  The flat spectrum radio quasar 3C 279 is a known $\gamma$-ray variable source
that has recently exhibited minute-scale variability at energies $>100$ MeV.
One-zone leptonic models for blazar emission are severely constrained by the
short timescale variability that implies a very compact emission region at a
distance of hundreds of Schwarzschild radii from the central black hole. Here,
we investigate a hadronic scenario where GeV $\gamma$-rays are produced via
proton synchrotron radiation. We also take into account the effects of the
hadronically initiated electromagnetic cascades (EMC). For a $\gamma$-ray
emitting region in rough equipartition between particles and kG magnetic
fields, located within the broad-line region (BLR), the development of EMC
redistributes the $\gamma$-ray luminosity to softer energy bands and eventually
leads to broad-band spectra that differ from the observed ones. Suppression of
EMC and energy equipartition are still possible, if the $\gamma$-ray emitting
region is located beyond the BLR, is fast moving with Doppler factor ($>70$),
and contains strong magnetic fields ($>100$ G). Yet, these conditions cannot be
easily met in parsec-scale jets, thus disfavouring a proton synchrotron origin
of the Fermi-LAT flare.
