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. 2008 Oct 3;4(10):e1000171. doi: 10.1371/journal.ppat.1000171

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This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration, which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

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(A) Based upon the structures of the unliganded SIV core gp120 [40] and the CD4-bound HIV-1 core [41], the movements of the co-receptor site bridging sheet beta-strands are highlighted in green. Note that 17b binds to the unliganded HIV-1 core with low affinity (∼1 uM) consistent with the “split orientation” of the bridging sheet revealed by the unliganded SIV core structure. 17b can also recognize full-length gp120 with high affinity [6] and the gp140 trimers as demonstrated in this study (see Figs 2 and 3). These observations suggest that the bridging sheet may not be in exactly the same conformation in the full-length protein context as it is in the original gp120 core protein structures with and without CD4. (B) A model of how the co-receptor region (green) is recognized by the naïve B cell receptor repertoire. In the left and middle panel, the co-receptor binding site is not formed. Following binding to high-affinity primate CD4 (likely on the surface of CD4⁺ cells), the bridging sheet is formed and locked into a single, CD4-dependent conformation, which then allows elicitation of co-receptor-site-directed, CD4i antibodies (right panel). The somatically mutated and affinity-matured 17b antibody can recognize either conformation, presumably inducing a proper fit of the unliganded bridging sheet conformation.