  The paper addresses the problem of suppressing naturally the unsafe d=4 as
well as the color-triplet mediated and/or gravity-linked d=5 proton-decay
operators, which generically arise in SUSY-unification. It also attempts to
give light masses to the neutrinos, of the type suggested by current
experiments. It is noted that neither the symmetries in $SO(10)$, nor those in
$E_6$, suffice for the purpose -- especially in the matter of suppressing
naturally the d=5 proton-decay operators. By contrast, it is shown that a
certain {\it string-derived symmetry}, which cannot arise within conventional
grand unification, but which does arise within a class of three-generation
string-solutions, suffices, in conjuction with $B-L$, to safeguard
proton-stability from all potential dangers, including those which may arise
through higher dimensional operators and the color-triplets in the infinite
tower of states. At the same time, the symmetry in question permits neutrinos
to acquire appropriate masses. This shows that {\it string theory plays an
essential role in ensuring natural consistency of SUSY-unification with two
low-energy observations -- proton-stability and light masses for the
neutrinos}. The correlation between the masses of the extra $Z'$-boson (or
bosons), which arise in these models, and proton-decay rate is noted.
