Identification of mitogen-activated protein kinase docking sites in enzymes that metabolize phosphatidylinositols and inositol phosphates

We found that several, but not all, of these enzymes contain identifiable D-domain sequences. Further, we found a high degree of conservation of these sequences and their location in human and mouse proteins; notable exceptions were PI 3-kinase C2-γ, PI 4-kinase type IIβ, and inositol polyphosphate 1-phosphatase.The results indicate that there may be extensive crosstalk between MAPK signaling and signaling pathways that are regulated by cellular levels of PIs or IPs.MAPKs catalyze the transfer of the γ-phosphate of adenosine triphosphate (ATP) to serine (S) or threonine (T) residues that precede proline (P) [1,2]; thus, these enzymes are termed proline-directed serine/threonine kinases. Although the sequences ST and TP are sufficient for phosphorylation to occur, the optimal sequence for phosphorylation by a MAPK is PX(S/T)P [1,3]. The majority of cellular proteins contain an SP or a TP sequence, yet, many of these proteins are not MAPK substrates [4], indicating that a mechanism exists for achieving substrate specificity for the MAPKs. This specificity is conferred by the substrate through a docking domain. In addition to underlying specificity, these docking interactions increase the catalytic efficiency of substrate phosphorylation [5-7].A MAPK docking site, distinct from the phosphoacceptor site, was first identified in c-Jun [8,9], a c-Jun N-terminal kinase (JNK) substrate; this site was designated the "δ domain". Subsequently, a JNK binding site in the transcription factor ATF-2 [10,11] and a motif termed the "d-box" of Elk-1 that binds ERK2 [4,12] were noted to be similar in sequence to the JNK binding site in c-Jun. Related motifs have been identified in a number of other proteins and have been given various names, including DEJL (docking sites for ERK and JNK, LXL) domain [4], kinase interaction motif (KIM) [13,14], MAPK-docking site [15,16], D box [5,12], D-site [17] and D-domain [6,18-20]. It is important to note that, although these domains were identifi