Specificity is perhaps the most enigmatic property of hormonemediated signaling pathways,
especially when one considers that more than 30 hormones employ the ubiquitous second
messengers, Ca2, phospholipid, or cAMP, to relay messages from the cell membrane to
intracellular effectors. In most cases the net signaling effect is to activate protein kinases or
phosphoprotein phosphatases, which, in turn, alter the phosphorylation state of cellular
target proteins. Although both enzyme classes have been intensely researched since the …