UNCOUPLED EXPRESSION OF MOLONEY MURINE LEUKEMIA-VIRUS ENVELOPE POLYPEPTIDES SU AND TM - A FUNCTIONAL-ANALYSIS OF THE ROLE OF TM DOMAINS IN VIRAL ENTRY

Moloney murine leukemia virus ecotropic envelope expression plasmids w ere used to demonstrate that the synthesis of the retroviral envelope SU and TM polypeptides can be uncoupled with retention of biologic act ivity. By substituting a glycosyl-phosphatidylinositol (GPI) membrane anchor for part or all of the retroviral envelope transmembrane protei n and creating several deletion variants of the TM subunit, we have be gun to dissect the role of the TM protein in envelope function. We sha w that a GPI-anchored envelope can be incorporated into virions and bi nds receptor. We found that the envelope cytoplasmic tail, while not r equired, influences the efficiency of retroviral transduction at some step after membrane fusion (possibly by interacting with core). The me mbrane-spanning domain of TM is involved in membrane fusion, and this function is distinct from its role as a membrane anchor. As few as eig ht amino acids of the putative membrane-spanning domain are sufficient to achieve membrane anchoring of envelope but not to mediate membrane fusion. In addition, though not required, the membrane-spanning domai n may have some direct role in the incorporation of envelope into viri ons. Finally, the extraceIlular domain of TM, besides containing the p utative fusion domain and interacting with SU, may influence the synth esis or stability and the glycosylation of envelope, possibly by affec ting oligomerization of the complex and proper intracellular transit.