Not1 mediates recruitment of the deadenylase Caf1 to mRNAs targeted for degradation by tristetraprolin.

The carbon catabolite repressor protein 4 (Ccr4)-Negative on TATA (Not) complex controls gene expression at two levels. In the nucleus, it regulates the basal transcription machinery, nuclear receptor-mediated transcription and histone modifications. In the cytoplasm, the complex is required for messenger RNA (mRNA) turnover through its two associated deadenylases, Ccr4 and Caf1. Not1 is the largest protein of the Ccr4-Not complex and serves as a scaffold for other subunits of the complex. Here, we provide evidence that human Not1 in the cytoplasm associates with the C-terminal domain of tristetraprolin (TTP), an RNA binding protein that mediates rapid degradation of mRNAs containing AU-rich elements (AREs). Not1 shows extensive interaction through its central region with TTP, whereas binding of Caf1 is restricted to a smaller central domain within Not1. Importantly, Not1 is required for the rapid decay of ARE-mRNAs, and TTP can recruit the Caf1 deadenylase only in presence of Not1. Thus, cytoplasmic Not1 provides a platform that allows a specific RNA binding protein to recruit the Caf1 deadenylase and thereby trigger decay of its target mRNAs.

Control of mRNA decay by phosphorylation of tristetraprolin.

TTP (tristetraprolin) is an RNA-binding protein that suppresses inflammation by accelerating the degradation of cytokine mRNAs. TTP binds to an AU-rich element in the 3'-untranslated region of its target mRNAs. In macrophages, the induction of cytokine expression requires activation of the p38-MAPK (mitogen-activated protein kinase)-MK2 [MAPKAP (MAPK-activated protein) kinase-2] kinase cascade. MK2 directly phosphorylates TTP and thereby contributes to transient stabilization of cytokine mRNAs. In the present review, we address the target specificity of TTP, summarize TTP-interacting proteins and discuss how phosphorylation regulates the activity, localization and stability of TTP.