Major envelope protein present in abundant amounts in the virion envelope. Mediates virion sialic acid-dependent attachment the sialoadhesin receptor SIGLEC1. This attachment induces virion internalization into alveolar macrophages predominantly through clathrin-dependent endocytosis. Heterodimer with the membrane protein; disulfide-linked. This heterodimerization is required for transport to the Golgi complex (By similarity). Interacts with host SIGLEC1; this interaction plays a role in virus entry into host cell (By similarity). Interacts with glycoprotein 4. N-glycosylated. Belongs to the arteriviridae GP5 protein family.