During metastasis, cancer cells can break free from primary tumor sites and migrate into the extracellular matrix, through the basement membrane, travel in the blood or lymph, and settle in distant sites. Actin reorganization, specifically in branched actin networks at the leading edge, is responsible for cell motility at various stages of these metastatic processes. A seven-protein complex called the Arp2/3 complex is responsible for the new actin filament growth and branching at the leading edge required for motility and proteins in this complex are often upregulated in cancer. In addition to this transcriptional upregulation, microRNAs can also control expression Arp2/3 complex components, affecting migratory potential. One protein in particular, ARPC5, is known to be regulated transcriptionally and by miRNAs in many cancers including head and neck squamous cell carcinoma, non-small cell lung cancer, bladder cancer, colorectal cancer, breast cancer, renal cell carcinoma, and prostate cancer, suggesting its key role in cancer cell migration. Given the endogenous suppression of expression by miRNAs and also the possible expression of an alternate isoform of ARPC5, ARPC5L, we propose that Arp2/3 complexes with no ARPC5, ARPC5, or ARPC5L can create versions of the actin-binding complex that are tuned for various cell functions. To address this, in Aim 1, we will purify and perform in vitro and in vivo characterization of complexes from a model system that naturally lacks ARPC5, a green alga that performs other typical Arp2/3 functions outside of surface motility. In Aim 2, we will directly test the functional consequences of ARPC5 and/or ARPC5L deletion in triple negative breast cancer cell line isolates that vary in their migration potential. Successful completion of this project will determine the molecular and cellular consequences of ARPC5-dependent Arp2/3 composition as well as the potential impact of modulating ARPC5 composition in the context of metastatic cancer. The BioMT COBRE will provide essential mentoring and collaboration from experts with decades of actin biochemistry and specifically Arp2/3 experience as well as in tumor progression. The COBRE will also provide core facility support in biochemistry and microscopy for the completion of these aims. Ultimately, this will allow the PI to expand work from her lab outside core projects in microtubule-based cytoskeletal structures into a new area of investigation related to actin regulation.