Seems to play a role in optimizing the host cell environment for viral replication without causing cell death by apoptosis. Enhances virus infectivity and pathogenicity. Probably involved in viral immune evasion mechanisms (By similarity). In infected CD4(+) T-lymphocytes, down-regulates cell surface expression of CD4, CD28, CD3, and MHC-I or MHC-II molecules. Interferes with TCR signaling from the cell membrane. Interacts with CD247/TCRZ (TCR zeta chain) and exert potent down-regulation of cell surface TCR/CD3 complexes (By similarity). Homodimer. Interacts with host CD247/TCRZ; this interaction induces down-regulation of cell surface TCR/CD3 complexes. Associates with the inner plasma membrane through its N-terminal domain. The N-terminal domain is composed of the N-myristoyl glycine and of a cluster of positively charged amino acids. It is required for inner plasma membrane targeting of Nef (By similarity). The 155 isolate is from a monkey imported from Kenya. Belongs to the lentivirus primate group Nef protein family.