| The compartmentalization of organelles in eukaryotic cells has necessitated their development of the means to transport organelles where and when they are needed. During cell division, the goal of transport is to insure that both daughter cells will contain a full complement of organelles once cytokinesis is complete. The molecular mechanisms of organelle inheritance typically involve elements of the cytoskeleton, either microtubules or actin microfilaments. In the budding yeast, Saccharomyces cerevisiae, the inheritance of mitochondria is a function of the actin cytoskeleton. Mitochondria move along bundles of actin filaments, called cables, from the mother cell into the bud. Mdm20p is a yeast protein required for actin cable integrity and thus, for mitochondrial inheritance as well. Mutations that resulted in the suppression of mdm20 phenotypes were identified in ACT1 and TPM1, genes encoding actin and tropomyosin, a protein that stabilizes actin filaments within cables. Characteristics of the mdm20-suppressing mutations in ACT1 and TPM1 suggested that they might enhance actin-tropomyosin interactions. By extension, it was suggested that the role of Mdm20p might be to regulate actin-tropomyosin binding. Tropomyosin must be N-terminally acetylated to bind actin. Using mass spectrometry, it was shown that Tpm1p purified from wildtype cells is acetylated while Tpm1p from mdm20 mutants is not. Therefore the function of Mdm20p is required for Tpm1p-actin binding, the formation of stabilized actin cables and the successful inheritance of mitochondria. |
