  Hidden sector scenarios in which dark matter (DM) interacts with the Standard
Model matter fields through the exchange of massive Z' bosons are well
motivated by certain string theory constructions. In this work, we thoroughly
study the phenomenological aspects of such scenarios and find that they present
a clear and testable consequence for direct DM searches. We show that such
string motivated St\"uckelberg portals naturally lead to isospin violating
interactions of DM particles with nuclei. We find that the relations between
the DM coupling to neutrons and protons for both, spin-independent (fn/fp) and
spin-dependent (an/ap) interactions, are very flexible depending on the charges
of the quarks under the extra U(1) gauge groups. We show that within this
construction these ratios are generically different from plus and minus 1 (i.e.
different couplings to protons and neutrons) leading to a potentially
measurable distinction from other popular portals. Finally, we incorporate
bounds from searches for dijet and dilepton resonances at the LHC as well as
LUX bounds on the elastic scattering of DM off nucleons to determine the
experimentally allowed values of fn/fp and an/ap.
