Nitric oxide (NO) is implicated in neuronal cell survival. However, excessive NO production mediates neuronal cell death, in part via mitochondrial dysfunction. Here, we report that the mitochondrial ubiquitin ligase, MITOL, protects neuronal cells from mitochondrial damage caused by accumulation of S-nitrosylated microtubuleassociated protein 1B-light chain 1 (LC1). S-nitrosylation of LC1 induces a conformational change that serves both to activate LC1 and to promote its ubiquination by MITOL, indicating that microtubule stabilization by LC1 is regulated through its interaction with MITOL. Excessive NO production can inhibit MITOL, and MITOL inhibition resulted in accumulation of S-nitrosylated LC1 following stimulation of NO production by calcimycin and N-methyl-D-aspartate. LC1 accumulation under these conditions resulted in mitochondrial dysfunction and neuronal cell death. Thus, the balance between LC1 activation by S-nitrosylation and down-regulation by MITOL is critical for neuronal cell survival. Our findings may contribute significantly to an understanding of the mechanisms of neurological diseases caused by nitrosative stress-mediated mitochondrial dysfunction.neurodegenerative diseases | oxidative stress N itric oxide (NO) is a signaling molecule implicated in a variety of physiological processes (1). However, excessive NO mediates neuronal cell death by nitrosative stress, potentially contributing to the pathology of many neurodegenerative disorders (2). In the nervous system, NO is synthesized mainly by neuronal NO synthases (nNOS), which are regulated by calcium mobilization (3). One way of propagating NO signals is the Snitrosylation of proteins, which involves the covalent attachment of a nitrogen monoxide group to the thiol side chain of a cysteine residue (4). S-nitrosylation of several proteins causes neurotoxicity, in part by triggering protein misfolding and aggregation and mitochondrial dysfunction (5, 6). Neurons are particularly vulnerable to mitochondrial dysfunction because they require high levels of energy for their survival and synaptic function (7). Accumulating evidence suggests that many mitochondrial proteins or regulatory proteins for mitochondrial function are physiological substrates for S-nitrosylation under nitrosative stress (8). Snitrosylation of mitochondrial fission protein, Drp1, has been shown to link excessive mitochondrial fission to neuronal injury in neurodegeneration (9). Thus, nitrosative stress because of the generation of excessive NO can mediate neurotoxicity, in part via the S-nitrosylation of mitochondria-related proteins.Microtubule-associated protein 1B (MAP1B) is a protein complex that consists of a heavy chain and light chain (LC1) (10). MAP1B plays an important role in the stability of the cytoskeleton and may have other cellular functions. In particular, MAP1S (C19ORF5), a sequence homolog of MAP1A/MAP1B, has been shown to accumulate in mitochondria and induce mitochondrial aggregation and genome destruction via a specific domain named MAGD (11). Simil...