To identify the interacting protein, we usedin vitrotranscribed biotinylatedmper23-UTR to isolate RNA-bound proteins, which were separated by SDS-PAGE (data not shown)

To identify the interacting protein, we usedin vitrotranscribed biotinylatedmper23-UTR to isolate RNA-bound proteins, which were separated by SDS-PAGE (data not shown). regulation ofmper2proposes that post-transcriptional mRNA decay mediated by PTB is usually a fine-tuned regulatory mechanism that includes dampening-down effects during circadian mRNA oscillations. == INTRODUCTION == The suprachiasmatic nucleus (SCN) in the hypothalamus governs circadian clock oscillation in mammals (1,2). Circadian oscillations in response to numerous stimuli have been demonstrated not only at the SCN in the brain FX1 but also in various peripheral tissues, and even in cultured cell lines (3). These oscillation mechanisms are regulated by a transcriptional unfavorable opinions loop including binding of the BMAL1/CLOCK heterodimer to the E/E box in FX1 the promoter region of clock genes and opinions inhibition by accumulated Per-Cry protein heterodimer, and by post-translational modification of protein products by cellular kinases and degradation by proteasomes (4). Among the core clock gene family, period 2 and period 1 are responsive to photic entrainment from the retina to the SCN (1,2,5,6). Robust circadian oscillations ofper2mRNA and Per2 protein are not only evident in the brain, but are also present in various peripheral tissues (7,8). period 2 is also known to have a physiological role in tumor suppression (9,10) and feeding/energy metabolism (11). For these reasons, period 2 regulation has been studied more than any other circadian molecule at the translational or post-translational level; however, no reports have demonstrated regulation at the level of mRNA degradation. In general, mRNA degradation is controlled by two major components, the mRNA binding motif (cis-acting element, CAE) and its binding partners (trans-acting factors, TAFs) (12); thus mRNAs containing a CAE in their 3-UTR may be regulated by various binding factors. The AU-rich element (ARE) in the 3-UTR is well characterized and is the most commonly used CAE in post-transcriptional regulation. The fate of the mRNA is therefore largely determined by associations between the ARE and its binding factors (13). The polypyrimidine tract-binding protein (PTB), also known as heterogeneous nuclear ribonucleoprotein (hnRNP) I, is a cellular RNA-binding protein that is abundant in many tissues. PTB binds to oligopyrimidine tracts in introns and regulates negative or positive exon definition in the differential splicing of various cellular mRNAs (1417). More recently, PTB was shown to be involved in other aspects of RNA metabolism, such as modulation of polyadenylation efficiency (18) and the FX1 stability of nitric oxide synthase mRNA during inflammation (19). Moreover, PTB represents the prototype of a heterogeneous group of cellular RNA-binding proteins that are directly involved in the regulation of translation, a function that was discovered during investigation of translation of picornaviruses (20,21). Activated cytoplasmic PTB appears to affect the nutrient-dependent stabilization of mRNAs involved in insulin BMP6 synthesis and secretion (22,23). In addition, regulation of protein kinase A by various hormone signaling pathways promotes nucleocytoplasmic relocalization of PTB (24), suggesting a direct role of PTB in the regulation of mRNA stability. Circadian rhythms have been studied using FX1 transgenic or knock-out animal models to investigate the molecular mechanisms underlying the control of gene expression at the transcriptional level (2528), whereas transcriptional feedback regulation is considered a representative model for understanding the phenomenum of circadian oscillation, little is known about the role of post-transcriptional regulation in circadian rhythm. Several reports have demonstrated FX1 oscillatory mechanisms mediated by post-transcriptional regulation in the fruit fly (29,30) and recent studies in mammalian systems revealed that regulation of mRNA decay is actively involved in circadian oscillation (31,32). Kojimaet al.also reported post-transcriptional regulation of the core clock gene PER1; however, this occurred through modulation of the translational process. Regulatory mechanisms acting at many different stages from core clock gene transcription to protein activation allow finely-tuned rhythmic oscillation. With the ongoing addition of new molecular components to the core model, our understanding.