Fork-headed domains

Weigel, D. and H. Jackie (1990). "The fork head domain a novel DNA binding motif of eukaryotic transcription factors " Cell 63(3) 455-456. 

Other negative correlations are simpler to interpret. Two functionally antagonistic enzymes, namely, protein kinases and protein phosphatases, have, to date, not been found in the same protein. Similarly, WW and SH3 domains that both bind the similar polyproline-containing substrates are never found together. This last finding, however, is curious since 216 proteins that contain either two or more WW, or two or more SH3, domains are known. Finally, it appears that proteins with domains that bind phosphoserine or phosphothreonine (FHA, fork-head-associated domains) never contain domains that bind phosphoty-rosine (SH2, PTB, and PTBI domains). This indicates that cytoplasmic signaling via phosphoserine or phosphothreonine occurs via pathways distinct from those signaling via phosphotyrosine. 

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