Themes in Signaling   Molecular surface representation of the conformation of the activation loops of Insulin receptor PTK domain (IRK) and tyrosine phosphorylated PTK domain of Insulin receptor (IRK3P). The ac tivation loops are colored green, the catalytic loops are in orange and the peptide substrate is pink. Carbon atoms are colored white, nitrogen atoms blue, oxygen atoms red and phosphorous atoms yellow. AMP-PNP is also shown in the IRK3P structure (Courte sy of S.R. Hubbard).