Arginine methylation of RNA-binding proteins regulates cell function and differentiation

Abstract

Arginine methylation is a post-translational modification that regulates protein function. RNA-binding proteins are an important class of cell-function mediators, some of which are methylated on arginine. Early studies of RNA-binding proteins and arginine methylation are briefly introduced, and the enzymes that mediate this post-translational modification are described. We review the most common RNA-binding domains and briefly discuss how they associate with RNAs. We address the following groups of RNA-binding proteins: hnRNP, Sm, Piwi, Vasa, FMRP, and HuD. hnRNPs were the first RNA-binding proteins found to be methylated on arginine. The Sm proteins function in RNA processing and germ cell specification. The Piwi proteins are largely germ cell specific and are also required for germ cell production, as is Vasa. FMRP participates in germ cell formation in Drosophila, but is more widely known for its neuronal function. Similarly, HuD plays a role in nervous system development and function. We review the effects of arginine methylation on the function of each protein, then conclude by addressing remaining questions and future directions of arginine methylation as an important and emerging area of regulation.

Figure 1. Arginine methylation reactions performed by PRMTs

A side chain of arginine is depicted, and the resulting methylation additions applied to it are indicated in red. Identified type I-IV PRMTs from Trypanosoma brucei (Fisk et al. 2009), Saccharomyces cerevisiae (Gary et al. 1996; Niewmierzycka and Clarke 1999), Schizosaccharomyces pombe (Bachand and Silver 2004), Caenorhabditis elegans (Takahashi et al. 2011; Yang et al. 2009), Drosophila melanogaster (Boulanger et al. 2004), and mammals (Krause et al. 2007) are presented. See text for a complete description of the PRMT types and their specific reactions.

Figure 2. Model for role of arginine methylation on RNA-binding protein function

hnRNPs are shown in pink and the Sm proteins are shown in green. Methylation of hnRNP A2 facilitates nuclear import; methylation of hnRNP A1 locks it into a non-specific RNA-binding mode. SmD1, SmD3 and SmB are methylated by PRMT5 (Brahms et al. 2000). Methylation increases the affinity of the Sm proteins for the SMN complex in the cytoplasm (Brahms et al. 2001; Friesen et al. 2001; Selenko et al. 2001). Curled lines represent RNA, unless otherwise noted. Arrows signify movement. Cyt -cytoplasm and Nuc- nucleus.

Figure 3. Impact of methylation on pole plasm formation during Drosophila oogenesis

oskar RNA (red) is held in a translationally inactive state while being transported to the posterior of the oocyte. SmD3 and proper posterior microtubule depolymerization, via the piRNA pathway, are required for normal oskar mRNA localization. By stage 10 of oogenesis, Oskar (Osk) is localized at the posterior pole, and is required for localization of Tudor (Tud), Vasa, and Valois (Val). SmB and SmD3 are methylated by Dart5 (red), which also localizes to the pole, and methylated SmB/D3 interacts with Tudor. Additional mRNAs besides oskar, gurken, and nanos, also localize to the pole, but are not indicated in the diagram. White regions indicate nurse cells; curled lines represent oskar mRNA. Dart 5 is also known as Capsuleen.

Figure 4. Methylation-dependent activities of RNA-binding proteins FMRP and HuD in neurons

FMRP is shown in blue, HuD is shown in brown. FMRP enters the nucleus to bind mRNAs (Kim et al. 2009). In its unmethylated form, FMRP binds RNAs containing a G-quadruplex structure (sc1-like RNA, black). Upon activation, PRMT1 (yellow) methylates FMRP, rendering it unable to bind G-quadruplex-containing structures, but still able to bind other RNAs, like AATYK (green) (Blackwell et al. 2011), which participate in neurite outgrowth (Tomomura et al. 2007). PRMT1 is modulated by cell cycle (Kim et al. 2010) and is required for neurite outgrowth (Miyata et al. 2008). Thus, methylation may direct the differentiation program of neurons by modulating the identity of FMRP-associated RNAs. Methylation of HuD decreases mRNA half-life by reducing its affinity for HuD (Fujiwara et al. 2006). Curled lines represent RNA, unless otherwise noted. Arrows signify movement. Cyt, cytoplasm; Nuc, nucleus; //, indicates distance down the neuronal process to the site of translation.