  Recently it was argued that proton lifetime limits impose that the scale of
quantum gravity must be above 10^{16} GeV. By studying the proton stability in
the context of realistic heterotic-string models, I propose that proton
longevity necessitates the existence of an additional U(1)_{Z'} symmetry, which
is of non-GUT origin and remains unbroken down to intermediate, or low,
energies. It is shown that the realistic string models frequently give rise to
U(1) symmetries, which suppress the proton decay mediating operators, with, or
without, the possibility of R-parity violation. By studying the F- and D-flat
directions, I examine whether the required symmetries remain unbroken in the
supersymmetric vacuum and show that in some examples they can, whereas in
others they cannot. The proton decay rate is proportional to the U(1)_{Z'}
symmetry breaking scale. Imposing the proton lifetime limits I estimate the
U(1)_{Z'} breaking scale and show that if substantial R-parity violation is
present the associated Z' is within reach of forthcoming collider experiments
