Centractins represent a family of highly-conserved mammalian genes whose sequences indicate a significant degree of similarity (>70%) to actin. Members of this family have been localized to centrosomes and large vesicular structures by morphological analyses. In addition, biochemical studies have shown that centractins are associated with a 20S cytosolic proteinaceous particle known as the p150(Glued), or dynactin complex. This complex has been implicated in the function of cytoplasmic dynein, a molecular motor which is responsible for retrograde vesicular transport along microtubules. As at least two isoforms of centractin have been characterized in our laboratory, as well as a novel yeast actin-related protein (ARP) which may represent a functional homologue of centractin. It is the aim of the research described here to use biochemical, morphological and genetic techniques to answer the most fundamental questions pertinent to the role of centractins in cellular function. These include: As centractin isoforms have individual subcellular localizations, how do they differ functionally? Are centractins capable of polymerizing into multimeric complexes or filaments, in a fashion homologous to actin, or to serve as actin-binding proteins? What is the "centractin phenotype" in mammalian cells whose centractin expression or integrity has been compromised? Is the yeast ARP, ACT3, the functional homologue of centractin, and if so what is its role in this organism?