PKA-activation mechanism revised Many hormone receptors stimulate production of cyclic AMP (adenosine monophosphate), which activates PKA (protein kinase A). The textbook view suggests that activation releases the catalytic subunit of the enzyme from its complex with the regulatory subunit. Smith et al. closely monitored activation of PKA in cultured human cells and found that dissociation of the holoenzyme was not necessary for activation. The kinase, which binds anchoring proteins that localize it in the cell, appears to be restricted to acting within about 200 Å of such anchoring proteins. Thus, PKA activity is more precisely targeted within the cell than previously anticipated. Science , this issue p. 1288