There are two major areas of research in this laboratory;
the Sec-dependent protein export pathway of Escherichia
coli, and the mechanism of action of a novel regulatory
protein, BipA, in E. coli.
Sec-dependent Protein Export
The Sec translocon is a highly conserved heterotrimeric
structure (the SecY complex) responsible for translocation
of a large number of substrate proteins across biological
membranes. In Escherichia coli, the SecY complex
is the major apparatus for export across the cytoplasmic
membrane as well as for insertion of inner membrane proteins.
This lab has used genetic and biochemical analyses to
investigate the Sec pathway, with an emphasis on the structure
of the SecY complex and the interactions amongst the Sec
proteins. The recent elucidation of the crystal structure
of the SecY complex from the archaeon, Methanococcus
jannaschii, has provided fresh impetus to our studies
in directions previously unavailable. We are using this
structural information to investigate the roles of individual
amino acid residues within the translocation complex through
site directed mutagenesis and phenotypic analysis.
Although most inner membrane and exported proteins utilize
the SecY complex to traverse the inner membrane, the routing
pathways by which these proteins are targeted to SecY
vary. Some secretory proteins depend on recognition by
SecB, others utilize SRP, and still others appear to require
only YidC or YidC in combination with SecYEG. We are exploring
the determinants that differentiate between the various
routing pathways.
Many secreted or inner membrane proteins are localized
to specific domains of the bacterial cell, i.e polar or
mid-cell locations. However, SecY is dispersed circumferentially
about the cell. Therefore, the question arises as to whether
all proteins are secreted at random SecY complexes and
then diffuse to their final location or if secretion occurs
at SecY complexes that are localized to the region of
the secretory protein’s final destination. We are
ivestigating whether there are distinct populations of
SecY complexes dedicated to particular functions.
The Novel Regulatory Protein, BipA
BipA is a unique regulatory protein with an unprecedented
mechanism of action. BipA has been implicated in the regulation
of several diverse pathways in the last few years. It
is a conserved protein, encoded by the genomes of a large
number of bacterial species, including several important
human pathogens. While dispensable under normal laboratory
conditions, BipA is important for robust bacterial growth
at low temperatures, contributes to the ability of pathogenic
strains to direct host cell cytoskeletal rearrangements,
and modulates expression of many genes that contribute
to a variety of bacterial functions. The current hypothesis
predicts that BipA interacts with the ribosome and affects
the efficiency of translation for substrate mRNAs (Owens
et al, 2004). These target mRNAs may be notable for an
extended region of homology to the ribosomal 16S rRNA,
resulting in formation of a hybrid duplex between the
mRNA and rRNA that interferes with initiation of translation.
BipA is proposed to disrupt this hybrid duplex and allow
translation to commence. Our studies will examine the
role of BipA in the common laboratory strain of Escherichia
coli K12, but will be relevant to the many pathogens
that also encode this novel regulator.
