  We calculate the high and low scale threshold corrections to the D=6 proton
decay mode in supersymmetric SU(5) grand unified theories with
higher-dimensional representation Higgs multiplets. In particular, we focus on
a missing-partner model in which the grand unified group is spontaneously
broken by the 75-dimensional Higgs multiplet and the doublet-triplet splitting
problem is solved. We find that in the missing-partner model the D=6 proton
decay rate gets suppressed by about 60%, mainly due to the threshold effect at
the GUT scale, while the SUSY-scale threshold corrections are found to be less
prominent when sfermions are heavy.
