Unique Structure and Distinctive Properties of the Ancient and Ubiquitous Gamma-Type Envelope Glycoprotein

Abstract

After the onset of the AIDS pandemic, HIV-1 (genus Lentivirus) became the predominant model for studying retrovirus Env glycoproteins and their role in entry. However, HIV Env is an inadequate model for understanding entry of viruses in the Alpharetrovirus, Gammaretrovirus and Deltaretrovirus genera. For example, oncogenic model system viruses such as Rous sarcoma virus (RSV, Alpharetrovirus), murine leukemia virus (MLV, Gammaretrovirus) and human T-cell leukemia viruses (HTLV-I and HTLV-II, Deltaretrovirus) encode Envs that are structurally and functionally distinct from HIV Env. We refer to these as Gamma-type Envs. Gamma-type Envs are probably the most widespread retroviral Envs in nature. They are found in exogenous and endogenous retroviruses representing a broad spectrum of vertebrate hosts including amphibians, birds, reptiles, mammals and fish. In endogenous form, gamma-type Envs have been evolutionarily coopted numerous times, most notably as placental syncytins (e.g., human SYNC1 and SYNC2). Remarkably, gamma-type Envs are also found outside of the Retroviridae. Gp2 proteins of filoviruses (e.g., Ebolavirus) and snake arenaviruses in the genus Reptarenavirus are gamma-type Env homologs, products of ancient recombination events involving viruses of different Baltimore classes. Distinctive hallmarks of gamma-type Envs include a labile disulfide bond linking the surface and transmembrane subunits, a multi-stage attachment and fusion mechanism, a highly conserved (but poorly understood) "immunosuppressive domain", and activation by the viral protease during virion maturation. Here, we synthesize work from diverse retrovirus model systems to illustrate these distinctive properties and to highlight avenues for further exploration of gamma-type Env structure and function.

Keywords: Env; Gp2; R-peptide; alpharetrovirus; deltaretrovirus; filovirus; gammaretrovirus; immunosuppressive domain; reptarenavirus; retrovirus; syncytin.

Figure 1

Gammaretroviral Replication Cycle. Schematic of a typical replication cycle for gammaretroviruses.

Figure 2

Domains of the gammaretroviral envelope glycoprotein. (a) Linear depiction of a gammaretrovirus Env highlighting conserved features. S–S indicates a disulfide bond between the CXXC and C(X)₆CC motifs. HR1 and HR2 are the two heptad repeat domains of the transmembrane subunit. HR1 overlaps slightly with the immunosuppressive domain (ISD). (b left) Schematic of an envelope glycoprotein trimer, composed of three heterodimers comprising SU and TM subunits. (b right) Depiction of an Env monomer, highlighting predicted locations of conserved regions including the ISD, HR1 and HR2, the fusion peptide and cytoplasmic tail domains. SU is linked to TM through a disulfide bond, depicted here by a pink line.

Figure 3

Crystal structures of gammaretrovirus receptor binding domains. Crystal structures of the receptor binding domain (RBD) of Friend MLV (PDB 1A0L), FeLV B (PDB 1LCS) and ENVp(b)1 (PDB 6W5Y). Defined variable regions A, B and C (VRA, VRB, VRC) are shown in blue for MLV and FeLV B. Approximate locations are indicated for EnvP(b)1 based on structurally similar locations (McCarthy et al., 2020). Variable regions are believed to provide receptor specificity among RBDs that recognize different receptors.

Figure 4

Binding and fusion of a Gamma-type Env protein with a host cell membrane. Env is shown as a single heterodimer rather than a trimer for simplicity. The receptor binding domain (RBD) is shown in blue, and the proline rich region (PRR) is shown in white. After binding of the SU subunit to a receptor, isomerization of the disulfide bond occurs, resulting in conversion to an intra-subunit bond within the CXXC motif of the SU subunit, releasing the SU subunit and ultimately resulting in fusion mediated by the TM subunit. The conformational change that TM undergoes is irreversible, and TM cannot revert to carry out fusion again.

Figure 5

Conservation of CXXC, ISD and CX₆CC motifs of gamma-type Envs. Geneious amino acid alignment of 27 gammaretroviral and gamma-like Env sequences and the CKS-17 peptide. Endogenous sequences are shown in red while exogenous viruses are shown in black. On the left is a section from SU containing the CXXC motif (which is absent from alpharetroviral and filoviral entry proteins). The CXXC contributes to an intersubunit disulfide bond with the third cysteine in the CX₆CC motif in TM. On the right is a section of sequence from TM that contains the highly conserved region of the immunosuppressive domain (ISD). The CKS-17 peptide was used to originally define the region of the ISD; also highlighted is the intra-subunit bond that forms within the CX₆CC motif. RSV and ALV are classified as alpharetroviruses, SRV3 is a betaretrovirus with a gamma-like Env, and HTLV is a deltaretrovirus; also notable is the gamma-like glycoprotein of Ebola virus which is a member of Filoviridae. Accession #s in order of figure: AFV99542.1; BAD98245.1; Q05320.1; ACI62863.1; AGI61275.1; P07575.1; ABS71857.1; QXV86750.1; P31796.1; NP_041262.1; NP_001291475.1; Q9UQF0.1; XP_022270294.1; AAM34209.1; XP_040289417.1; XP_025743503.1; QXP50143.1; XP_015345157.1; AAAU04934.1; AYG96595.1; ACD35952.1; AAQ88184.1; P26804.1; AEI59727.1; ALX81658.1; ALV83307.1; AAB03092.1.

Figure 6

Receptor Interference (Superinfection Resistance). Env expression in infected, virus-producing cells induces receptor interference. Proposed mechanisms include: (1) blocking trafficking of the receptor to the cell surface; (2) binding of the receptor at the cell surface, thereby blocking new viral particles from binding, and (3) inducing endocytosis and subsequent degradation of the receptor.