KLHL40 deficiency destabilizes thin filament proteins and promotes nemaline myopathy

Mutations in nebulin, a giant muscle protein with 185 actin-binding nebulin repeats, are the major cause of nemaline myopathy in humans. Nebulin sets actin thin filament length in sarcomeres, potentially by stabilizing thin filaments in the I-band, where nebulin and thin filaments coalign. However, the precise role of nebulin in setting thin filament length and its other functions in regulating power output are unknown. Here, we show that Lasp, the only member of the nebulin family in Drosophila melanogaster, acts at two distinct sites in the sarcomere and controls thin filament length with just two nebulin repeats. We found that Lasp localizes to the Z-disc edges to control I-band architecture and also localizes at the A-band, where it interacts with both actin and myosin to set proper filament spacing. Furthermore, introducing a single amino acid change into the two nebulin repeats of Lasp demonstrated different roles for each domain and established Lasp as a suitable system for studying nebulin repeat function.

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Thin Filament Proteins and Thin Filament-Linked Regulation of Vertebrate Muscle Contractio

Critical Reviews in Biochemistry ◽

10.3109/10409238409108717 ◽

1984 ◽

Vol 16 (3) ◽

pp. 235-305 ◽

Cited By ~ 301

Author(s):

Paul C. Leavis ◽

John Gergely ◽

Andrew G. Szent-Gyorgyi

Keyword(s):

Thin Filament ◽

Vertebrate Muscle ◽

Thin Filament Proteins

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Myosin MgADP release rate decreases at longer sarcomere length to prolong myosin attachment time in skinned rat myocardium

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.00555.2015 ◽

2015 ◽

Vol 309 (12) ◽

pp. H2087-H2097 ◽

Cited By ~ 9

Author(s):

Bertrand C. W. Tanner ◽

Jason J. Breithaupt ◽

Peter O. Awinda

Keyword(s):

Release Rate ◽

Thin Filament ◽

Molecular Mechanisms ◽

Sarcomere Length ◽

Cardiac Contractility ◽

Force Production ◽

Cross Bridge ◽

Thin Filament Proteins ◽

Filament Activation ◽

Ventricular Filling

Cardiac contractility increases as sarcomere length increases, suggesting that intrinsic molecular mechanisms underlie the Frank-Starling relationship to confer increased cardiac output with greater ventricular filling. The capacity of myosin to bind with actin and generate force in a muscle cell is Ca2+ regulated by thin-filament proteins and spatially regulated by sarcomere length as thick-to-thin filament overlap varies. One mechanism underlying greater cardiac contractility as sarcomere length increases could involve longer myosin attachment time ( t on) due to slowed myosin kinetics at longer sarcomere length. To test this idea, we used stochastic length-perturbation analysis in skinned rat papillary muscle strips to measure t on as [MgATP] varied (0.05–5 mM) at 1.9 and 2.2 μm sarcomere lengths. From this t on-MgATP relationship, we calculated cross-bridge MgADP release rate and MgATP binding rates. As MgATP increased, t on decreased for both sarcomere lengths, but t on was roughly 70% longer for 2.2 vs. 1.9 μm sarcomere length at maximally activated conditions. These t on differences were driven by a slower MgADP release rate at 2.2 μm sarcomere length (41 ± 3 vs. 74 ± 7 s−1), since MgATP binding rate was not different between the two sarcomere lengths. At submaximal activation levels near the pCa50 value of the tension-pCa relationship for each sarcomere length, length-dependent increases in t on were roughly 15% longer for 2.2 vs. 1.9 μm sarcomere length. These changes in cross-bridge kinetics could amplify cooperative cross-bridge contributions to force production and thin-filament activation at longer sarcomere length and suggest that length-dependent changes in myosin MgADP release rate may contribute to the Frank-Starling relationship in the heart.

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Ubiquitylation by Trim32 causes coupled loss of desmin, Z-bands, and thin filaments in muscle atrophy

The Journal of Cell Biology ◽

10.1083/jcb.201110067 ◽

2012 ◽

Vol 198 (4) ◽

pp. 575-589 ◽

Cited By ~ 105

Author(s):

Shenhav Cohen ◽

Bo Zhai ◽

Steven P. Gygi ◽

Alfred L. Goldberg

Keyword(s):

Muscle Atrophy ◽

Ubiquitin Ligase ◽

Thin Filament ◽

Thick Filament ◽

Myofibrillar Proteins ◽

Rapid Loss ◽

Thin Filaments ◽

Thin Filament Proteins ◽

Rapid Destruction ◽

Fiber Atrophy

During muscle atrophy, myofibrillar proteins are degraded in an ordered process in which MuRF1 catalyzes ubiquitylation of thick filament components (Cohen et al. 2009. J. Cell Biol. http://dx.doi.org/10.1083/jcb.200901052). Here, we show that another ubiquitin ligase, Trim32, ubiquitylates thin filament (actin, tropomyosin, troponins) and Z-band (α-actinin) components and promotes their degradation. Down-regulation of Trim32 during fasting reduced fiber atrophy and the rapid loss of thin filaments. Desmin filaments were proposed to maintain the integrity of thin filaments. Accordingly, we find that the rapid destruction of thin filament proteins upon fasting was accompanied by increased phosphorylation of desmin filaments, which promoted desmin ubiquitylation by Trim32 and degradation. Reducing Trim32 levels prevented the loss of both desmin and thin filament proteins. Furthermore, overexpression of an inhibitor of desmin polymerization induced disassembly of desmin filaments and destruction of thin filament components. Thus, during fasting, desmin phosphorylation increases and enhances Trim32-mediated degradation of the desmin cytoskeleton, which appears to facilitate the breakdown of Z-bands and thin filaments.

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