View Program - asbmb

36
Exocytosis of large cargo: a lesson from coronaviruses
Carolyn Machamer 1 , Travis Ruch 1 .
Johns Hopkins University School of Medicine, Baltimore MD 21205 1
Abstract:
Coronaviruses are an interesting model for studying exocytosis of large cargo. These enveloped
viruses assemble by budding into the lumen of the ER-Golgi intermediate compartment, and are
released by exocytosis. However, the virions (~100 nm) are larger than the typical secretory
vesicle. How do cells handle this large cargo? We have evidence that one of the viral envelope
proteins, E, modifies the microenvironment of the Golgi complex to enhance release of
infectious virus. The E protein helps orchestrate budding and is incorporated into the virus
envelope at low levels, but is made in excess infected cells and is thought to have additional
functions. E is a small protein with a single hydrophobic domain, and has ion channel activity in
vitro when inserted into planar bilayers. To test the role of the potential ion channel function,
we constructed a recombinant coronavirus containing E protein with a heterologous
hydrophobic domain. The mutant virus assembles normally, but release of infectious virus is
severely compromised. Virions accumulate intracellularly in vacuolar structures where partial
proteolysis leads to release of non-infectious particles. When overexpressed from cDNA, the
coronavirus E protein induces morphological changes in the Golgi complex and alters cargo
traffic. There are two critical residues in the hydrophobic domain required for these effects,
suggesting that ion channel function may be critical for productive exocytosis of infectious
virions. Elucidation of the mechanism by which the E protein promotes the release of infectious
virions should help dissect the process by which cells handle other large cargo.