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.
