Plays a crucial role in virion assembly and budding. Expressed late in the virus life cycle, it acts as an inhibitor of viral transcription and RNA synthesis by interacting with the viral polymerase L. Presumably recruits the NP encapsidated genome to cellular membranes at budding sites via direct interaction with NP. The budding of the virus progeny occurs after association of protein Z with the viral glycoprotein complex SSP-GP1-GP2 at the cell periphery, step that requires myristoylation of protein Z (By similarity). Interacts with protein N; this interaction probably directs the encapsidated genome to budding sites. Interacts (via RING domain) with polymerase L; this interaction inhibits viral transcription and replication. Interacts with the glycoprotein complex; this interaction plays a role in virion budding. Mainly perinuclear. During budding, associates at the inner side of the plasma membrane of infected cells (By similarity). The RING finger domain is essential for the inhibitory activity of protein Z in transcription and RNA replication. Belongs to the arenaviridae Z protein family.