sentences:  cAMP is a second messenger that influences multiple cellular functions via the activation of two downstream effector molecules, protein kinase A (PKA) and the exchange proteins directly activated by cAMP (Epac-1 and -2).Epac-1 and PKA are both important factors involved in the inhibition of AM bacterial killing.The effects of PKA results from its ability to phosophorylate serine and threonine residues on many cellular proteins, especially transcription factor cAMP response element binding protein (CREB).