e implement the Witten’s loop mechanism for the right-handed Majorana neutrino mass generation within the framework of the non-SUSY unification based on $SU(5)\times U(1)$ gauge group -- so called flipped SU(5). Since in this model the mass matrices of up-type quarks and Dirac neutrinos differ only by transposition, the mixing matrix relevant for proton decay computation is closely related to lepton mixing. With the recent data on PMNS mixing angles at hand, bounds on the proton lifetime are computed. Finally, our model is compared with SUSY flipped SU(5). This is joint work with Michal Malinsky and Carolina Arbelaez Rodriguez.