  Differential production cross sections of $K^{\pm}$ mesons have been measured
in $p$ + C and $p$ + Au collisions at 1.6, 2.5 and 3.5 GeV proton beam energy.
At beam energies close to the production threshold, the $K^-$ multiplicity is
strongly enhanced with respect to proton-proton collisions. According to
microscopic transport calculations, this enhancement is caused by two effects:
the strangeness exchange reaction $NY \to K^- NN$ and an attractive in-medium
$K^-N$ potential at saturation density.
