  The activity of cell membrane inclusions (such as ion channels) is influenced
by the host lipid membrane, to which they are elastically coupled. This
coupling concerns the hydrophobic thickness of the bilayer (imposed by the
length of the channel, as per the hydrophobic matching principle) but also its
slope at the boundary of the inclusion. However, this parameter has never been
measured so far. We combine small-angle x-ray scattering data and a complete
elastic model to measure the slope for the model gramicidin channel and show
that it is surprisingly steep, in two membrane systems with very different
elastic properties. This conclusion is confirmed and generalized by the
comparison with recent results in the simulation literature and with
conductivity measurements.
