Triplet-doublet Splitting, Proton Stability and Extra Dimension

We propose a new possibility to reconcile the coupling unification scenario with the triplet-doublet mass splitting and the proton stability based on 5-dimensional supersymmetric model with an SU(5) gauge symmetry. It is shown that the minimal supersymmetric standard model is derived on a 4-dimensional wall through compactification on $S^1/(Z_2 \times Z_2')$.