Evolutionary conserved motifs

SNAREs comprise a superfamily of proteins that function in all membrane fusion steps of the secretory pathway within eukaryotic cells. They are small proteins that vary in structure and size (see Section 1.1), but share an evolutionary conserved stretch of 60-70 amino acids containing eight heptad repeats, which is termed SNARE motif (Brunger 2005). The number of different SNAREs varies between different organisms, ranging from 25 in yeast, 36 in mammals, to over 50 in plants. Each fusion step requires a specific set of four different SNARE motifs that is contributed by three or four different SNAREs, and each of the membranes destined to fuse contains at least one SNARE with a membrane anchor. 

Wuchty, S., Z. N. Oltvai, and A. L. Barabasi. (2003). Evolutionary conservation of motif constituents in the yeast protein interaction network. Nat Genet 35 176-179 Oct... 

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... 

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