An Unusual Two-Step Control of CPEB Destruction by Pin1

Abstract

Cytoplasmic polyadenylation is a conserved mechanism that controls mRNA translation and stability. A key protein that promotes polyadenylation-induced translation of mRNAs in maturing Xenopus oocytes is the cytoplasmic polyadenylation element binding protein (CPEB). During this meiotic transition, CPEB is subjected to phosphorylation-dependent ubiquitination and partial destruction, which is necessary for successive waves of polyadenylation of distinct mRNAs. Here we identify the peptidylprolyl cis-trans isomerase Pin1 as an important factor mediating CPEB destruction. Pin1 interacts with CPEB in an unusual manner in which it occurs prior to CPEB phosphorylation and prior to Pin1 activation by serine 71 dephosphorylation. Upon induction of maturation, CPEB becomes phosphorylated, which occurs simultaneously with Pin1 dephosphorylation. At this time, the CPEB-Pin1 interaction requires cdk1-catalyzed CPEB phosphorylation on S/T-P motifs. Subsequent CPEB ubiquitination and destruction are mediated by a conformational change induced by Pin1 isomerization of CPEB. Similar to M phase progression in maturing Xenopus oocytes, the destruction of CPEB during the mammalian cell cycle requires Pin1 as well. These data identify Pin1 as a new and essential factor regulating CPEB degradation.

Structural Analysis of the Pin1-CPEB1 interaction and its potential role in CPEB1 degradation

“…We attribute the line broadening to these residues being in proximity with the ligand, with binding occurring in the intermediate exchange regime on the NMR time scale. Overall, the data obtained by NMR and ITC corroborate the interactions revealed in experiments using mammalian cells 14 and indicate that the WW domain of Pin1 and the unphosphorylated CPEB1 N-terminus interact with weak affinity ( Fig. 1b,c ).…”

“…1a ). The boundaries of these constructs were chosen to be similar to that used by Nechama et al 14 , and were extended to include Ser210, known to be essential for degradation after phosphorylation by Cdc2 9 . Since target recognition of Pin1 is achieved by the interaction of the WW domain with the protein partners, we prepared an 15 N-labeled WW domain sample and monitored the chemical shift perturbations induced in the WW domain by the presence of the ligands (1:1 WW: ligand ratio) using HSQC experiments ( Fig.…”

“…Pin1 interacts with phosphorylated CPEB1 in Xenopus oocytes, as well as in mammalian cells 14 . In addition, several studies have indicated that the inter-domain communication between the WW and the catalytic domain in Pin1 is important for the activity in vivo 15 .…”

“…In vivo , the processes of phosphorylation and ubiquitination occur very rapidly 10 with the trans configuration of the pSer-Pro motifs being crucial for SCF β-TrCP ubiquitin ligase recognition 11 12 13 . In this context, it is not surprising that phosphorylated CPEB1 has recently been identified as a target for the peptidyl-prolyl isomerase Pin1 and that the isomerase protein regulates CPEB1 destruction 14 . It has also been observed that the interaction of CPEB1 and Pin1 can occur even in the absence of detectable CPEB phosphorylation 14 suggesting that the pre-association of Pin1 with its unphosphorylated target protein might be essential for ensuring the presence of Pin1 to interact with the phosphorylated substrate when it is required.…”

Functional Integration of mRNA Translational Control Programs

“…De-repression and activation of CPEB1-mediated translation is initiated through Mos-dependent MAP kinase signaling, leading to the translation of the cyclin B1 and CPEB4 mRNAs [ 45 , 54 , 55 ]. Through the subsequent downstream action of cyclin B-CDK signaling and the peptidyl-prolyl cis-trans isomerase, Pin1, CPEB1 undergoes partial degradation [ 56 , 57 , 58 ]. Maintenance of CPEB-mediated translation activation after initiation of CPEB1 degradation is mediated by the compensatory action of CPEB4 [ 46 ].…”