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

doi:10.1073/pnas.0404341102

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

^*Carolina Cardiovascular Biology Center and ^‡Departments of Medicine, Pharmacology, and Cell and Developmental Biology, University of North Carolina, Chapel Hill, NC 27599-7126; and ^†Department of Medicine, Duke University, Durham, NC 27710

Edited by Eric N. Olson, University of Texas Southwestern Medical Center, Dallas, TX, and approved November 17, 2004 (received for review June 18, 2004)

Abstract

Muscle-specific RING finger protein 1 (MuRF1) is a sarcomere-associated protein that is restricted to cardiac and skeletal muscle. In skeletal muscle, MuRF1 is up-regulated by conditions that provoke atrophy, but its function in the heart is not known. The presence of a RING finger in MuRF1 raises the possibility that it is a component of the ubiquitin–proteasome system of protein deg-radation. We performed a yeast two-hybrid screen to search for interaction partners of MuRF1 in the heart that might be targets of its putative ubiquitin ligase activity. This screen identified troponin I as a MuRF1 partner protein. MuRF1 and troponin I were found to associate both in vitro and in vivo in cultured cardiomyocytes. MuRF1 reduced steady-state troponin I levels when coexpressed in COS-7 cells and increased degradation of endogenous troponin I protein in cardiomyocytes. The degradation of troponin I in cardiomyocytes was associated with the accumulation of ubiquitylated intermediates of troponin I and was proteasome-dependent. In vitro, MuRF1 functioned as a ubiquitin ligase to catalyze ubiquitylation of troponin I through a RING finger-dependent mechanism. In isolated cardiomyocytes, MuRF1 reduced indices of contractility. In cardiomyocytes, these processes may determine the balance between hypertrophic and antihypertrophic signals and the regulation of myocyte contractile responses in the setting of heart failure.

cardiomyocyte

Footnotes

↵ § To whom correspondence should be addressed. E-mail: cpatters{at}med.unc.edu.
Author contributions: V.K., H.M., H.A.R., and C.P. designed research; V.K., H.M., R.A., and H.-H.L. performed research; V.K. and H.-H.L. contributed new reagents/analytic tools; V.K., H.M., R.A., H.-H.L., H.A.R., and C.P. analyzed data; and V.K. and C.P. wrote the paper.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: MuRF1, muscle-specific RING finger protein 1; HA, hemagglutinin; NRVM, neonatal rat ventricular myocytes; PE, phenylephrine; moi, multiplicity of infection.