Domain families evolution sequence motifs

Many examples of recurring domain or motif structures are available, and these reveal that protein tertiary structure is more reliably conserved than primary sequence. The comparison of protein structures can thus provide much information about evolution. Proteins with significant primary sequence similarity, and/or with demonstrably similar structure and function, are said to be in the same protein family. A strong evolutionary relationship is usually evident within a protein family. For example, the globin family has many different proteins with both structural and sequence similarity to myoglobin (as seen in the proteins used as examples in Box 4-4 and again in the next chapter). Two or more families with little primary sequence similarity sometimes make use of the same major structural... 

The P propeller domain is a widespread protein organizational motif. Typically, p-propeller proteins are encoded by repeated sequences where each repeat unit corresponds to a twisted P-sheet structural motif these P-sheets are arranged in a circle around a central axis to generate the p-propeller structure. Two superfamilies of P-propeller proteins, the WD-repeat and Kelch-repeat families, exhibit similarities not only in struaure, but, remarkably, also in the types of molectdar functions they perform. Whde it is unlikely that WL) and Kelch repeats evolved from a common ancestor, their evolution into diverse families of similar function may reflect the evolutionary advantages of the stable core P-propeller fold. In this chapter, we examine the relationships between these two widespread protein families, emphasizing recently published work relating to the structure and funrtion of both Kelch and WD-repeat proteins. 

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