ASSEMBLY AND RELEASE OF SIV ENV PROTEINS WITH FULL-LENGTH OR TRUNCATED CYTOPLASMIC DOMAINS

We have used recombinant vaccinia viruses expressing full-length or tr uncated gag or env genes of SIVmac239 to investigate the requirements for assembly of SIV proteins. We observed that assembly of virus-like particles (VLPs) was found to be 3- to 6-fold higher with full-length Env than with the truncated forms, or than VLPs containing only Gag pr oteins, in primary monkey cells or various human cell lines. When cell s expressing Env proteins in the absence of Gag were examined by immun oelectron microscopy, clusters of Env protein and membrane vesicles co ntaining Env proteins were observed at cell surfaces. A low level of v esicles was released from cells expressing full-length Env, but about a 10-fold higher level was released in cells expressing a truncated fo rm of Env [Env733(t)] in which the cytoplasmic domain is only 17 amino acids in length. Another truncated protein, Env718(t), with a short c ytoplasmic tail of 3 aa, was also incorporated into VLPs at a 10-fold higher level than the full-length Env protein and was more efficiently released in vesicles. The mature SU and TM proteins were predominantl y incorporated into VLPs with full-length Env, but both cleaved and un cleaved precursor proteins were present in VLPs with truncated Env as well as in Env and Env(t) vesicles; A more prominent layer of spikes w as seen by electron microscopy in VLPs with truncated Env than in VLPs containing full-length Env. These results indicate that truncated Env proteins have the ability to self-associate on the cell surface and a re assembled into a more closely packed array than full-length Env, wh ich could explain the preferential incorporation of Env proteins with short cytoplasmic tails into virions. (C) 1996 academic Press, Inc.