Deciphering the assembly pathway of Sm‐class U snRNPs

Abstract

The assembly of the Sm-class of uridine-rich small nuclear ribonucleoproteins (U snRNPs), albeit spontaneous in vitro, has recently been shown to be dependent on the aid of a large number of assisting factors in vivo. These factors are organized in two interacting units termed survival motor neuron (SMN)-and protein arginine methyltransferase 5 (PRMT5)-complexes, respectively. While the PRMT5-complex acts early in the assembly pathway by activating common proteins of U snRNPs, the SMN-complex functions to join proteins and RNA in a highly ordered, apparently regulated manner. Here, we summarize recent progress in the understanding of this process and discuss the influence exerted by the aforementioned trans-acting factors. The ability to reconstitute macromolecular complexes from purified single components in vitro has fostered the concept of self-assembly, one of the central principles of molecular biology. From a theoretical, ''in vitro'' point of view, the formation of macromolecular complexes depends on diffusiondriven, random, and reversible encounters of the cognate subunits [1,2]. The stability of the resulting entities is determined by the ratio of the respective association and dissociation rate constants. While these considerations hold true for idealized, aqueous solutions, the situation in vivo is markedly different. In cells, the local concentration of individual components of macromolecular complexes and other proteins is usually relatively low, yet as a whole they occupy a significant fraction of the total volume. Hence, the possibility for unspecific interactions arises, which hinder the assembly pathway. Individual molecules therefore need to be directed to the site of complex assembly, in order to increase their local concentration and protect them against unfavorable interactions. Furthermore, a separation of both the site of assembly and the site of function should help to prevent assembly intermediates from adversely affecting the function of fully assembled macromolecular complexes. Taking these considerations into account, it is not surprising that cells have evolved strategies to ensure the faithful generation of macromolecular assemblies [1]. Among others, three features seem to predominate: (1) the segregation of biosynthesis of individual components and their assembly into higher-order structures into different subcellular compartments; (2) the evolution of molecular chaperones, which promote formation of intermediates, shielding these intermediates from adverse, premature interactions with substrate molecules of the finally assembled macromolecular complexes; and (3) trans-acting factors, working as scaffolds to coordinate several processes of the assembly reaction. A number of biological processes follow, at least in part, these principles and well known examples are the assembly of proteasomes and nucleosomes [3,4].In this review, we summarize recent experimental advances in the understanding of the in vivo biogenesis pathway of macromolecular RNA-protein complexes found in...

Recruitment of the NineTeen Complex to the activated spliceosome requires AtPRMT5

“…PRMT5 plays important roles in U snRNP assembly, in which U1, 2, 4, 5 snRNP Sm proteins Sm B/B′, D1, D3, and U6 snRNPspecific Sm-like protein LSm4 are symmetrically dimethylated by PRMT5 (16,17). These methylated Sm proteins can be recognized by the survival motor neuron (SMN) complex or Tudor staphylococcal nuclease (Tudor-SN) and loaded onto U snRNAs, forming U snRNPs (18)(19)(20).…”

“…PRMT5 plays important roles in U snRNP assembly, in which U1, 2, 4, 5 snRNP Sm proteins Sm B/B′, D1, D3, and U6 snRNPspecific Sm-like protein LSm4 are symmetrically dimethylated by PRMT5 (16,17). These methylated Sm proteins can be recognized by the survival motor neuron (SMN) complex or Tudor staphylococcal nuclease (Tudor-SN) and loaded onto U snRNAs, forming U snRNPs (18)(19)(20).The snRNPs assemble dynamically on a pre-mRNA along with the non-snRNP splicing factors and catalyze two sequential transesterification reactions producing a mature mRNA. Briefly, U1 snRNP first recognizes the conserved 5′ splice site sequence in the intron by base pairing.…”

Arginine methylation mediated by the Arabidopsis homolog of PRMT5 is essential for proper pre-mRNA splicing

“…Such methylation can increase the binding affinity of these Sm proteins for the downstream recipient, Survival Motor Neuron, the spinal muscular atrophy disease gene product (17,18). Subsequently, the PRMT5-and Survival Motor Neuroncomplexes cooperate to load the Sm proteins onto U snRNAs, forming U snRNPs (19,20). As such, it is assumed that symmetric arginine dimethylation is essential for pre-mRNA splicing; however, only in vitro biochemical evidence has emerged (21), and to what extent PRMT5 affects splicing in vivo remains elusive.…”

Intracellular Bacterial Pathogens Trigger the Formation of U Small Nuclear RNA Bodies (U Bodies) through Metabolic Stress Induction

“…In eukaryotes, the machinery driving the splicing process depends on the spliceosome, a set of core catalytic RNA-protein complexes and more than 150 other protein splicing factors (1). Uridine-rich small nuclear ribonucleic acid (U snRNA) molecules function as building blocks of the spliceosome and associate with specific sets of proteins to form complexes known as uridine-rich small nuclear ribonucleoproteins (U snRNPs) 3 (2)(3)(4). The survival of motor neuron (SMN) complex, which consists of nine different proteins including SMN and Gemin proteins 2-8, is directly involved in the cytoplasmic maturation of U snRNAs (5).…”