  Extensions of the standard model with low-energy supersymmetry generically
allow baryon- and lepton-number violating operators of dimension four and five,
yielding rapid proton decay. The dimension-four operators are usually forbidden
by matter parity. We investigate to what extent the appearance of
dimension-five operators at the Planck scale may be constrained by the
different grand-unified gauge groups. Dimension-five operators are suppressed
in models based on E_6 and SU(3)_C x SU(3)_L x SU(3)_R, where four matter
fields do not form a gauge singlet. An intermediate scale offers the
possibility to sufficiently suppress these dimension-five operators.
