AGC protein kinase phosphorylation and protein kinase C

Protein kinase cascades feature in many signal transduction pathways. For t hose discussed here, a single upstream protein kinase appears to be respons ible for the control of multiple downstream targets. So how is specificity introduced into these events? For the downstream kinases (substrates) descr ibed here, it would appear that specificity is determined by substrate-dire cted events that are permissive for phosphorylation. There are also distinc tions relating to the turnover of these phosphorylations providing a furthe r element of specificity.