Teraflop 73 - Novembre - cesca

The aminoterminal
extracellular domain
A point mutant rendering the TSHr abnormally
sensitive to hCG was identified
in a woman presenting with severe
hyperthyroidism on the occasion of
each of her pregnancies. The structural
consequences of the mutation were
analysed in the lights of a structural
model of the LRR segment and opened
the way to the study of binding interactions
in all three GPHRs. Key residues
have been identified in the TSHr and
FSHr which, upon mutation to their
LH/CGr counterpart, render these receptor
sensitive to hCG. Strikingly, substitution
of eight residues of the TSHr
and three residues of the FSHr make
these mutant receptors responsive to
hCG with an EC50 similar to the that of
the wt LH/CGr. In the absence of direct
structural information of the hormonereceptor
complexes, the molecular
modeling studies realized in the frame
of this study provide the best current
picture of the interaction of glycoprotein
hormones with their cognate receptors
(Smits et al. 2003).
The serpentine portion
The TSHr can be activated constitutively
by a surprisingly large number of
point mutations, the majority of which
affects the serpentine portion of the receptor.
This characteristic provides a
unique opportunity to identify key interactions
responsible for the maintenance
of the receptor in its inactive state. Following
the identification of an interaction
between residues of the sixth and
seven transmembrane helices (D6.44
and N7.49), the breaking of which activates
the receptor, it was found that
freeing of the side chain of residue
N7.49 is required for activation of the
receptor by all other mutations as well
as by its natural agonist, TSH. Molecular
REFERENCES
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modeling of the serpentine harboring
mutations provides a rationale to the activating
effect of a series of them. This
approach is expected to contribute to
our understanding of the general mechanisms
of activation of rhodopsin-like
GPCRs (Govaerts et al 2001).
Interactions between the extracellular
and the serpentine portions
Activating mutations affecting residues
of the extracellular domain and the extracellular
loops led to the suggestion
that a silencing interaction would exist
between these two domains. This hypothesis
was demonstrated by showing
an increase in the constitutive activity of
"beheaded" receptors. However, the
truncated constructs were far from fully
activated; they could be further activated
by mutations affecting the transmembrane
segments of the serpentine.
Interestingly, activating mutations located
in extracellular loops with a strong
effect on the holoreceptor were without
effect on the truncated constructs. The
data are compatible with a model in
which the unliganded extracellular domain
of the GPHRs would function as a
tethered inverse agonist of their
serpentine. When bound to their hormones,
or when harboring an activating
mutation, the extracellular domains
would switch to a tethered full agonist
(Vlaeminck et al 2002).
Relation between recognition
specificity and constitutive activity
Patients with spontaneous ovarian hyperstimulation
syndrome were shown to
harbor point mutations in their FSHr
gene rendering the receptor abnormally
sensitive to hCG. Unexpectedly, these
mutations were located in the serpentine
portion of the receptor, outside the
hormone binding domain. The observation
that these mutations cause constitutive
activation when engineered in the
glycoprotein hormone receptors points
to an unexpected relation between
recognition specificity and basal activity
of the FSHr (Smits et al. 2003). ■
Acknowledgments
These studies are the result of a tight collaboration
between several research teams at
IRIBHM, headed by M. Parmentier and S.
Costagliola, Euroscreen SA, and the group
of L. Pardo (U. Barcelona).
Structure of the ectodomain of
glycoprotein hormone receptors,
modeled on the crystal structure
of the Ribonuclease Inhibitor.
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TERAFLOP
Novembre 2003