Assembly and Budding of Negative-Strand RNA Viruses

Assembly of negative-strand RNA viruses occurs by budding from host plasma membranes. The budding process involves association of the viral core or nucleocapsid with a region of cellular membrane that will become the virus budding site, which contains the envelope glycoproteins and matrix protein. This region of membrane then buds out and pinches off to become the virus envelope. This review will address the questions of what are the mechanisms that bring the nucleocapsid and envelope glycoproteins together to form the virus budding site, and how does this lead to release of progeny virions? Recent evidence supports the idea that viral envelope glycoproteins and matrix proteins are organized into membrane microdomains that coalesce to form virus budding sites. There has also been substantial progress in understanding the last step in virus release, referred to as the "late budding function," which often involves host proteins of the vacuolar protein sorting apparatus. Key questions are raised as to the mechanism of the initial steps in formation of virus budding sites: How are membrane microdomains brought together and how are nucleocapsids selected for incorporation into these budding sites, particularly in the case of viruses for which genome RNA sequences are important for envelopment of nucleocapsids?