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An extensive shuffling of blocks of protein sequence (protein modules) has occurred during the evolution of proteins. Those portions of a protein denoted by the same shape and color are evolutionarily related but not identical. (A) The bacterial catabolite gene activator protein (CAP) contains one domain ( blue triangle) that binds a specific DNA sequence and a second domain ( red rectangle) that binds cyclic AMP (see Figure 3-35). The DNA-binding domain here is related to the DNA-binding domains of many other gene regulatory proteins, including the lac repressor and cro repressor proteins. In addition, two copies of the cyclic-AMP-binding domain are found in eucaryotic protein kinases regulated by the binding of cyclic nucleotides. (B) Serine proteases like chymotrypsin are formed from two domains ( brown). In some related proteases that are highly regulated and more specialized, the two protease domains are connected to one or more domains homologous to domains found in epidermal growth factor ( green hexagon), to a calcium-binding protein ( yellow triangle), or to a "kringle" domain ( blue square) that contains three internal disufide bridges.