  We measured the magnetoresistance of the 2D electron liquid formed at the
(111) LaAlO$_3$/SrTiO$_3$ interface. The hexagonal symmetry of the interface is
manifested in a six-fold crystalline component appearing in the anisotropic
magnetoresistance (AMR) and planar Hall data, which agree well with symmetry
analysis we performed. The six-fold component increases with carrier
concentration, reaching 15% of the total AMR signal. Our results suggest the
coupling between higher itinerant electronic bands and the crystal as the
origin of this effect and demonstrate that the (111) oxide interface is a
unique hexagonal system with tunable magnetocrystalline effects.
