Understanding the driving forces governing protein assembly requires the characterization of interactions at molecular level. We focus on two homologous oppositely charged proteins, lysozyme and alpha-lactalbumin, which can assemble into microspheres. The assembly early steps were characterized through the identification of interacting surfaces monitored at residue level by NMR chemical shift perturbations by titrating one N-15-labeled protein with its unlabeled partner. While a-lactalbumin has a narrow interacting site, lysozyme has interacting sites scattered on a broad surface. The further assembly of these rather unspecific heterodimers into tetrarners leads to the establishment of well-defined interaction sites. Within the tetramers, most of the electrostatic charge patches on the protein surfaces are shielded. Then, hydrophobic interactions, which are possible because alpha-lactalbumin is in a partially folded state, become preponderant, leading to the formation of larger oligomers. This approach will be particularly useful for rationalizing the design of protein assemblies as nanoscale devices.