Summary Functional micropeptides can be concealed within RNAs that appear to be non-coding. We discovered a conserved micropeptide, that we named myoregulin (MLN), encoded by a skeletal muscle-specific RNA annotated as a putative long non-coding RNA. MLN shares structural and functional similarity with phospholamban (PLN) and sarcolipin (SLN), which inhibit SERCA, the membrane pump that controls muscle relaxation by regulating Ca2+ uptake into the sarcoplasmic reticulum (SR). MLN interacts directly with SERCA and impedes Ca2+ uptake into the SR. In contrast to PLN and SLN, which are expressed in cardiac and slow skeletal muscle in mice, MLN is robustly expressed in all skeletal muscle. Genetic deletion of MLN in mice enhances Ca2+ handling in skeletal muscle and improves exercise performance. These findings identify MLN as an important regulator of skeletal muscle physiology and highlight the possibility that additional micropeptides are encoded in the many RNAs currently annotated as non-coding.

The enveloped negative-stranded RNA virus measles virus (MeV) is an important human pathogen. The nucleoprotein (N 0 ) assembles with the viral RNA into helical ribonucleocapsids (NC) which are, in turn, coated by a helical layer of the matrix protein.The viral polymerase complex uses the NC as its template. The N 0 assembly onto the NC and the activity of the polymerase are regulated by the viral phosphoprotein (P). In this study, we pulled down an N 0 1-408 fragment lacking most of its C-terminal tail domain by several affinity-tagged, N-terminal P fragments to map the N 0 -binding region of P to the first 48 amino acids. We showed biochemically and using P mutants the importance of the hydrophobic interactions for the binding. We fused an N 0 binding peptide, P 1-48 , to the C terminus of an N 0 21-408 fragment lacking both the N-terminal peptide and the C-terminal tail of N protein to reconstitute and crystallize the N 0 -P complex. We solved the X-ray structure of the resulting N 0 -P chimeric protein at a resolution of 2.7 Å. The structure reveals the molecular details of the conserved N 0 -P interface and explains how P chaperones N 0 , preventing both self-assembly of N 0 and its binding to RNA. Finally, we propose a model for a preinitiation complex for RNA polymerization. IMPORTANCEMeasles virus is an important, highly contagious human pathogen. The nucleoprotein N binds only to viral genomic RNA and forms the helical ribonucleocapsid that serves as a template for viral replication. We address how N is regulated by another protein, the phosphoprotein (P), to prevent newly synthesized N from binding to cellular RNA. We describe the atomic model of an N-P complex and compare it to helical ribonucleocapsid. We thus provide insight into how P chaperones N and helps to start viral RNA synthesis. Our results provide a new insight into mechanisms of paramyxovirus replication. New data on the mechanisms of phosphoprotein chaperone action allows better understanding of virus genome replication and nucleocapsid assembly. We describe a conserved structural interface for the N-P interaction which could be a target for drug development to treat not only measles but also potentially other paramyxovirus diseases. Measles virus (MeV) belongs to the Paramyxoviridae family, which includes several other human pathogens, like respiratory syncytial (RSV), mumps, and parainfluenza viruses. It has a helical ribonucleocapsid (NC) containing a nonsegmented singlestrand RNA (ssRNA) genome wrapped around the outside the nucleoprotein (N) helix (1). The helical NC is active in both transcription and replication. During virus assembly, the matrix protein forms an additional helix covering the majority of the NC, potentially inhibiting transcription and promoting packaging into progeny virions (2). There are still only limited data on the detailed molecular interactions required to go from replication initiation to packaging of nascent RNA. The availability of N in a chaperoned, assembly-competent state with the phosphoprot...

The crystal structure of the full-length rat peroxisomal multifunctional enzyme, type 1 (rpMFE1), has been determined at 2.8 Å resolution. This enzyme has three catalytic activities and two active sites. The N-terminal part has the crotonase fold, which builds the active site for the ⌬

The multifunctional enzyme, type-1 (MFE1) is involved in several lipid metabolizing pathways. It catalyses: (a) enoyl-CoA isomerase and (b) enoyl-CoA hydratase (EC 4.2.1.17) reactions in its N-terminal crotonase part, as well as (3) a 3S-hydroxy-acyl-CoA dehydrogenase (HAD; EC 1.1.1.35) reaction in its C-terminal 3S-hydroxy-acyl-CoA dehydrogenase part. Crystallographic binding studies with rat peroxisomal MFE1, using unbranched and branched 2E-enoyl-CoA substrate molecules, show that the substrate has been hydrated by the enzyme in the crystal and that the product, 3S-hydroxy-acyl-CoA, remains bound in the crotonase active site. The fatty acid tail points into an exit tunnel shaped by loop-2. The thioester oxygen is bound in the classical oxyanion hole of the crotonase fold, stabilizing the enolate reaction intermediate. The structural data of these enzyme product complexes suggest that the catalytic base, Glu123, initiates the isomerase reaction by abstracting the C2-proton from the substrate molecule. Subsequently, in the hydratase reaction, Glu123 completes the catalytic cycle by reprotonating the C2 atom. A catalytic water, bound between the OE1-atoms of the two catalytic glutamates, Glu103 and Glu123, plays an important role in the enoyl-CoA isomerase and the enoyl-CoA hydratase reaction mechanism of MFE1. The structural variability of loop-2 between MFE1 and its monofunctional homologues correlates with differences in the respective substrate preferences and catalytic rates. DatabaseThe structures have been deposited in the Protein Data Bank under accession numbers: 3ZW8 (MFE1 apo), 3ZW9 (MFE1 2S-methyl-3S-hydroxy-butanoyl-CoA complex), 3ZWA (MFE1 3S-hydroxy-hexanoyl-CoA complex), 3ZWB (MFE1-E123A 2E-hexenoyl-CoA complex) and 3ZWC (MFE1 3S-hydroxy-decanoyl-CoA complex). Structured digital abstract

Multifunctional enzyme, type‐1 ( MFE 1) is a monomeric enzyme with a 2E‐enoyl‐CoA hydratase and a 3S‐hydroxyacyl‐CoA dehydrogenase ( HAD ) active site. Enzyme kinetic data of rat peroxisomal MFE 1 show that the catalytic efficiencies for converting the short‐chain substrate 2E‐butenoyl‐CoA into acetoacetyl‐CoA are much lower when compared with those of the homologous monofunctional enzymes. The mode of binding of acetoacetyl‐CoA (to the hydratase active site) and the very similar mode of binding of NAD + and NADH (to the HAD part) are described and compared with those of their monofunctional counterparts. Structural comparisons suggest that the conformational flexibility of the HAD and hydratase parts of MFE 1 are correlated. The possible importance of the conformational flexibility of MFE 1 for its biocatalytic properties is discussed. Database Structural data are available in PDB database under the accession number 5MGB .