Introduction MicroRNAs are small noncoding RNA molecules that negatively regulate gene expression via degradation or translational repression of their targeted mRNAs. It is known that aberrant microRNA expression can play important roles in cancer, but the role of microRNAs in autoimmune diseases is only beginning to emerge. In this study, the expression of selected microRNAs is examined in rheumatoid arthritis.

Jasplakinolide paradoxically stabilizes actin filaments in vitro, but in vivo it can disrupt actin filaments and induce polymerization of monomeric actin into amorphous masses. A detailed analysis of the effects of jasplakinolide on the kinetics of actin polymerization suggests a resolution to this paradox. Jasplakinolide markedly enhances the rate of actin filament nucleation. This increase corresponds to a change in the size of actin oligomer capable of nucleating filament growth from four to approximately three subunits, which is mechanistically consistent with the localization of the jasplakinolide-binding site at an interface of three actin subunits. Because jasplakinolide both decreases the amount of sequestered actin (by lowering the critical concentration of actin) and augments nucleation, the enhancement of polymerization by jasplakinolide is amplified in the presence of actin-monomer sequestering proteins such as thymosin ␤ 4 . Overall, the kinetic parameters in vitro define the mechanism by which jasplakinolide induces polymerization of monomeric actin in vivo. Expected consequences of jasplakinolide function are consistent with the experimental observations and include de novo nucleation resulting in disordered polymeric actin and in insufficient monomeric actin to allow for remodeling of stress fibers.Jasplakinolide is a cyclic peptide isolated from the marine sponge, Jaspis johnstoni, that we have previously shown to bind to and stabilize filamentous actin in vitro (1). In vivo data suggests that jasplakinolide-treated prostate cancer cells have both decreased labeling of F-actin and decreased amounts of rhodamine-phalloidin bound to cell extracts (2), results that could be explained by the observation that jasplakinolide and phalloidin bind competitively to actin (1). In addition, however, in vivo data also convincingly show that jasplakinolide disrupts actin filaments with alterations in cellular architecture (2, 3), an effect that cannot be explained simply by competitive binding. We now present kinetic data characterizing the steady state and time-dependent in vitro interactions between jasplakinolide and actin that provide a plausible explanation for the effects of jasplakinolide on actin distribution in cultured cells. EXPERIMENTAL PROCEDURESMaterials-Rabbit skeletal muscle actin was prepared from frozen muscle (Pel-Freez, Rogers, AR) in buffer G (5.0 mM Tris, 0.2 mM ATP, 0.2 mM dithiothreitol, 0.1 mM CaCl 2 , and 0.01% sodium azide, pH 7.8) (4). Non-muscle actin from bovine brain was prepared by the method of Ruscha and Himes (5). Muscle and non-muscle pyrenyl-labeled actins 1 were prepared with 0.67-0.95 mol of label/mol of protein using the method of Kouyama and Mihashi (6). Labeled and unlabeled actins were further purified by gel filtration on Superose 12 (Amersham Pharmacia Biotech). Thymosin ␤ 4 cDNA was a gift from Dr. Vivian Nachmias and was inserted in a pET-11a vector, expressed in BL21(DE3) Escherichia coli, and purified as described previously (7). Jasplakinolide was a gift from ...

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2 SUMMARYLatrunculin A is used extensively as an agent to sequester monomeric actin in living cells. We hypothesize that additional activities of latrunculin A may be important for its biological activity. Our data are consistent with the formation of a one to one stoichiometric complex with equilibrium dissociation constant of 0.2 to 0.4 µM, and provide no evidence that the actin-latrunculin A complex participates in the elongation of actin filaments. Profilin and latrunculin A bind independently to actin, whereas binding of thymosin β 4 to actin is inhibited by latrunculin A.Potential implications of this differential effect on actin-binding proteins are discussed. From a structural perspective, if latrunculin A binds to actin at a site that sterically influences binding by thymosin β 4 , then the observation that latrunculin A inhibits nucleotide exchange on actin implies an allosteric effect on the nucleotide binding cleft. Alternatively, if as previously postulated, latrunculin A binds in the nucleotide cleft of actin, then its ability to inhibit binding by thymosin β 4 is a surprising result that suggests that significant allosteric changes affect the thymosin β 4 binding site. We show that latrunculin A and actin form a crystalline structure with orthorhombic space group P2 1 2 1 2 1 and diffraction to 3.10 Å. A highresolution structure with optimized crystallization conditions should provide insight regarding these remarkable allosteric properties.