Insights into the Fusion Mechanism of Herpes Simplex Virus Type 1 Glycoprotein B.
Herpesviruses are a diverse family of enveloped double-stranded DNA viruses that infect
a wide range of animal hosts, including humans. These viruses enter cells by fusion of
the viral envelope with a host cell membrane using conserved fusion glycoprotein B (gB)
and its heterodimeric activator gH/gL plus additional nonconserved proteins. This
complexity sets herpesviruses apart from... read moremost other enveloped viruses, which require
only one or two proteins for entry, and resembles cellular fusion processes. However,
the mechanism of herpesvirus entry is not well defined. Herpes Simplex virus (HSV)
causes mucocutaneous lesions as well as severe conditions such as ocular infections and
encephalitis. HSV has provided an excellent system for studying herpesvirus entry.
Structural, biochemical and functional characterization of the four viral proteins
required for HSV membrane fusion, gD, gB and gH/gL, has defined their functions, but the
mechanism remains elusive. In this work, we set out to answer several questions about
how gB and gH/gL are regulated. It is presumed that gB catalyzes fusion by undergoing
conformational changes that provide the energy for this process, and that gH/gL plays a
role in activating gB, likely via direct interaction of their ectodomains. Additional
sources of regulation may come from the intraviral cytoplasmic domain of gB, which is
thought to repress fusion, and the short cytoplasmic tail of gH, which may be involved
in activating gB, possibly via a direct interaction with the gB cytoplasmic domain.
However, the potential regulatory mechanisms of these intraviral domains remains
unknown. Here, we show that the cytoplasmic domain of gB represses its fusion activity
via a unique mechanism requiring sequences that exhibit membrane binding activity and
preservation of the membrane-bound conformation. We provide the first structural model
of this native conformation and propose that structural studies of the cytoplasmic
domain must involve membrane. Additionally, we propose that the gH cytoplasmic tail
regulates the fusion activity of gB via an indirect mechanism, perhaps involving
membrane interaction. Finally, we show that the postfusion conformation of gB tolerates
several mutations believed to destabilize this conformation, supporting previous
conclusions that the postfusion form of gB is highly stable. Most importantly, these
experiments tell us that membrane plays a critical role in gB
Thesis (Ph.D.)--Tufts University, 2013.
Submitted to the Dept. of Molecular Microbiology.
Advisor: Ekaterina Heldwein.
Committee: Joan Mecsas, Ralph Isberg, and Linc Sonenshein.
Keywords: Virology, Biochemistry, and Molecular biology.read less