Many proteins exhibit a fourth level of organization, referred to as quaternary structure. This involves noncovalent association between individual protein molecules, which may themselves have multiple domains, to form defined aggregated structures. These may be long chains of globular proteins, as in the muscle protein actin (Figure 1). More usual are symmetrical arrays of a small number of monomer units, often called protomer units: arrays of 2, 4, and 6 protomers are common, but cases of up to 48 units are known. Some examples are shown in Figure 2. Quaternary structures can be considered special cases of the very general phenomenon of protein–protein interactions. Such interactions may be quite stable, resulting in what we call quaternary assemblies or multisubunit proteins. But they may, in other cases, be transitory but nonetheless vital to the dynamics of the cell. How complex such patterns can be is only now being recognized, in a discipline called bioinformatics.
Fig1. Proteins can form filaments of identical or nonidentical subunits. (A) Actin, the monomer globular actin, G-actin, and the helical polymer of actin filaments (F-actin). G-actin is globular in structure; the polymer is a two-stranded helical arrangement of G-actin. Polymerization is induced upon binding of ATP to G-actin; ATP hydrolysis occurs, but ADP stays bound to the filament. The asymmetry of the individual subunits leads to polarity of the filament, defined as + and – ends; the two ends grow at different rates. (B) Tropocollagen, the basic unit of collagen fibers, is a helix of three polypeptide chains, each ~1000 residues in length. Each chain is a left-handed helix with ~3.3 residues/turn. The three helices wrap around each other in a right-handed sense; the structure is stabilized by hydrogen bonds. The structure imposes constraints on the primary sequence, with glycine-proline and hydroxyproline forming a repetitive motif. The image of collagen fibers on the right is taken by atomic force microscopy, AFM. (C) Microtubule, a hollow cylinder. Along the microtubule axis, α- and β-tubulin heterodimers are joined end-to-end to form protofilaments with alternating α- and β-subunits. Staggered assembly of 13 protofilaments yields a helical arrangement of tubulin heterodimers in the cylinder wall. [B, left, from Mathews CK, van Holde KE, Appling DR et al. (2012) Biochemistry, 4th ed. With permission from Pearson Prentice Hall. Illustration rendered by Graham Johnson. Right, from J Gross, Biozentrum / Science Photo Library. C, bottom, from Li H, DeRosier DJ, Nicholson WV et al. (2002) Structure 10: 1317–1328. With permission from Elsevier.]
Fig2. A gallery of proteins whose quaternary structures contain identical or highly similar protein subunits. (A) Topoisomerase VI is an example of proteins that contain two identical subunits. (B) PCNA (proliferating cell nuclear antigen) is formed by three identical subunits. (C) Hemoglobin contains two α- and two β-subunits.
The protomers comprising a quaternary assembly may be the same or of several different kinds; if only one type is involved, they are usually arranged in some type of point-group symmetry, with one or more axes of symmetry. In Figure 2, topoisomerase shows twofold symmetry; rotation of the molecule 180° about an axis in the plane of this page will yield exactly the same image. Proliferating cell nuclear antigen, PCNA, has a threefold symmetry axis, perpendicular to the plane of the page. Hemoglobin is said to have pseudo-twofold symmetry, because the two kinds of subunits are almost, but not quite, identical.
Only rarely is molecular size itself a reason for a protein to have a defined quaternary structure. More often than not, such arrangements facilitate allosteric regulation of protein function. Consider the difference between hemoglobin (see Figure 2) and myoglobin, which is very much like a protomer of hemoglobin. Myoglobin will bind oxygen, and does so in tissues. But hemoglobin is much better suited to the delivery of oxygen because it binds and releases oxygen cooperatively: the presence of oxygen on one protomer facilitates its binding to another. Furthermore, the binding of other allosteric effectors to hemoglobin modifies oxygen affinity, allowing adaption to varied circumstances. For a deeper understanding of allostery.
In some cases, quaternary structure becomes so elaborate and extensive that a molecule is described as a multiprotein complex. Later chapters feature many examples of quaternary complexes involving as many as a dozen or more different kinds of protomers, grouped together to fulfill some complex function and its regulation.
Such complexes are often associated with fundamental cell processes, like replication of DNA, transcription of DNA into RNA, or synthesis of proteins. Many of the protomer types in such complexes bind allosteric effectors, which in turn modulate the function of the whole complex. More and more, it is becoming clear that the cell is a network of interacting molecules. In many respects, quaternary structure and domain structure are complementary ways of achieving the same goals. Quaternary assembly, however, has the advantage that functional units can be exchanged for varied needs.
Quaternary structure is stabilized by the same kinds of noncovalent interactions that account for secondary and tertiary structure in proteins (see Table 1). As these involve specific groups on the surfaces of the protomers, we know that the quaternary arrangement is dictated by the protomer tertiary structure, and ultimately by the sequence of the polypeptide chain or chains.
Table1. Molecular interactions determining protein secondary, tertiary, and quaternary structure.
