Proteins Associate into Multimeric Structures and Macromolecular Assemblies

Multimeric proteins consist of two or more polypeptides or subunits. A fourth level of structural organization, quaternary structure, describes the number (stoichiometry) and relative positions of the subunits in multimeric proteins. Hemagglu-tinin, for example, is a trimer of three identical subunits held together by noncovalent bonds (Figure 3-7b). Other multimeric proteins can be composed of any number of identical or different subunits. The multimeric nature of many proteins is critical to mechanisms for regulating their function. In addition, enzymes in the same pathway may be associated as subunits of a large multimeric protein within the cell, thereby increasing the efficiency of pathway operation.

The highest level of protein structure is the association of proteins into macromolecular assemblies. Typically, such structures are very large, exceeding 1 mDa in mass, approaching 30-300 nm in size, and containing tens to hundreds of polypeptide chains, as well as nucleic acids in some cases. Macromolecular assemblies with a structural function include the capsid that encases the viral genome and bundles of cytoskeletal filaments that support and give shape to the plasma membrane. Other macromolecular assemblies act as molecular machines, carrying out the most complex cellular processes by integrating individual functions into one coordinated process. For example, the transcriptional machine that initiates the synthesis of messenger RNA (mRNA) consists of RNA polymerase, itself a multimeric protein, and at least 50 additional components including general transcription factors, promoter-binding proteins, helicase, and other protein complexes (Figure 3-9). The transcription factors and promoter-binding proteins correctly position a poly-merase molecule at a promoter, the DNA site that determines where transcription of a specific gene begins. After helicase unwinds the double-stranded DNA molecule, polymerase simultaneously translocates along the DNA template strand and synthesizes an mRNA strand. The operational details of this complex machine and of others listed in Table 3-1 are discussed elsewhere.

TABLE 3-1 Selected Molecular Machines

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