X-ray Structures of the Post-fusion 6-Helix Bundle of the Human Syncytins and their Functional Implications

Abstract

The retroviral envelope-derived proteins syncytin-1 and syncytin-2 (syn1 and syn2) drive placentation in humans by forming a syncytiotophoblast, a structure allowing for an exchange interface between maternal and fetal blood during pregnancy. Despite their essential role, little is known about the molecular mechanism underlying the syncytins' function. We report here the X-ray structures of the syn1 and syn2 transmembrane subunit ectodomains, featuring a 6-helix bundle (6HB) typical of the post-fusion state of gamma-retrovirus and filovirus fusion proteins. Contrary to the filoviruses, for which the fusion glycoprotein was crystallized both in the post-fusion and in the spring-loaded pre-fusion form, the highly unstable nature of the syncytins' prefusion form has precluded structural studies. We undertook a proline-scanning approach searching for regions in the syn1 6HB central helix that tolerate the introduction of helix-breaker residues and still fold correctly in the pre-fusion form. We found that there is indeed such a region, located two a-helical turns downstream a stutter in the central coiled-coil helix -precisely where the breaks of the spring-loaded helix of the filoviruses map. These mutants were fusion-inactive as they cannot form the 6HB, similar to the "SOSIP" mutant of HIV Env that allowed the high-resolution structural characterization of its labile pre-fusion form.These results now open a new window of opportunity to engineer more stable variants of the elusive pre-fusion trimer of the syncytins and other gamma-retroviruses envelope proteins for structural characterization.

Structure of the Core Postfusion Porcine Endogenous Retrovirus Fusion Protein

“…The stutter breaks the periodicity of the heptad repeat by adding an extra “defg” repeat into the motif ( abcdefg defg abcdefg ). The four-residue stutter identified on the postfusion PERV TM structure is also observed in the 6HB structures of Mason-Pfizer monkey virus (MPMV), XMRV, bovine leukemia virus (BLV), and human syncytin 1 and syncytin 2 ( 20 ). HR-disrupting stutters have been identified in the coiled-coil domains of several other nonretroviral class I fusion proteins, including those of EBOV and lymphocytic choriomeningitis virus (LCMV) ( 21 ).…”

“…The interruption of the hydrophobic core induces a local unwinding of the superhelical coiled-coil in the two helical turns downstream of the stutter ( 22 ). As previously described, this local unwinding causes a deviation from ideal geometry that manifests in a helical twist ( 20 ). These four residues that disrupt the HR impart structural flexibility to the long coiled-coils that are otherwise rigid due to the knob-in-hole packing of the HRs ( 22 ).…”

Human Endogenous Retrovirus Type W Envelope from Multiple Sclerosis Demyelinating Lesions Shows Unique Solubility and Antigenic Characteristics

“…The first one, located around aa300, corresponds to the fusion peptide, unmasked during the pre- to post-fusion conformational transition, inducing cell–cell membrane fusion. The second one, located around aa430, is a regular anchoring transmembrane domain (Ruigrok et al 2019 ). Interestingly, pHERV-W ENV has only one membrane-spanning site that corresponds to the site of cell membrane anchoring domain.…”

Pregnancy complications and Interferon-induced transmembrane proteins (IFITM): balancing antiviral immunity and placental development

“…The capture of syncytin genes (Syncytin-1 and -2 in humans and Syncytins-A and -B in mice), occurred independently from different ERV in diverse mammalian lineages 10 to 85 million years ago [8]. The mechanism mediating cell-cell fusion in the placenta is therefore similar to virus-cell fusion [11,12]. These viral genes have become an integral part of our development and are required for healthy placental and fetal development [13,14].…”

“…il y a 10 à 85 millions d'années [8]. Le mécanisme qui régit la fusion cellule-cellule dans le placenta est donc similaire à la fusion virus-cellule [11,12]. Ces gènes viraux font désormais partie intégrante de notre patrimoine génétique et sont nécessaires au bon développement du placenta et du foetus [13,14].…”