Figure 1. Domain architecture of proteins. The top panel shows the amino acid sequence of hematopoietic cell kinase. Comparison of this sequence with other proteins reveals three distinct conserved regions, termed SH3, SH2 and tyrosine kinase (TyrKc) domains. These are shown schematically in the middle panel. Each of these domains corresponds to a distinct entity in the three-dimensional structure (lower panel). These regions can be recombined and found with other combinations of domains in other proteins.

Figure 2. Example of protein repeats: (a) the quinoprotein ethanol dehydrogenase contains a repeating ‘PQQ’ region. These do not correspond to stable structures in their own right, but come together to form a compact 3D structure, as shown in (b). Additional insertions in each repeat disrupt the overall symmetry of the molecule.

Figure 3. Trees and function as part of phylogenetic analysis. Functional features common to ‘A’ and ‘C’ are also likely to be shared by ‘B’, whereas they will not be shared by ‘D’ if they have arisen at the point indicated by the circle.