  Higher-dimensional models of grand unification allow us to relate the top
Yukawa coupling y_t to the gauge coupling g. The tree level relation y_t=g at
the scale of grand unification implies, in the framework of the MSSM, a rather
small ratio of Higgs expectation values tan beta. We find that, in the presence
of localized Fayet-Iliopoulos terms, y_t is suppressed against g because the
bulk fields acquire non-trivial profiles whose overlap is smaller than in the
case of flat profiles. This increases the prediction for tan beta to moderately
large values. Thus tan beta is related to the geometry of compact space. We
also discuss explicit realizations of such settings in orbifold
compactifications of the heterotic string. It turns out that anisotropic
compactifications, allowing for an orbifold GUT interpretation, are favored.
