TOP mRNPs: Molecular Mechanisms and Principles of Regulation

Abstract

The cellular response to changes in the surrounding environment and to stress requires the coregulation of gene networks aiming to conserve energy and resources. This is often achieved by downregulating protein synthesis. The 5' Terminal OligoPyrimidine (5' TOP) motif-containing mRNAs, which encode proteins that are essential for protein synthesis, are the primary targets of translational control under stress. The TOP motif is a cis-regulatory RNA element that begins directly after the m7G cap structure and contains the hallmark invariant 5'-cytidine followed by an uninterrupted tract of 4-15 pyrimidines. Regulation of translation via the TOP motif coordinates global protein synthesis with simultaneous co-expression of the protein components required for ribosome biogenesis. In this review, we discuss architecture of TOP mRNA-containing ribonucleoprotein complexes, the principles of their assembly, and the modes of regulation of TOP mRNA translation.

Keywords: 5’ Terminal Oligopyrimidine; LARP1; RNA; RNA binding proteins; translation regulation.

Figure 1

Cap-dependent translation initiation and regulation. Upper panel: Under normal conditions, mTORC1 phosphorylates and inactivates 4E-BPs. eIF4E binds to the m7G cap of the transcript and recruits eIF4G, eIF4A (4A), and eIF4B (4B) forming the eIF4F cap binding complex. eIF4F recruits the 43S pre-initiation complex (PIC) consisting of eIF3, the ternary complex (eIF2, GTP, and the initiator Methionine tRNA), and the 40S small ribosomal subunit to form the 48S PIC. Lower Panel: During nutrient deprivation, mTORC1 is inactivated resulting in hypo-phosphorylated 4E-BP. 4E-BP can then bind to eIF4E and block assembly of eIF4F, in turn, halting translation initiation. The eIF2α kinases are activated during stress responses and phosphorylate eIF2α. This phosphorylation interferes with GDP exchange and renders the ternary complex inactive.

Figure 2

Schematics of 5’ TOP motif binding proteins and their RNA binding domains. La possesses the La Module consisting of the La Motif (LaM, orange) as well as two RNA recognition motifs (RRM, green). LARP1 also has the La module but with a poly(A) binding protein (PABP)-interacting motif (PAM2, purple). There is also the DM15 (red) domain in the C-terminal region that is required for 5’ TOP motif and cap binding. Cellular Nucleic Acid Binding Protein (CNBP) contains seven CCHC-zinc finger repeats (1-7, blue) as well as an arginine/glycine rich motif (RGG, yellow). AUF1 has four isoforms, p45 is the chosen canonical isoform shown here. AUF1 contains two RRMs as well as an RGG motif. All four isoforms contain the RRMs. In p42 and p45, the RGG is interrupted by an alternatively spliced exon.

Figure 3

LARP1 regulates translation of 5’ TOP motif containing transcripts. Upper panel: Under normal nutrient conditions, the DM15 domain of LARP1 is unable to bind to the m7G cap allowing eIF4E binding, eIF4F assembly, and procession of translation initiation. The La module of LARP1 binds to the poly(A) tail, PABP, and the 5’ TOP motif in a cap independent manner. These interactions promote circularization of the TOP transcript and enhance translation. Lower panel: Nutrient deprivation leads to inactivation of mTORC1 and hypophosphorylation of residues in or near the DM15 domain allowing it to bind to the 5’ TOP motif and the m7G cap. DM15 blocks interactions of eIF4E with the cap and halts the downstream steps required for cap-dependent translation initiation thereby repressing translation.

Figure 4

Multiple proteins bind to 5’ TOP motifs and affect translation. Schematic depicting potential interactions of RNPs with the 5’ TOP motif. Red arrows depict inhibition of expression and blue arrows depict promoting effects on expression. Different part of LARP1 (green) can interact with the TOP motif and can both promote or repress translation depending on the context. La (red) and TIA1/R (blue) bind to U-rich regions of the TOP motif and are thought to play an inhibitory role in TOP expression. AUF1 (brown) also binds to U-rich regions of the TOP motif but appears to have a promoting effect on expression. Furthermore, La, TIA1/R, and AUF1 may compete for binding sites. CNBP interacts with G-rich regions near the distal part of the TOP motif. CNBP as well as AUF1 may associate with secondary structures and potential G4 quadraplexes that may form in this region of some TOP motifs through their RGG motifs.