We are studying the molecular organization of protein arrays using two-dimensional streptavidin crystals bound to biotinylated lipid monolayers at the air-water interface. We constructed a mutant form of the streptavidin protein that successfully alters the molecular organization of the streptavidin crystals. Cocrystallization of streptavidin carrying this single targeted point mutation with wild-type streptavidin yields two-dimensional crystals displaying a chiral morphology with molecular coexistence, indicating a solid-phase transition. The phase coexistence and resulting morphologies are reminiscent of two-dimensional crystal behavior of wild-type streptavidin near its isoelectric point, and this analogy is discussed. These results demonstrate the potential to manipulate protein array formation through point mutagenesis and cocrystallization.