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We present crystal structures of alpha - COP and beta ' - COP bound to a series of naturally occurring retrieval motifs - encompassing KKxx , KxKxx and non - canonical RKxx and viral KxHxx sequences .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
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