May down-regulate host tetherin (BST2) by lysosomal degradation, thereby counteracting its antiviral activity. Attaches the virion to the cell membrane by interacting with host receptor, initiating the infection (By similarity). Binding to human ACE2 and CLEC4M/DC-SIGNR receptors and internalization of the virus into the endosomes of the host cell induces conformational changes in the S glycoprotein. Proteolysis by cathepsin CTSL may unmask the fusion peptide of S2 and activate membrane fusion within endosomes. Mediates fusion of the virion and cellular membranes by acting as a class I viral fusion protein. Under the current model, the protein has at least three 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. Acts as a viral fusion peptide which is unmasked following S2 cleavage occurring upon virus endocytosis. Homotrimer; each monomer consists of a S1 and a S2 subunit. The resulting peplomers protrude from the virus surface as spikes (By similarity). Binds to human and palm civet ACE2 and human CLEC4M/DC-SIGNR. Interacts with the accessory proteins 3a and 7a. Accumulates in the endoplasmic reticulum-Golgi intermediate compartment, where it participates in virus particle assembly. Colocalizes with S in the host endoplasmic reticulum-Golgi intermediate compartment (PubMed:20861307). Some S oligomers are transported to the host plasma membrane, where they may mediate cell-cell fusion. The KxHxx motif seems to function as an ER retrieval and binds COPI in vitro. Fusion peptide 1 (FP1) and fusion peptide 2 (FP2) function cooperatively and have a membrane-ordering effect on lipid headgroups and shallow hydrophobic regions of target bilayers. They are considered as two domains of an extended, bipartite FP. The membrane-ordering activity is calcium-dependent and also dependent on correct folding, which is maintained by an internal disulfide bond in FP2. The cytoplasmic Cys-rich domain is palmitoylated. Spike glycoprotein is digested by cathepsin CTSL within endosomes. Specific enzymatic cleavages in vivo yield mature proteins. The precursor is processed into S1 and S2 by host cell furin or another cellular protease to yield the mature S1 and S2 proteins. Additionally, a second cleavage leads to the release of a fusion peptide after viral attachment to host cell receptor. The cytoplasmic Cys-rich domain is palmitoylated. Spike glycoprotein is digested within host endosomes. Tor2 is the prototype of the virus isolated during the severe SARS outbreak in 2002-2003. GD03 has been isolated from the second mild SARS outbreak in winter 2003-2004. SZ3 has been isolated from palm civet, the presumed animal reservoir. The spike proteins from those three isolates display a strong affinity for palm civet ACE2 receptor, whereas only the Tor2 spike protein efficiently binds human ACE2. This may explain the high pathogenicity of Tor2 virus, whose spike is highly adapted to the human host. Therefore, the lack of severity of disease during the 2003-2004 outbreak could be due to the incomplete adaptation of GD03 virus to bind human ACE2. Mutation Asn-479 and Thr-487 in palm civet coronavirus seems necessary and sufficient for the virus to acquire the ability to efficiently infect humans. Belongs to the betacoronaviruses spike protein family.