  We show that one loop GUT scale threshold corrections to gauge couplings are
a significant constraint on the GUT symmetry breaking sector of the theory. The
one loop threshold corrections relate the prediction for $\alpha_s(M_Z)$ to the
proton lifetime. We have calculated these corrections in a new complete SO(10)
SUSY GUT. The results are consistent with the low energy measurement of
$\alpha_s(M_Z)$. We have also calculated the proton lifetime and branching
ratios in this model. We show that proton decay rates provide a powerful test
for theories of fermion masses.
