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Mitochondrial tRNA 3'-end metabolism is critical for the formation of functional tRNAs. Deficient mitochondrial tRNA 3'-end metabolism is linked to an array of human diseases, including optic neuropathy, but their pathophysiology remains poorly understood. In this report, we investigated the molecular mechanism underlying the Leber's hereditary optic neuropathy (LHON)-associated tRNA Ala 5587A>G mutation, which changes a highly conserved adenosine at position 73 (A73) to guanine (G73) on the 3'-end of the tRNA acceptor stem. The m.5587A>G mutation was identified in three Han Chinese families with suggested maternal inheritance of LHON. We hypothesized that the m.5587A>G mutation altered tRNA Ala 3'-end metabolism and mitochondrial function. In vitro processing experiments showed that the m.5587A>G mutation impaired the 3'-end processing of tRNA Ala precursors by RNase Z and inhibited the addition of CCA by tRNA nucleotidyltransferase (TRNT1). Northern blot analysis revealed that the m.5587A>G mutation perturbed tRNA Ala aminoacylation, as evidenced by decreased efficiency of aminoacylation and faster electrophoretic mobility of mutated tRNA Ala in these cells. The impact of m.5587A>G mutation on tRNA Ala function was further supported by increased melting temperature, conformational changes, and reduced levels of this tRNA. Failures in tRNA Ala metabolism impaired mitochondrial translation, perturbed assembly and activity of oxidative phosphorylation complexes, diminished ATP production and membrane potential, and increased production of reactive oxygen species. These pleiotropic defects elevated apoptotic cell death and promoted mitophagy in cells carrying the m.5587A>G mutation, thereby contributing to visual impairment. Our findings may provide new insights into the pathophysiology of LHON arising from mitochondrial tRNA 3'-end metabolism deficiency., Competing Interests: Conflict of interest All authors declare that they have no conflict of interest with contents of this article., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

Recent progress has been made on the role of oncoproteins c-Ski and related SnoN in the control of cellular transformation. c-Ski/SnoN potently repress transforming growth factor-beta (TGF-beta) antiproliferative signaling through physical interaction with signal transducers called Smads. Overexpression of c-Ski/SnoN also induces skeletal muscle differentiation, but how c-Ski/SnoN function in myogenesis is largely unknown. During our investigation on the role of sumoylation in TGF-beta signaling, we inadvertently found that SnoN is modified by small ubiquitin-like modifier-1 (SUMO-1). Here, we biochemically characterize SnoN sumoylation in detail and report the physiological function of the modification. Sumoylation occurs primarily at lysine 50 (Lys-50). PIAS1 and PIASx proteins physically interact with SnoN to stimulate its sumoylation, thus serving as SUMO-protein isopeptide ligases (E3) for SnoN sumoylation. SnoN sumoylation does not alter its metabolic stability or its ability to repress TGF-beta signaling. Notably, loss of sumoylation in the Lys-50 site (via a Lys-to-Arg point mutation) potently activates muscle-specific gene expression and enhances myotube formation. Our study suggests a novel role for SUMO modification in the regulation of myogenic differentiation.

Smads are important intracellular effectors in signaling pathways of the transforming growth factor-beta (TGF-beta) superfamily. Upon activation by TGF-beta, receptor-phosphorylated Smads form a complex with tumor suppressor Smad4/DPC4, and the Smad complexes then are imported into the nucleus. Although diverse pathways regulate the activity and expression of receptor-phosphorylated and inhibitory Smads, cellular factors modulating the activity of the common Smad4 remain unidentified. Here we describe the involvement of the small ubiquitin-like modifier-1 (SUMO-1) conjugation pathway in regulating the growth inhibitory and transcriptional responses of Smad4. The MH1 domain of Smad4 was shown to associate physically with Ubc9, the ubiquitin carrier protein (E2) conjugating enzyme in sumoylation. In cultured cells, Smad4 is modified by SUMO-1 at the endogenous level. The sumoylation sites were identified as two evolutionarily conserved lysine residues, Lys-113 and Lys-159, in the MH1 domain. We found that the mutations at Lys-113 and Lys-159 did not alter the ability of Smad4 to form a complex with Smad2 and FAST on the Mix.2 promoter. Importantly, SUMO-1 overexpression enhanced TGF-beta-induced transcriptional responses. These findings identify sumoylation as a unique mechanism to modulate Smad4-dependent cellular responses.