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.