The actin-related protein Act3p of Saccharomyces cerevisiae is located in the nucleus

Deconstructing the nucleus: global architecture from local interactions

Recent advances in fluorescence in situ hybridization and three-dimensional microscopy have revealed a high degree of large-scale order in the nucleus, indicating that the position of each gene within the nucleus is not random. As with any other biological phenomenon, this large-scale organization must ultimately be specified by molecular interactions. Biochemical and molecular investigations have revealed a small set of local molecular-scale interactions that can be used together in a combinatorial fashion to establish a global large-scale nuclear architecture.

Centractin (ARP1) associates with spectrin revealing a potential mechanism to link dynactin to intracellular organelles

Centractin (Arp1), an actin-related protein, is a component of the dynactin complex. To investigate potential functions of the protein, we used transient transfections to overexpress centractin in mammalian cells. We observed that the overexpressed polypeptide formed filamentous structures that were significantly longer and more variable in length than those observed in the native dynactin complex. The centractin filaments were distinct from conventional actin in subunit composition and pharmacology as demonstrated by the absence of immunoreactivity of these filaments with an actin-specific antibody, by resistance to treatment with the drug cytochalasin D, and by the inability to bind phalloidin. We examined the transfected cells for evidence of specific associations of the novel centractin filaments with cellular organelles or cytoskeletal proteins. Using immunocytochemistry we observed the colocalization of Golgi marker proteins with the centractin polymers. Additional immunocytochemical analysis using antibodies to non-erythroid spectrin (fodrin) and Golgi-spectrin (beta I sigma *) revealed that spectrin colocalized with the centractin filaments in transfected cells. Biochemical assays demonstrated that spectrin was present in dynactin-enriched cellular fractions, was coimmunoprecipitated from rat brain cytosol using antibodies to dynactin subunits, and was coeluted with dynactin using affinity chromatography. Immunoprecipitations and affinity chromatography also revealed that actin is not a bona fide component of dynactin. Our results indicate that spectrin is associated with the dynactin complex. We suggest a model in which dynactin associates with the Golgi through an interaction between the centractin filament of the dynactin complex and a spectrin-linked cytoskeletal network.

Epigenetic effects on yeast transcription caused by mutations in an actin-related protein present in the nucleus

Gene inactivation can result when a delta element of the Ty1 transposon inserted into the yeast HIS4 promoter (his4-912delta) alters the transcription initiation site. Previous work has identified mutations that suppress this transcriptional defect by restoring the transcription start site to the native position, and these mutations have been implicated in transcriptional regulation and chromatin structure. We show that in a sin4 mutant such suppression is incompletely penetrant, such that genetically identical yeast cells (sin4 his4-912delta) show either of two distinct phenotypic states, His+ or His-. To study this type of potential epigenetic control of gene expression, we constructed a strain with ADE2 expression under the control of the his4-912delta promoter, as colony color provides a convenient assay for ADE2 expression. We isolated mutations in the ACT3 gene that show variegated expression of this ADE2 reporter. The act3 his4delta-ADE2 colonies display both white and red sectors, showing that the two different phenotypes are possible in a single colony. The two phenotypic states can be inherited during clonal growth, yet are reversible. Analysis of RNA isolated from individual colonies of either red or white color demonstrates that it is the state of the promoter, as either On or Off, that is inherited and is responsible for the colony color. An act3 mutation also affects expression of the HIS4 and LYS2 genes; thus, Act3p is not a delta element-specific transcriptional regulator. Immunofluorescence microscopy experiments demonstrate that the Act3p protein is present in the nucleus. Act3p shows clear homology to actin, and possible roles for an actin-related protein in transcription are discussed.

The actin-related proteins

A family of proteins has been discovered over the past three years whose members have clear sequence homology to actin but are distinguished from actin by their structural and functional diversity. The ranks of this family, whose members are known as the actin-related proteins (arps), are expanding rapidly. Arps are but one branch of a larger superfamily which includes the actins, hsp/hsc70s, sugar kinases and several cell cycle proteins from bacteria. The existence of the superfamily has been inferred from tertiary structural data. In the case of the arps, their identification and classification has been based upon primary structural data. Placing the arps in a functional context is proving a slower process, although genetic and biochemical analyses are converging in several cases. In the past year, different arps have been linked to functions mediated by actin filaments (arp2 and arp3), microtubules (arp1) and the structural elements of chromatin (arp4 and arp6).