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Titin kinase is an inactive pseudokinase scaffold that supports MuRF1 recruitment to the sarcomeric M-line.

Bogomolovas J, Gasch A, Simkovic F, Rigden DJ, Labeit S, Mayans O - Open Biol (2014)

Bottom Line: Inactivity is the result of two atypical residues in TK's active site, M34 and E147, that do not appear compatible with canonical kinase patterns.While not mediating stretch-dependent phospho-transfers, TK binds the E3 ubiquitin ligase MuRF1 that promotes sarcomeric ubiquitination in a stress-induced manner.Finally, we suggest that an evolutionary dichotomy of kinases/pseudokinases has occurred in TK-like kinases, where invertebrate members are active enzymes but vertebrate counterparts perform their signalling function as pseudokinase scaffolds.

View Article: PubMed Central - PubMed

Affiliation: Department of Integrative Pathophysiology, Medical Faculty Mannheim, University of Heidelberg, Mannheim 68167, Germany Institute of Integrative Biology, Biosciences Building, University of Liverpool, Crown St., Liverpool L69 7ZB, UK.

ABSTRACT
Striated muscle tissues undergo adaptive remodelling in response to mechanical load. This process involves the myofilament titin and, specifically, its kinase domain (TK; titin kinase) that translates mechanical signals into regulatory pathways of gene expression in the myofibril. TK mechanosensing appears mediated by a C-terminal regulatory tail (CRD) that sterically inhibits its active site. Allegedly, stretch-induced unfolding of this tail during muscle function releases TK inhibition and leads to its catalytic activation. However, the cellular pathway of TK is poorly understood and substrates proposed to date remain controversial. TK's best-established substrate is Tcap, a small structural protein of the Z-disc believed to link TK to myofibrillogenesis. Here, we show that TK is a pseudokinase with undetectable levels of catalysis and, therefore, that Tcap is not its substrate. Inactivity is the result of two atypical residues in TK's active site, M34 and E147, that do not appear compatible with canonical kinase patterns. While not mediating stretch-dependent phospho-transfers, TK binds the E3 ubiquitin ligase MuRF1 that promotes sarcomeric ubiquitination in a stress-induced manner. Given previous evidence of MuRF2 interaction, we propose that the cellular role of TK is to act as a conformationally regulated scaffold that functionally couples the ubiquitin ligases MuRF1 and MuRF2, thereby coordinating muscle-specific ubiquitination pathways and myofibril trophicity. Finally, we suggest that an evolutionary dichotomy of kinases/pseudokinases has occurred in TK-like kinases, where invertebrate members are active enzymes but vertebrate counterparts perform their signalling function as pseudokinase scaffolds.

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Distribution of active kinases and inactive pseudokinases in titin-like filaments from vertebrate and invertebrate muscle. For each filament, the following domains are shown: Ig (blue circles), Fn3 (green circles), active kinases (magenta boxes) and inactive pseudokinases (grey boxes). Pseudokinases where inactivity is tentatively suggested but cannot be as reliably predicted are indicated with a question mark.
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RSOB140041F6: Distribution of active kinases and inactive pseudokinases in titin-like filaments from vertebrate and invertebrate muscle. For each filament, the following domains are shown: Ig (blue circles), Fn3 (green circles), active kinases (magenta boxes) and inactive pseudokinases (grey boxes). Pseudokinases where inactivity is tentatively suggested but cannot be as reliably predicted are indicated with a question mark.

Mentions: Finally, the inactivity of vertebrate TK contrasts markedly with the high levels of in vitro catalysis of its invertebrate homologues, TwcK, TTN-1 and projectin. Interestingly, this difference correlates with a similar contrast in obscurin kinases, also members of the TK kinase family. Obscurin (and the invertebrate homologue UNC-89) contains two kinases, PK1 and PK2. In vertebrates, PK1 and PK2 have canonical active sites and are catalytically active [45,46]. In invertebrates, PK1 is inactive as it lacks catalytic residues and PK2 has a degenerated active site indicating that its activity might be modest [46]. This inverse correlation in titin-like and obscurin-like kinases in vertebrate and invertebrate organisms (figure 6) suggests that stretch-activated phospho-transfer might reside in different filament systems in muscles across animal phyla. Future studies are required to understand the patterns of activity and scaffolding in these kinases in terms of muscle responses to stress stimuli.FigureĀ 6.


Titin kinase is an inactive pseudokinase scaffold that supports MuRF1 recruitment to the sarcomeric M-line.

Bogomolovas J, Gasch A, Simkovic F, Rigden DJ, Labeit S, Mayans O - Open Biol (2014)

Distribution of active kinases and inactive pseudokinases in titin-like filaments from vertebrate and invertebrate muscle. For each filament, the following domains are shown: Ig (blue circles), Fn3 (green circles), active kinases (magenta boxes) and inactive pseudokinases (grey boxes). Pseudokinases where inactivity is tentatively suggested but cannot be as reliably predicted are indicated with a question mark.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4042850&req=5

RSOB140041F6: Distribution of active kinases and inactive pseudokinases in titin-like filaments from vertebrate and invertebrate muscle. For each filament, the following domains are shown: Ig (blue circles), Fn3 (green circles), active kinases (magenta boxes) and inactive pseudokinases (grey boxes). Pseudokinases where inactivity is tentatively suggested but cannot be as reliably predicted are indicated with a question mark.
Mentions: Finally, the inactivity of vertebrate TK contrasts markedly with the high levels of in vitro catalysis of its invertebrate homologues, TwcK, TTN-1 and projectin. Interestingly, this difference correlates with a similar contrast in obscurin kinases, also members of the TK kinase family. Obscurin (and the invertebrate homologue UNC-89) contains two kinases, PK1 and PK2. In vertebrates, PK1 and PK2 have canonical active sites and are catalytically active [45,46]. In invertebrates, PK1 is inactive as it lacks catalytic residues and PK2 has a degenerated active site indicating that its activity might be modest [46]. This inverse correlation in titin-like and obscurin-like kinases in vertebrate and invertebrate organisms (figure 6) suggests that stretch-activated phospho-transfer might reside in different filament systems in muscles across animal phyla. Future studies are required to understand the patterns of activity and scaffolding in these kinases in terms of muscle responses to stress stimuli.FigureĀ 6.

Bottom Line: Inactivity is the result of two atypical residues in TK's active site, M34 and E147, that do not appear compatible with canonical kinase patterns.While not mediating stretch-dependent phospho-transfers, TK binds the E3 ubiquitin ligase MuRF1 that promotes sarcomeric ubiquitination in a stress-induced manner.Finally, we suggest that an evolutionary dichotomy of kinases/pseudokinases has occurred in TK-like kinases, where invertebrate members are active enzymes but vertebrate counterparts perform their signalling function as pseudokinase scaffolds.

View Article: PubMed Central - PubMed

Affiliation: Department of Integrative Pathophysiology, Medical Faculty Mannheim, University of Heidelberg, Mannheim 68167, Germany Institute of Integrative Biology, Biosciences Building, University of Liverpool, Crown St., Liverpool L69 7ZB, UK.

ABSTRACT
Striated muscle tissues undergo adaptive remodelling in response to mechanical load. This process involves the myofilament titin and, specifically, its kinase domain (TK; titin kinase) that translates mechanical signals into regulatory pathways of gene expression in the myofibril. TK mechanosensing appears mediated by a C-terminal regulatory tail (CRD) that sterically inhibits its active site. Allegedly, stretch-induced unfolding of this tail during muscle function releases TK inhibition and leads to its catalytic activation. However, the cellular pathway of TK is poorly understood and substrates proposed to date remain controversial. TK's best-established substrate is Tcap, a small structural protein of the Z-disc believed to link TK to myofibrillogenesis. Here, we show that TK is a pseudokinase with undetectable levels of catalysis and, therefore, that Tcap is not its substrate. Inactivity is the result of two atypical residues in TK's active site, M34 and E147, that do not appear compatible with canonical kinase patterns. While not mediating stretch-dependent phospho-transfers, TK binds the E3 ubiquitin ligase MuRF1 that promotes sarcomeric ubiquitination in a stress-induced manner. Given previous evidence of MuRF2 interaction, we propose that the cellular role of TK is to act as a conformationally regulated scaffold that functionally couples the ubiquitin ligases MuRF1 and MuRF2, thereby coordinating muscle-specific ubiquitination pathways and myofibril trophicity. Finally, we suggest that an evolutionary dichotomy of kinases/pseudokinases has occurred in TK-like kinases, where invertebrate members are active enzymes but vertebrate counterparts perform their signalling function as pseudokinase scaffolds.

Show MeSH