Proline Kinase Library
Serine or threonine residues preceding proline (Ser/Thr-Pro) are the major regulatory phosphorylation motifs that function in diverse cellular processes. Enzymes responsible for such phosphorylation belong to a large family of so-called Pro-directed protein kinases, including cyclin-dependent protein kinases (CDKs), mitogen-activated protein kinases (MAPKs), Jun N-terminal protein kinases (JNKs) and glycogen synthase kinase-3 (GSK-3). Enzymes that reverse the phosphorylation step are Ser/Thr phosphatases, including phosphatase 2A (PP2A), RNA polymerase II C-terminal domain (RNA Pol II CTD) phosphatase (FCP1) and calcineurin. These kinases and phosphatases play a crucial role in diverse cellular processes such as the cell cycle, transcription and various signal-transduction pathways, as well as in human diseases such as cancer and Alzheimer’s disease. Ser/Thr phosphorylation is regulated by inducing conformational changes. 
 K. P. Lu, Y. C. Liou, and X. Z. Zhou, “Pinning down proline-directed phosphorylation signaling,” Trends Cell Biol., vol. 12, no. 4, pp. 164–172, 2002, doi: 10.1016/S0962-8924(02)02253-5.