Troponin I (TnI) and the two tropomyosins Tm1 and Tm2 are part of a complex that has a well-characterised role in regulating muscle sarcomere contraction. Some evidence suggests, however, that these proteins might also have a role in other cell types and processes. Now, Alberto Ferrús and colleagues () provide new evidence that the Tn-Tm complex has important functions in the nucleus during Drosophila embryogenesis. The authors first determine that TnI can translocate between the cytoplasm and the nucleus in the S2 cell line, and propose that its subcellular localisation might depend on the physiological state of the cells. They then show that embryos carrying mutations in TnI, Tm1 or Tm2 display nuclear defects at the early pre-cellular stage, such as abnormal localisation of actin, altered tubulin structure and fragmented chromosomes. At the onset of cellularisation, the mutant embryos show a disruption of apicobasal polarity, and mislocalisation of the polarity proteins Dlg and Pins. The authors conclude, therefore, that the Tn-Tm complex is required for proper actin function in the Drosophila embryo, and thereby supports proper nuclear divisions and the correct localisation of polarity proteins that are essential during development.
