  We consider a non-supersymmetric $E_6$ Grand Unified Theory (GUT) with
intermediate trinification symmetry $SU(3)_C \times SU(3)_L \times SU(3)_R
\times D$ (D denoted as D-parity for discrete left-right symmetry) and study
the effect of one-loop threshold corrections arising due to every class of
superheavy particles (scalars, fermions and vectors). It is observed that, the
intermediate mass scale $M_I$ and $\sin^2\theta_W$ remain unaffected by GUT
threshold contributions. The threshold modified unification mass scale $M_U$ is
in excellent agreement with the present experimental proton decay constraint.
The novel feature of the model is that GUT threshold uncertainty of $M_U$ is
found to be controlled by superheavy scalars only, leading to a very predictive
scenario for proton decay, which can be verifiable within the foreseeable
experiments.
