The existence of actin in the nucleus, although questioned for many decades, has become more widely accepted and its functions are beginning to be defined. We have used Xenopus egg extracts to investigate the involvement of actin in nuclear assembly and disassembly. To observe nuclear actin without preparing nuclear matrix, rhodamine-actin was added to cell free reactions and acquisition of actin observed directly during nuclear assembly. Actin fluorescent signals were detected when sperm pronuclei began to decondense and remained during nuclear maturation. To test if actin is essential for nuclear structure, latrunculin A was added to nuclear assembly reactions and also to mature nuclei. Latrunculin A, which sequesters G-actin monomers, prevented proper nuclear assembly at concentrations above 25 ?M and actin was excluded from latrunculinA-perturbed nuclei. When latrunculin A was incubated with nuclei preassembled in vitro, nuclear structure became distorted and chromatin collapsed. To explore interactions of nuclear actin-containing complexes, we focussed on protein 4.1 which has nuclear isoforms containing an actin binding domain. By double-label immunofluoresence and whole-mount electron microscopy of mammalian cells, protein 4.1 epitopes and actin epitopes were observed to be associated in nuclear matrix. In coimmunoprecipitation and pulldown experiments in mammalian or Xenopus extracts, protein 4.1 interacted with the BRG1 subunit of the actin-containing BAF chromatin remodeling complex. Our results suggest that nuclear actin is essential both for proper assembly of nuclei as well as for maintenance of interphasic nuclear structure and that a nuclear actin /BRG1 complex interacts with protein 4.1.