Figure 1. (a) A. tumefaciens Tram homodimer; (b) vesicular stomatitis virus G protein homotrimer; (c) chimaeric heterotrimeric G protein; (d) obligatory homodimerization of globin and globin in human haemoglobin; (e) nonobligatory heterodimer of human growth hormone complexed with human growth hormone receptor.

Figure 2. Affinity tag pulldown experiments allow selective capture of a tagged protein (red circles) and binding partners (green circles) on to a solid support, such as a bead. Co-immunoprecipitation experiments are based on similar principles, except that antibodies are used to tether bait proteins to a solid support.

Figure 3. (a) Fluorophores (green circles) excited with polarized light emit polarized photons (black arrows emanating from fluorophores). Measuring fluorescence emission with polarizations parallel and perpendicular to the polarization of excitation provides an indication of the degree to which the fluorescently labelled protein molecules (blue circles) are tumbling in solution; (b) protein-binding partners (orange circles) interact with the fluorescently labelled proteins to form a complex that tumbles more slowly, increasing the polarization and the anisotropy of the fluorophores.

Figure 4. (a) To achieve fluorescence resonance energy transfer, the donor emission spectrum must overlap with the acceptor absorption spectrum; (b) a protein with a donor fluorophore (cyan circle) is excited with photons of wavelength ₁ and emits photons of wavelength ₂. When brought into close proximity to an acceptor fluorophore (yellow circle) due to protein complex formation, FRET results in emission by the acceptor fluorophore of photons of wavelength ₃.