most frequent disorder of sulfur metabolism. We present the structure of the human enzyme, discuss the unique arrangement of the CBS domains in the C-terminal region, and propose how they interact with the catalytic core of the complementary subunit to regulate access to the catalytic site. This arrangement clearly contrasts with other proteins containing the CBS domain including the recent Drosophila melanogaster CBS structure. The absence of large conformational changes and the crystal structure of the partially activated pathogenic D444N mutant suggest that the rotation of CBS motifs and relaxation of loops delineating the entrance to the catalytic site represent the most likely molecular mechanism of CBS activation by S-adenosyl-L-methionine. Moreover, our data suggest how tetramers, the native quaternary structure of the mammalian CBS enzymes, are formed. Because of its central role in transsulfuration, redox status, and H 2 S biogenesis, CBS represents a very attractive therapeutic target. The availability of the structure will help us understand the pathogenicity of the numerous missense mutations causing inherited homocystinuria and will allow the rational design of compounds modulating CBS activity.
Significance
Cystathionine β-synthase (CBS), the pivotal enzyme of the transsulfuration pathway, regulates flux through the pathway to yield compounds, such as cysteine, glutathione, taurine, and H
2
S, that control cellular redox status and signaling. Our crystal structure of an engineered human CBS construct bound to
S
-adenosylmethionine (AdoMet) reveals the unique binding site of the allosteric activator and the architecture of the human CBS enzyme in its activated conformation. Together with the basal conformation that we reported earlier, these structures unravel the molecular mechanism of human CBS activation by AdoMet. Current knowledge will allow for modeling of numerous pathogenic mutations causing inherited homocystinuria and for design of compounds modulating CBS activity.
CD22 maintains a baseline level of B-cell inhibition to keep humoral immunity in check. As a B-cell-restricted antigen, CD22 is targeted in therapies against dysregulated B cells that cause autoimmune diseases and blood cancers. Here we report the crystal structure of human CD22 at 2.1 Å resolution, which reveals that specificity for α2-6 sialic acid ligands is dictated by a pre-formed β-hairpin as a unique mode of recognition across sialic acid-binding immunoglobulin-type lectins. The CD22 ectodomain adopts an extended conformation that facilitates concomitant CD22 nanocluster formation on B cells and binding to trans ligands to avert autoimmunity in mammals. We structurally delineate the CD22 site targeted by the therapeutic antibody epratuzumab at 3.1 Å resolution and determine a critical role for CD22 N-linked glycosylation in antibody engagement. Our studies provide molecular insights into mechanisms governing B-cell inhibition and valuable clues for the design of immune modulators in B-cell dysfunction.
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