receptor is the site on the endoplasmic reticulum of targeting of
ribosomes bound by SRP. These ribosomes are synthesizing
secreted and integral membrane proteins.
The eukaryotic SRP receptor is
composed of two subunits called SR a
and SR b.
In E. coli
the homologue of the SRP receptor is FtsY
We discovered that the SR a subunit is assembled on
the endoplasmic reticulum by binding to the SRb
subunit during an mRNA encoded pause in translation (Young
and Andrews, EMBOJ. 1996).
Both our lab and that of
Peter Walter have shown that SRb
is a conventional type I transmembrane protein that requires SRP and
SRP receptor to assemble into the endoplasmic reticulum membrane (Young
et al. JBC, 1995).
Both subunits of the SRP
receptor are GTPases. SRa
binds to the GTP bound form of SRb
suggesting a novel way in which SRb
contributes to protein targeting.(Legate
and Andrews, Biochem Cell Biol. 2001)
FtsY (which is strictly
speaking the homologue of only SRa)
binds directly to E. coli membranes by binding to
phosphatid-thanolamine. Although there is no homologue for SRb
in the E. coli genome, we have very good evidence that FtsY binds to
a protein in the E. coli membrane. (Millman
et al. JBC, 2001)
SRb is a unique GTP binding protein in that
it has no intrinsic GTP activity. (Legate
and Andrews, JBC, 2003)