Sequential activation of the three protomers in the Moloney murine leukemia virus Env

Area of Science:

• Virology
• Structural Biology
• Biochemistry

Background:

• Class I viral membrane fusion proteins are trimers composed of surface (SU) and transmembrane (TM) subunits.
• Understanding the activation mechanism of these proteins is crucial for viral entry and infection processes.
• Moloney murine leukemia virus envelope protein (Env) serves as a model for studying class I fusion protein activation.

Purpose of the Study:

• To investigate the sequential or simultaneous activation of protomeric units within the Moloney murine leukemia virus Env trimer.
• To elucidate the order of events during Env activation, focusing on subunit isomerization and release.

Main Methods:

• Biochemical methods were employed to monitor the isomerization of the SU-TM disulfide bond and subsequent SU release.
• In vitro activation was induced by depleting stabilizing calcium ions (Ca2+) from solubilized Env.
• Receptor-triggered activation on rat XC cells was also studied, with intermediates captured via alkylation of active thiols.

Main Results:

• The early activation step of Moloney murine leukemia virus Env was found to occur sequentially across the three protomers.
• This sequential activation generates two distinct asymmetric oligomer intermediates: (SU-TM)2TM and (SU-TM)TM2.
• The observed sequential activation mechanism holds true for both in vitro and receptor-triggered viral Env activation.

Conclusions:

• The activation of Moloney murine leukemia virus Env protomers proceeds in a sequential, rather than simultaneous, manner.
• This stepwise activation is a conserved mechanism, observed under different triggering conditions.
• The identification of asymmetric intermediates provides key insights into the conformational changes driving viral membrane fusion.