Fig. 9. MURF2 in relation to titin, myosin and the filamentous systems of actin and microtubules during sarcomere formation. (A) At initial stages of myofibril assembly, MURF2 (red ovals) is microtubule associated. Myosin (dark green rods) colocalises early with MURF-decorated microtubules (light-green). (B) Titin (red chains) localises in dot-like aggregates on actin SFLS (blue) with a spacing of around 1 µm. Black links, -actinin crosslinks. These structures co-align with MURF-decorated microtubules. (C) MURF2 and myosin colocalise strictly in nascent striated myofibrils, when the cross-striated pattern of myosin begins to form and titin Z-Z staining increases to the mature approximately 2 µm pattern. Transient colocalisation with A-band titin is observed at this stage. MURF-MURF heteromultimeres could link different sarcomeric components at this stage and result in the exact alignment of titin and myosin. (D) In mature myofibrils, myosin and actin are arranged in highly ordered cross-striated patterns with ordered polarity (actin pointed ends marked by arrows), and the titin molecule is extended, with the N-terminal portion remaining in the Z-disk and the C-terminus integrated into the M-band. Depending on differentiation state and/or muscle type, MURF2 can be present in the M-band or the nucleus. Bar, 1 µm.

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