It has been proposed that HIV-1-neutralizing 2F5 and 4E10 monoclonal antibodies recognize gp41 ectodomain pretransmembrane sequences in the context of the membrane interface. With the aim of evaluating lipid-bilayer surface effects on recognition, we use phospholipid model systems to investigate the ability of 2F5 and 4E10 to transfer into membrane interfaces and bind epitope sequences immersed therein. The experimental data support the predicted tendencies for partitioning and recognition of membrane-bound epitopes, albeit with lower affinity in the case of 2F5. Our findings support the existence of two different recognition mechanisms at membrane surfaces: the association of 2F5 with the interface possibly prevents epitope immersion into the bilayer, and 4E10 membrane association might allow recognition of the membrane-bound epitope. We discuss the relevance of these observations for the design of immunogens aimed at eliciting 2F5- and 4E10- like humoral responses.
SignificanceIon channels are proteins that mediate the flow of ions across cell membranes. Human genetic mutations of one type of ion channel, called hTRPM4, underlie a form of progressive familial heart block. Its distribution among many tissues, however, suggests that its functions are broad. We have solved the atomic structure of hTRPM4 to an overall resolution of 3.7 Å. The channel is composed of four identical subunits surrounding a central pore. We show the path of Na+ ions through the channel and point out aspects of the channel’s internal machinery that may affect its function. The structure will enable more directed experiments to understand the physiological function of this channel.
Virus infections can result in a variety of cellular injuries, and these often involve the permeabilization of host membranes by viral proteins of the viroporin family. Prototypical viroporin 2B is responsible for the alterations in host cell membrane permeability that take place in enterovirus-infected cells. 2B protein can be localized at the endoplasmic reticulum (ER) and the Golgi complex, inducing membrane remodeling and the blockade of glycoprotein trafficking. These findings suggest that 2B has the potential to integrate into the ER membrane, but specific information regarding its biogenesis and mechanism of membrane insertion is lacking. Here, we report experimental results of in vitro translation-glycosylation compatible with the transloconmediated insertion of the 2B product into the ER membrane as a double-spanning integral membrane protein with an N-/C-terminal cytoplasmic orientation. A similar topology was found when 2B was synthesized in cultured cells. In addition, the in vitro translation of several truncated versions of the 2B protein suggests that the two hydrophobic regions cooperate to insert into the ER-derived microsomal membranes.Virus infections can lead to a variety of cellular injuries, and usually these involve the restructuring of host membrane systems. Viroporins are a group of small virally encoded proteins that interact with cellular membranes to modify permeability and promote the release of viral particles. A typical feature exhibited by viroporins is the presence of at least one membrane-spanning helix anchoring the protein into membranes. After membrane insertion, their oligomerization creates hydrophilic channels or pores (22).Poliovirus is the enterovirus prototype member of the Picornaviridae family. This small, nonenveloped, icosahedral virus possesses a single-stranded 7.5-kb positive-sense RNA genome that encodes a single polyprotein. Polyprotein processing by virus-encoded proteases yields the structural P1 region proteins that encapsidate viral RNA and the nonstructural P2 and P3 region proteins involved in the replication of the viral RNA and membrane permeabilization (2). Nonstructural 2B protein is one of the products generated on processing the P2 region (62). Viroporin 2B has been identified as one of the viral proteins responsible for the alterations in host cell membrane permeability that take place in enterovirus-infected cells. Different 2B proteins expressed in cells have been localized at the endoplasmic reticulum (ER), Golgi complex, and, to a lesser extent, to the plasma and mitochondrial membranes (18,31,49,58). Biochemical and structural data indicate that viroporins form homo-oligomers that create pores in the ER and Golgi complex membranes (1,16,17,30,59). However, experimental data dealing with the mechanism of the membrane integration of the 2B product are lacking to date.The poliovirus 2B viroporin protein is hydrophobic overall and rather small (97 amino acids). Hydrophobicity within the viroporin 2B sequence seems to cluster in two main regions (F...
The amino-terminal region within the HIV-1 gp41 aromatic-rich pretransmembrane domain is an amphipathic-at-interface sequence (AIS). AIS is highly conserved between different viral strains and isolates and recognized by the broadly neutralizing 2F5 antibody. The atomic structure of the native Fab2F5-bound AIS appears to involve a nonhelical extended region and a beta-turn structure. We previously described how an immunogenic complex forms, based on the stereospecific interactions between AIS and the gp41 amino-terminal fusion peptide (FP). Here, we have analyzed the structure generated by these interactions using synthetic hybrids containing AIS and FP sequences connected through flexible tethers. The monoclonal 2F5 antibody recognized FP-AIS hybrid sequences with an apparently higher affinity than the linear AIS. Indeed, these hybrids exhibited a weaker capacity to destabilize membranes than FP alone. A combined structural analysis, including circular dichroism, infrared spectroscopy, and two-dimensional infrared correlation spectroscopy, revealed the existence of specific conformations in FP-AIS hybrids, predominantly involving beta-turns. Thermal denaturation studies indicated that FP stabilizes the nonhelical folded AIS structure. We propose that the assembly of the FP-AIS complex may act as a kinetic trap in halting the capacity of FP to promote fusion.
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