Stabilization of TM trimer interactions during activation of moloney murine leukemia virus Env

The transmembrane subunit (TM) of the trimeric retrovirus Env complex is thought to direct virus-cell membrane fusion by refolding into a cell membrane-interacting, extended form that subsequently folds back on itself into a very stable trimer of hairpin-like TM polypeptides. However, so far there is only limited evidence for the formation of a stable TM trimer during Env activation. Here we have studied the oligomer composition and stability of an intermediate and the fully activated form of Moloney murine leukemia virus (Mo-MLV) Env. Activation of Mo-MLV Env is controlled by isomerization of its intersubunit disulfide. This results in surface subunit (SU) dissociation and TM refolding. If activation is done in the presence of an alkylator, this will modify the isomerization-active thiol in the SU of Env and arrest Env at an intermediate stage, the isomerization-arrested state (IAS) of its activation pathway. We generated IAS and fully activated Envs in vitro and in vivo and studied their states of oligomerization by two-dimensional blue native polyacrylamide gel electrophoresis (PAGE) and nonreducing sodium dodecyl sulfate (SDS)-PAGE. The IAS Env was composed of trimers of SU-TM complexes, whereas the activated Env consisted of SU monomers and TM trimers. When the oligomers were subjected to mild SDS treatment the TM trimer was found to be 3.5 times more resistant than the IAS oligomer. Thus, this demonstrates that a structural conversion of TM takes place during activation, which results in the formation of a stable TM trimer.