  We show how the bosonic sector of the radion supermultiplet plus d=4, N=1
supergravity emerge from a consistent braneworld Kaluza-Klein reduction of D=5
M--theory. The radion and its associated pseudoscalar form an SL(2,R)/U(1)
nonlinear sigma model. This braneworld system admits its own brane solution in
the form of a 2-supercharge supersymmetric string. Requiring this to be free of
singularities leads to an SL(2,Z) identification of the sigma model target
space. The resulting radion mode has a minimum length; we suggest that this
could be used to avoid the occurrence of singularities in brane-brane
collisions. We discuss possible supersymmetric potentials for the radion
supermultiplet and their relation to cosmological models such as the cyclic
universe or hybrid inflation.
