Muscle-specific RING finger-1 interacts with titin to regulate sarcomeric M-line and thick filament structure and may have nuclear functions via its interaction with glucocorticoid modulatory element binding protein-1

Abstract

The COOH-terminal A168–170 region of the giant sarcomeric protein titin interacts with muscle-specific RING finger-1 (MURF-1). To investigate the functional significance of this interaction, we expressed green fluorescent protein fusion constructs encoding defined fragments of titin's M-line region and MURF-1 in cardiac myocytes. Upon expression of MURF-1 or its central region (containing its titin-binding site), the integrity of titin's M-line region was dramatically disrupted. Disruption of titin's M-line region also resulted in a perturbation of thick filament components, but, surprisingly, not of the NH2-terminal or I-band regions of titin, the Z-lines, or the thin filaments. This specific phenotype also was caused by the expression of titin A168–170. These data suggest that the interaction of titin with MURF-1 is important for the stability of the sarcomeric M-line region.MURF-1 also binds to ubiquitin-conjugating enzyme-9 and isopeptidase T-3, enzymes involved in small ubiquitin-related modifier–mediated nuclear import, and with glucocorticoid modulatory element binding protein-1 (GMEB-1), a transcriptional regulator. Consistent with our in vitro binding data implicating MURF-1 with nuclear functions, endogenous MURF-1 also was detected in the nuclei of some myocytes. The dual interactions of MURF-1 with titin and GMEB-1 may link myofibril signaling pathways (perhaps including titin's kinase domain) with muscle gene expression.

Muscle-specific RING finger 1 is a bona fide ubiquitin ligase that degrades cardiac troponin I

“…Interaction of titin with MuRF1 in sarcomeres has previously been reported (13,18). Our data and that of other groups indicate that MuRF1 exists in multiple locations within the cell, including the cytosol, along the sarcomere, and within the nucleus.…”

“…Previous studies have shown that MuRF1 is found in several niches within myocytes: the cytosol, nucleus, and along myofibrils (13,18). We also observed similar localization of MuRF1 in cardiomyocytes treated with or without PE (Fig.…”

“…These observations may be relevant to the role played by MuRF1 in the sarcomere. MuRF1 binds to titin and disrupts its interactions with the M-line without inducing the degradation of titin, thus partially disrupting sarcomere assembly (18). We hypothesize that MuRF1 may destabilize sarcomeres by binding to titin, which would in turn provide MuRF1 with access to its substrate troponin I. Troponin I could then be targeted for ubiquitylation and proteasome-dependent degradation by MuRF1.…”

XIAP Reduces Muscle Proteolysis Induced by CKD

“…46 MuRF-1 binds the giant sarcomeric protein titin, suggesting that MuRF-1 regulates the stability of the large structural protein. 47 In addition, Cohen et al 48 reported that MuRF-1 ubiquitinates MyBP-C, MyLC1, and MyLC2, including in myofibrils. Because these proteins stabilize the thick filament, their selective ubiquitination may facilitate thick filament disassembly.…”

Cellular mechanisms and local progenitor activation to regulate skeletal muscle mass

“…These two genes correspond to ubiquitinprotein ligases expressed solely in skeletal and cardiac muscle that mediate the processes of muscle remodeling and increase proteolysis of disuse atrophy. For instance, MURF1 was firstly identified as the ubiquitin-ligase responsible for the direct binding and degradation of titin, a huge myofibrillar protein, and myosin heavy chain (McElhinny et al 2002;Clarke et al 2007). Further confirm of the functional role of these atrogenes came from the experiments performed on MAFbx-/-and MURF1-/-transgenic mice, refractory to muscle-loss phenomena induced by denervation.…”