Failure to cleave murine leukemia virus envelope protein does not precludeits incorporation in virions and productive virus-receptor interaction

It is thought that complete cleavage of retroviral envelope protein into ma ture surface protein (SU) and transmembrane protein (TM) is critical for it s assembly into virions and the formation of infectious virus particles. He re we report the identification of highly infectious, cleavage-deficient en velope mutant proteins. Substitution of aspartate for lysine 104, arginines 124 and 126, or arginines 223 and 225 strongly suppressed cleavage of the envelope precursor and yet allowed efficient incorporation of precursor mol ecules as the predominant species in virions that were almost as infectious as the wild-type virus. These results indicate that cleavage of the envelo pe precursor into mature SU and TM is not necessary for assembly into virio ns. Moreover, they call into question how many mature envelope protein subu nits are required to complete virus entry, suggesting that a very few molec ules suffice. The failure of host cell proteases to cleave these mutant pro teins, whose substitutions are distal to the actual site of cleavage, sugge sts that the envelope precursor is misfolded, sequestering the cleavage sit e. In agreement with this, all cleavage mutant proteins exhibited significa nt losses of receptor binding, suggesting that these residues play roles in proper envelope protein folding. We also identified a charged residue, arg inine 102, whose substitution suppressed envelope cleavage and allowed prec ursor incorporation but resulted in virions that were virtually noninfectio us and that exhibited the greatest reduction in receptor binding. Placement of these cleavage mutations into envelope proteins of targeted retroviral vectors for human gene therapy may prevent loss of the modified surface pro teins from virions, improving their infectivity and storage hardiness.