Kaplan Gilad, Roitburd-Berman Anna, Lewis George K, Gershoni Jonathan M
Department of Cell Research and Immunology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel.
Division of Vaccine Research, The Institute of Human Virology, University of Maryland School of Medicine, Baltimore, Maryland, USA.
J Virol. 2016 Apr 14;90(9):4481-4493. doi: 10.1128/JVI.03206-15. Print 2016 May.
The HIV envelope binds cellular CD4 and undergoes a range of conformational changes that lead to membrane fusion and delivery of the viral nucleocapsid into the cellular cytoplasm. This binding to CD4 reveals cryptic and highly conserved epitopes, the molecular nature of which is still not fully understood. The atomic structures of CD4 complexed with gp120 core molecules (a form of gp120 in which the V1, V2, and V3 loops and N and C termini have been truncated) have indicated that a hallmark feature of the CD4-bound conformation is the bridging sheet minidomain. Variations in the orientation of the bridging sheet hairpins have been revealed when CD4-liganded gp120 was compared to CD4-unliganded trimeric envelope structures. Hence, there appears to be a number of conformational transitions possible in HIV-1 monomeric gp120 that are affected by CD4 binding. The spectrum of CD4-bound conformations has been interrogated in this study by using a well-characterized panel of conditional, CD4-induced (CD4i) monoclonal antibodies (MAbs) that bind HIV-1 gp120 and its mutations under various conditions. Two distinct CD4i epitopes of the outer domain were studied: the first comprises the bridging sheet, while the second contains elements of the V2 loop. Furthermore, we show that the unliganded extended monomeric core of gp120 (coree) assumes an intermediate CD4i conformation in solution that further undergoes detectable rearrangements upon association with CD4. These discoveries impact both accepted paradigms concerning gp120 structure and the field of HIV immunogen design.
Elucidation of the conformational transitions that the HIV-1 envelope protein undergoes during the course of entry into CD4(+)cells is fundamental to our understanding of HIV biology. The binding of CD4 triggers a range of gp120 structural rearrangements that could present targets for future drug design and development of preventive vaccines. Here we have systematically interrogated and scrutinized these conformational transitions using a panel of antibody probes that share a specific preference for the CD4i conformations. These have been employed to study a collection of gp120 mutations and truncations. Through these analyses, we propose 4 distinct sequential steps in CD4i transitions of gp120 conformations, each defined by antibody specificities and structural requirements of the HIV envelope monomer. As a result, we not only provide new insights into this dynamic process but also define probes to further investigate HIV infection.
HIV包膜与细胞CD4结合，并经历一系列构象变化，导致膜融合以及病毒核衣壳进入细胞质。这种与CD4的结合会暴露出隐蔽且高度保守的表位，其分子本质仍未完全明了。与gp120核心分子（一种gp120形式，其中V1、V2和V3环以及N和C末端已被截断）复合的CD4的原子结构表明，与CD4结合的构象的一个标志性特征是桥接片小结构域。当将与CD4结合的gp120与未与CD4结合的三聚体包膜结构进行比较时，发现桥接片发夹的方向存在差异。因此，HIV-1单体gp120中似乎存在多种受CD4结合影响的构象转变。在本研究中，通过使用一组特征明确的条件性、CD4诱导（CD4i）单克隆抗体（MAb）来研究与CD4结合的构象谱，这些抗体在各种条件下结合HIV-1 gp120及其突变体。研究了外结构域的两个不同的CD4i表位：第一个包括桥接片，而第二个包含V2环的元件。此外，我们表明，未结合配体的gp120延伸单体核心（coree）在溶液中呈现中间CD4i构象，在与CD4结合后会进一步发生可检测的重排。这些发现影响了关于gp120结构的公认范式以及HIV免疫原设计领域。
阐明HIV-1包膜蛋白在进入CD4+细胞过程中所经历的构象转变，对于我们理解HIV生物学至关重要。CD4的结合会触发一系列gp120结构重排，这可能为未来药物设计和预防性疫苗开发提供靶点。在这里，我们使用一组对CD4i构象具有特定偏好的抗体探针，系统地研究和审视了这些构象转变。这些探针已被用于研究一系列gp120突变和截短。通过这些分析，我们提出了gp120构象的CD4i转变中的4个不同的连续步骤，每个步骤由HIV包膜单体的抗体特异性和结构要求定义。因此，我们不仅为这一动态过程提供了新的见解，还定义了进一步研究HIV感染的探针。
