The surface protein (SU) attaches the virus to the host cell by binding to the cell receptor. This interaction triggers the refolding of transmembrane protein (TM) and is thought to activate its fusogenic potential by unmasking its fusion peptide (By similarity). The transmembrane protein (TM) acts as a class I viral fusion protein. Under the current model, the protein has at least 3 conformational states: pre-fusion native state, pre-hairpin intermediate state, and post-fusion hairpin state. During viral and target cell membrane fusion, the coiled coil regions (heptad repeats) assume a trimer-of-hairpins structure, positioning the fusion peptide in close proximity to the C-terminal region of the ectodomain. The formation of this structure appears to drive apposition and subsequent fusion of viral and target cell membranes. Membranes fusion leads to delivery of the nucleocapsid into the cytoplasm (By similarity). The leader peptide is a component of released, infectious virions and is required for particle budding. The mature envelope protein consists of a trimer of SU-TM heterodimers. The N-terminus of leader peptide specifically interacts with Gag protein. This specific interaction between Gag protein and Env glycoprotein may allow particle egress (By similarity). The polyprotein has a highly unusual biosynthesis for a retroviral glycoprotein. It is translated as a full-length precursor protein into the rough endoplasmic reticulum and initially has a type III protein configuration with both its N and C-termini located intracytoplasmically (By similarity). Its N-terminus is located inside the viral particle. The surface protein is not anchored to the viral envelope, but associates with the extravirion surface through its binding to TM. The ER retention signal plays an important role in establishing the intracellular site of budding. Envelope glycoproteins are synthesized as an inactive precursor that is processed by host furin or a furin-like protease to yield a functional hetero-oligomeric complex. The transmembrane protein and the surface protein are N-glycosylated. Mono- and polyubiquitinated leader peptide are found in viral particles. Ubiquitination may be involved in regulating the balance between viral and subviral particles release (By similarity). Foamy viruses are distinct from other retroviruses in many respects. Their protease is active as an uncleaved Pro-Pol protein. Mature particles do not include the usual processed retroviral structural protein (MA, CA and NC), but instead contain two large Gag proteins. Their functional nucleic acid appears to be either RNA or dsDNA (up to 20% of extracellular particles), because they probably proceed either to an early (before integration) or late reverse transcription (after assembly). Foamy viruses have the ability to retrotranspose intracellularly with high efficiency. They bud predominantly into the endoplasmic reticulum (ER) and occasionally at the plasma membrane. Budding requires the presence of Env proteins. Most viral particles probably remain within the infected cell.