Multisite Phosphorylation of NIPA at G2/M.

The regulated oscillation of protein expression is an essential mechanism of cell cycle control. The SCF class of E3 ubiquitin ligases is involved in this process by targeting cell cycle regulatory proteins for degradation by the proteasome, with the F-Box subunit of the SCF specifically recruiting a given substrate to the SCF core. We previously reported the cloning of NIPA (Nuclear Interaction Partner of ALK) in complex with constitutively active oncogenic fusions of ALK, which contributes to the development of lymphomas and sarcomas. Subsequently we characterized NIPA as a F-Box protein (FBP) that defines an oscillating ubiquitin E3 ligase. The SCFNIPA complex targets nuclear cyclin B1 for ubiquitination in interphase while phosphorylation of NIPA in late G2 phase and mitosis inactivates the complex to allow for accumulation of cyclin B1. Here, we identify the region of NIPA that mediates binding to its substrate cyclin B1. In addition to the recently described serine residue 354, we specify 2 new residues, Ser-359 and Ser-395, implicated in the phosphorylation process at G2M within this region. Moreover, we found cyclin B1/Cdk1 to phosphorylate NIPA at Ser-395 in mitosis. Mutation of both Ser-359 and Ser-395 impaired effective inactivation of the SCFNIPA complex, resulting in reduced levels of mitotic cyclin B1. Furthermore, we aimed to identify the kinases involved in the initial phosphorylation of Ser-345. Therefore, we tested a panel of different kinases active at the G2M transition such as GSK3?, Casein kinase 2, PLK-1 and Erk1. Effective in vitro phosphorylation of NIPA could only be demonstrated with Erk-1. Moreover, we demonstrate an interaction of Erk-1 and NIPA at G2M but not in interphase cells. Binding of Erk-1 and NIPA led to phosphorylation at Ser-354 in vivo and could be blocked by the MEK-1/MEK-2 inhibitor PD98059. Together these data suggest a process of sequential phosphorylation where NIPA is initially phosphorylated by Erk-1 leading to the dissociation of NIPA from the SCF core complex. Once Ser-354 is phosphorylated, cyclin B1/CDK1 amplifies phosphorylation of NIPA, thus contributing to the regulation of its own abundance in early mitosis. In ALK positive lymphomas enhanced phosphorylation of NIPA at Ser 354 can be observed. We demonstrate that NPM-ALK leads to the activation of Erk-1, thereby phosphorylating and inactivating the SCFNIPA E3 ligase. Inactivation of SCFNIPA may have an important impact on the cell cycle turnover of lymphoma cells and thus for the pathogenesis of NPM-ALK induced lymphomas.