During muscle development, de novo formed myosin and actin filaments assemble into the greatly organized sarcomeric structure critical for muscle function. Although sarcomerogenesis clearly involves the formation of novel actin filaments, it has so far been poorly understood how these filaments form. Two key steps of filament formation are nucleation and elongation. However, in muscle cells the essential actin nucleation and elongation factors, regulating actin filament formation, have not been clearly identified, and the mechanism that ensures sarcomeric thin filament assembly remained mysterious. Recently, we found that DAAM family formins, well known actin nucleation and elongation factors in nonmuscle cells, also play an essential role in sarcomerogenesis, whereas others identified the SALS protein as a key regulator of thin filament elongation. The major objective of our research is to investigate the molecular and cellular mechanisms of thin filament assembly during sarcomerogenesis by the detailed analysis of the functions of DAAM family formins and SALS. We aim to use the combination of genetic, cellular and in vitro assays (fluorescence spectroscopy, fluorescence microscopy, reconstituted biomimetic approach) to reveal the functional properties of these proteins, and to explore their molecular interactions with each other and with the known regulators of thin filament formation. We expect that the complex approach proposed will help us to gain deeper insights into the mechanism of myofibrillogenesis, especially into the mechanism of thin filament formation and the integration of the actin and myosin filament systems.
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