The endoplasmic
reticulum (ER) performs
myriad and essential
functions in the
development and maintenance
of cell structure
and function. In
addition, the ER
participates in a
number of critical
signal transduction
pathways activated
in the cellular response
to hypoxic/ischemic
stress, pathological
injury and in the
progression to regulated
cell death (apoptosis).
My laboratory
has a broad interest
in the the mechanisms
of ER function
in homeostasis
and in cell stress
and injury. We
are currently focused
on two related
areas of research.
The first concerns
the mechanisms
governing the partitioning
of mRNA and ribosomes
between the cytosol
and ER/nuclear
envelope compartments
of the cell. In
current views,
it is thought that
ribosomes and mRNAs
are partitioned
to the ER during
translation, via
the signal recognition
particle pathway.
Following translation,
ribosomes then
dissociate from
the ER and return
to the cytoplasmic
pool. In our recent
studies, we observed
that following
the termination
of protein synthesis
on the ER, 80S
ribosomes maintain
their association
with the ER protein
translocation machinery.
Interestingly,
post-termination,
ER-bound ribosomes
can participate
in de novo protein
synthesis and do
not distinguish
between mRNAs encoding
secretory/membrane
protein precursors
and mRNAs encoding
cytosolic proteins.
However, when a
membrane-bound
ribosome participates
in the synthesis
of a cytosolic
protein, the ribosome/mRNA/nascent
chain complex dissociates
from the ER membrane
to complete translation
in the cytosol.
This novel mechanism,
termed elongation-couple
ribosome release,
functions to maintain
the compartmental
segregation of
mRNAs characteristic
of the eukaryotic
cell. In our current
studies, we are
examining the the
regulation of mRNA
partitioning between
the cytosol and
ER compartments
and the mechanism
of mRNA localization
to the ER. These
studies use a broad
variety of cell
biological, molecular
and biochemical
techniques and
are being conducted
in tissue culture
systems.
ER-resident molecular
chaperones are integral
elements of the cellular
response to stress.
Among the various
ER chaperones, GRP94,
the ER Hsp90 chaperone,
serves a number of
interesting functions.
In addition to its
role in assisting
protein folding and
assembly, GRP94 has
recently been found
to function as an
immune modulator.
For example, when
isolated from tumor
tissue, GRP94 can
serve as a potent
anti-tumor vaccine.
The anti-tumor activity
of GRP94 is thought
to reflect a peptide
binding function,
and as the ER is
the site of peptide
loading onto MHC
Class I molecules,
GRP94 may have access
to the entire peptide
repertoire of the
cell. However, we
have recently discovered
that the immunogenic
activity of GRP94
is independent of
bound peptides and
thus reflects a novel “biological
adjuvant” function
of this protein.
In this role, GRP94
elicits the activation
of innate immune
responses, such as
cytokine release,
through interactions
with antigen presenting
cells. These observations
identify fundamental,
physiologically relevant
roles for this molecular
chaperone that extend
far beyond the regulation
of protein folding
and assembly. We
are applying biochemical,
cell biological and
immunological experimental
systems to our studies
on the molecular
basis of GRP94 function
in cell stress and
immune regulation.
