class I viral fusion protein that directs fusion of viral and host endosomal membranes, leading to delivery of the nucleocapsid into the cytoplasm. Membrane fusion is mediated by irreversible conformational changes induced upon acidification in the endosome.; Stable signal peptide (SSP): cleaved and functions as a signal peptide. In addition, it is also retained as the third component of the GP complex. The SSP is required for efficient glycoprotein expression, post-translational maturation cleavage of GP1 and GP2, glycoprotein transport to the cell surface plasma membrane, formation of infectious virus particles, and acid pH-dependent glycoprotein-mediated cell fusion.; interacts with the host receptor.