Latest Results

The HIV-1 envelope spike (Env) is the sole antigen on the virion surface that induces strong antibody responses in infected individuals; it is the key target for B-cell based HIV-1 vaccine development. The full-length Env contains a heavily glycosylated ectodomain, the highly conserved membrane-proximal external region (MPER), a single-pass transmembrane domain (TMD), and a large cytoplasmic tail (CT) of ~150 residues. Multiple lines of evidence suggest that the different regions of the Env are structurally coupled, and thus they all can play a role in modulating the antigenic properties of the Env. Historically, the transmembrane and membrane-proximal regions of single-pass membrane proteins have been difficult to visualize. In this study, we finally managed to solve an NMR structure of the full-length CT using a protein fragment comprising the TMD and the CT in large bicelles that closely mimic a lipid bilayer. By integrating the new NMR data and those acquired previously on other protein fragments, we derived a model of the entire membrane-interacting region of the Env. Moreover, Anne Brown lab of Virginia Tech performed all-atom molecular dynamics (MD) simulations (1 ms) in membrane with HIV lipid composition and showed how the remarkable CT structure can be accommodated in HIV membrane. The results are described in Piai et al, J Am Chem Soc 2021, doi: 10.1021/jacs.1c01762.

HIV-1 envelope glycoprotein (Env) mediates the fusion of viral and target cell membranes and is a major target for HIV vaccine development. We determined the NMR structure of an bicelle incorporated Env segment comprising the transmembrane domain (TMD) and a portion of the cytoplasmic tail (CT) and show that the CT folds into membrane attached amphipathic helices that wrap around the TMD thereby forming a support baseplate for the rest of Env, and we also provide insights into the dynamic coupling across the TMD between the ectodomain and CT. The results are published in Nature Communication, 11(1):2317 (2020).

The transmembrane (TM) anchors of cell surface proteins had been one of the “blind spots” in structural biology because they are generally very hydrophobic and sometimes dynamic, and are thus difficult targets for structural characterization. A plethora of examples showed that these membrane anchors are not merely anchors but can multimerize specifically to activate signaling receptors on the cell surface or to stabilize the envelope proteins in viruses. Through a series of studies of the TM domains of immune receptors and viral membrane proteins, we have established a robust protocol for determining atomic resolution structures of TM oligomers by nuclear magnetic resonance (NMR) in bicelles that closely mimic a lipid bilayer. Our protocol overcomes the hurdles typically encountered by other structural biology techniques such as x-ray crystallography and cryo-EM when studying small TM domains. The detailed protocol is now published in Nature Protocol, 14(8):2483-2520 (2019). 

Presentation of membrane proteins to host immune systems has been a challenging problem due to complexity arising from the poor in vivo stability of the membrane-mimetic media often used for solubilizing the membrane proteins. We report the use of functionalized, biocompatible nanoparticles as substrates to guide the formation of proteoliposomes that can present many copies of membrane proteins in a unidirectional manner. The approach was demonstrated to present the membrane-proximal region of the HIV-1 envelope glycoprotein. These nanoparticle-supported liposomes are broadly applicable as membrane antigen vehicles for inducing host immune responses. This study is published in Angewandte Chemie, 58(29):9866-9870 (2019).

This paper reports a new conceptual finding in receptor biology that the single-pass transmembrane anchor of a receptor is both necessary and sufficient for driving the signal transduction by forming a dimer-trimer higher-order network. Mechanism of receptor clustering has previously been attributed to the extracellular and/or the intracellular interactions, but no one has expected that the single-pass transmembrane helix alone in these receptors directly assembles a higher-order structure to drive signaling, and that this higher-order structure is in turn inhibited by the unliganded extracellular domain. We suspect that the transmembrane helix of many other receptors may utilize similar mechanisms to mediate receptor activation. This study therefore opens up a new concept in receptor signaling in general. This study is published in Cell, 176(6):1477-1489 (2019). PDF