  Parton distributions contain factorizable final state interaction effects
originating from the fast-moving struck quark interacting with the target
spectators in deeply inelastic scattering. We show that these interactions give
rise to gauge invariance of the transverse momentum-dependent parton
distributions. As compared to previous analyses, our study demonstrates the
existence of extra scaling contributions from transverse components of the
gauge potential at the light-cone infinity. They form a transverse gauge link
which is indispensable for restoration of the gauge invariance of parton
distributions in the light-cone gauge where the gauge potential does not vanish
asymptotically. Our finding helps to explain a number of features observed in a
model calculation of structure functions in the light-cone gauge.
