Mob4-dependent STRIPAK involves the chaperonin TRiC to coordinate myofibril and microtubule network growth

Abstract

Myofibrils of the skeletal muscle are comprised of sarcomeres that generate force by contraction when myosin-rich thick filaments slide past actin-based thin filaments. Surprisingly little is known about the molecular processes that guide sarcomere assembly in vivo, despite deficits within this process being a major cause of human disease. To overcome this knowledge gap, we undertook a forward genetic screen coupled with reverse genetics to identify genes required for vertebrate sarcomere assembly. In this screen, we identified a zebrafish mutant with a nonsense mutation in mob4. In Drosophila, mob4 has been reported to play a role in spindle focusing as well as neurite branching and in planarians mob4 was implemented in body size regulation. In contrast, zebrafish mob4^geh mutants are characterised by an impaired actin biogenesis resulting in sarcomere defects. Whereas loss of mob4 leads to a reduction in the amount of myofibril, transgenic expression of mob4 triggers an increase. Further genetic analysis revealed the interaction of Mob4 with the actin-folding chaperonin TRiC, suggesting that Mob4 impacts on TRiC to control actin biogenesis and thus myofibril growth. Additionally, mob4^geh features a defective microtubule network, which is in-line with tubulin being the second main folding substrate of TRiC. We also detected similar characteristics for strn3-deficient mutants, which confirmed Mob4 as a core component of STRIPAK and surprisingly implicates a role of the STRIPAK complex in sarcomerogenesis. Within muscle, highly organised filaments slide over each other to generate the force required for the movement of our bodies. Assembly of this contractile apparatus is not well understood and its regulation remains enigmatic. To gain novel insights into these processes we facilitated a zebrafish screen that resulted in the discovery of a novel molecule involved in the coordination of the contractile apparatus assembly: The highly conserved MOB family member Mob4. Whereas loss of mob4 led to impaired actin biogenesis and defects in the contractile apparatus assembly, which resembled aspects of human myopathy disorders, gain of mob4 function resulted in a higher amount of contractile apparatus. Further analyses of strn3-deficient mutants demonstrated that Mob4 functions within a protein complex called striatin-interacting phosphatases and kinases (STRIPAK). Mob4 also involves another protein complex called TRiC, which is required for actin and tubulin biogenesis. Whereas actin is the main component of the muscle’s thin filaments, tubulin constitutes the microtubule network essential for neuronal axons. Accordingly, all analysed mutants for mob4 and strn3 featured neuronal as well as muscle defects. We thus conclude that the two protein complexes STRIPAK and TRiC interact through Mob4 to coordinate growth of the myofibril and microtubule network.