 Protein complexes . A 2006 study purified 4 @,@ 339 proteins from cultures of strain K @-@ 12 and found interacting partners for 2 @,@ 667 proteins , many of which had unknown functions at the time . A 2009 study found 5 @,@ 993 interactions between proteins of the same E. coli strain , though these data showed little overlap with those of the 2006 publication . 
