Download Structures of the bacteriophage Sf6 terminase large subunit reveal a

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Structures of the bacteriophage Sf6 terminase large
subunit reveal a molecular clamp that captures DNA
for cleavage
Haiyan Zhao1, Yvonne Kamau1, Theodore Christensen1, Liang Tang1
1
University of Kansas
Many tailed double-stranded DNA bacteriophages employ a bipartite molecular machine
called terminase to package viral genome into preformed capsids. The terminase large
subunit makes an initiation cleavage on concatemeric viral DNA, and pumps it into the
capsid fueled by ATP hydrolysis. When an appropriate amount of DNA is inserted, the
terminase large subunit makes a second cut on DNA, and brings the remaining DNA to
next capsid for next packaging cycle. The terminase large subunit contains an N-terminal
ATPase domain and a C-terminal nuclease domain. We have determined the structures of
Shigella virus terminase large subunit gp2 C-terminal nuclease domain (gp2-CTD),
revealing a structural platform and an active site containing three aspartate residues,
reminiscent of those of RNases H and topoisomerases. Structures of gp2-CTD in the
absence and presence of catalytic ligand Mg2+ at 1.9 and 2.1 Å resolution reveal an open
and closed conformation for the active site, representing an catalytically inactive and
active state respectively. Upon binding of Mg2+, the loop of residues 245-255 that forms a
lid above the active site transitions from the open to the closed conformation, driven by
coordination of the main chain oxygen atom of residue S246 with Mg2+. These studies
suggest a mechanism of gp2-CTD nuclease activity upon initiation and completion of viral
DNA packaging, which begins with the open conformation, followed by docking of the DNA
substrate onto the active site and binding of Mg2+, which triggers the conformational
change of the loop 245-255, leading to a closed conformation that holds the DNA in place
for cleavage.