Core component of the dynactin complex which assists cytoplasmic dynein by increasing its processivity and by regulation of its cargo binding. The dynactin complex is required for the spindle translocation late in anaphase and is involved in a cell wall synthesis checkpoint. Arp1 forms the backbone filament of the dynactin rod structure and serves as the scaffold for the remaining subunits. Required for proper orientation of the mitotic spindle (By similarity). Self-associates to form an actin-like filament of 8-10 monomers. Component of the dynactin complex. Belongs to the actin family. ARP1 subfamily.