  We analyse proton decay in the context of simple supersymmetric SU(5) grand
unified models with an extra compact spatial dimension described by the
orbifold S^1/(Z_2 x Z_2'). Gauge and Higgs degrees of freedom live in the bulk,
while matter fields can only live at the fixed point branes. We present an
extended discussion of matter interactions on the brane. We show that proton
decay is naturally suppressed or even forbidden by suitable implementations of
the parity symmetries on the brane. The corresponding mechanism does not affect
the SU(5) description of fermion masses also including the neutrino sector,
where Majorana mass terms remain allowed.
