The design of proteins that self-assemble into well-defined, higher order structures is an important goal that has potential applications in synthetic biology, materials science, and medicine. We previously designed a two-component protein system, designated A-(+) and A-(−), in which self-assembly is mediated by complementary electrostatic interactions between two coiled-coil sequences appended to the C-terminus of a homotrimeric enzyme with C3 symmetry. The coiled-coil sequences are attached through a short, flexible spacer sequence providing the system with a high degree of conformational flexibility. Thus, the primary constraint guiding which structures the system may assemble into is the symmetry of the protein building block.