Actin is one of the most abundant and conserved eukayotic proteins.  Actin nucleation, defined as the formation of actin oligomers from G-actin monomers, is the rate-limiting step for de novo actin assembly in many fundamental cellular processes.  In cells, actin nucleation is precisely and tightly regulated through a diverse set of actin nucleators.  However, due to their fleeting nature, actin nuclei have long eluded structural investigation.  Our lab has developed a novel double-mutant strategy, which allows the capture of actin nuclei in action.  In the seminar, I will discuss the molecular mechanisms of actin nucleation based on the crystal structures and functional studies of three eukaryotic tandem-actin-binding nucleators.