Anomalous U(1)'s and proton stability in brane models

We consider the most general generation-independent U(1) gauge symmetry con sistent with the presence of Yukawa couplings for all quarks and leptons in the SUSY version of the Standard Model. This U(1) has generically mixed an omalies with SM groups, which can not be cancelled by the Green-Schwarz mec hanism of heterotic D = 4 strings. We argue that these anomalies can in pri nciple be cancelled by the generalized Green-Schwarz mechanism present in f ield theories corresponding to D-branes at singularities. Moreover, unlike the heterotic case, once the U(1) symmetry is broken it may remain as an ex act perturbative global symmetry in the low energy theory. Applying this sc heme to the SUSY SM we find that gauging such a general U(1): i) B and L vi olating operators at least up to dim = 3; 4; 5; 6 are generically forbidden ; ii) The mu-term is generically supressed. We also study the properties of a U(1)(x) symmetry whose mixed anomalies with the different SM gauge group s are in the ratio of the beta function coefficients beta(a). This relation has been shown to hold in certain orientifold models. In all cases the U(1 ) remains as a global symmetry at the orientifold singularity, the SM Higgs can break it at the electroweak scale, making possible to relate the blowi ng-up of the singularity with electroweak symmetry breaking.