ABSTRACT
Completion of the human genome project has opened up tremendous opportunities for the study of complex biological processes at the molecular level.1 We now know that only about 1%–2% of the genome encodes for proteins.2 These gene products perform all cellular functions from metabolism to developmental control to apoptosis and cell death. In order to comprehend how the cell works and thus the whole organism, it is important to know the detailed functions of these proteins. From the start, we need to elucidate their three-dimensional (3D) structures and their relationships and interactions with other proteins. Some proteins, such as myoglobin, exist freely in the cytosol as monomers. Others associate into protein complexes, the simplest of which are dimers, either with another identical protein subunit (homodimer), for example, malate dehydrogenase, or a different protein subunit (heterodimer), for example, creatine kinase.3 Still others may associate into higher architectural organizations such as tetramers, pentamers, hexamers, and larger multicomponent aggregates. Examples of these organizations are shown in Figure 1.1.4-8 As the number of components increases, so do the complexities of the protein interactions. It then becomes more difŠcult to elucidate the sites of protein contacts and the 3D dispositions of the individual subunits.
