Formin proteins participate in a wide range of cytoskeletal processes in all eukaryotes. The defining feature of formins is a highly conserved approximately 400 residue region, the Formin Homology-2 (FH2) domain, which has recently been found to nucleate actin filaments. Here we report crystal structures of the S. cerevesiae Bni1p FH2 domain. The mostly alpha-helical FH2 domain forms a unique "tethered dimer" in which two elongated actin binding heads are tied together at either end by an unusual lasso and linker structure. Biochemical and crystallographic observations indicate that the dimer is stable but flexible, with flexibility between the two halves of the dimer conferred by the linker segments. Although each half of the dimer is competent to interact with filament ends, the intact dimer is required for actin nucleation and processive capping. The tethered dimer architecture may allow formins to stair-step on the barbed end of an elongating nascent filament.

Formins are required for cell polarization and cytokinesis, but do not have a defined biochemical activity. In Saccharomyces cerevisiae, formins and the actin-monomer-binding protein profilin are specifically required to assemble linear actin structures called 'actin cables'. These structures seem to be assembled independently of the Arp2/3 complex, the only well characterized cellular mediator of actin nucleation. Here, an activated yeast formin was purified and found to promote the nucleation of actin filaments in vitro. Formin-dependent actin nucleation was stimulated by profilin. Thus, formin and profilin mediate actin nucleation by an Arp2/3-independent mechanism. These findings suggest that distinct actin nucleation mechanisms may underlie the assembly of different actin cytoskeletal structures.

Formins are conserved Rho-GTPase effectors that communicate Rho-GTPase signals to the cytoskeleton. We found that formins were required for the assembly of one of the three budding yeast actin structures: polarized arrays of actin cables. A dominant-active formin induced the assembly of actin cables. The activation and localization of the formin Bni1p required components of the polarisome complex. These findings potentially define the cellular function of formins in budding yeast and explain their involvement in the generation of cell polarity. A requirement for formins in constructing specific actin structures might be the basis for the diverse activities of formins in development.

Formins have conserved roles in cell polarity and cytokinesis and directly nucleate actin filament assembly through their FH2 domain. Here, we define the active region of the yeast formin Bni1 FH2 domain and show that it dimerizes. Mutations that disrupt dimerization abolish actin assembly activity, suggesting that dimers are the active state of FH2 domains. The Bni1 FH2 domain protects growing barbed ends of actin filaments from vast excesses of capping protein, suggesting that the dimer maintains a persistent association during elongation. This is not a species-specific mechanism, as the activities of purified mammalian formin mDia1 are identical to those of Bni1. Further, mDia1 partially complements BNI1 function in vivo, and expression of a dominant active mDia1 construct in yeast causes similar phenotypes to dominant active Bni1 constructs. In addition, we purified the Bni1-interacting half of the cell polarity factor Bud6 and found that it binds specifically to actin monomers and, like profilin, promotes rapid nucleotide exchange on actin. Bud6 and profilin show additive stimulatory effects on Bni1 activity and have a synthetic lethal genetic interaction in vivo. From these results, we propose a model in which Bni1 FH2 dimers nucleate and processively cap the elongating barbed end of the actin filament, and Bud6 and profilin generate a local flux of ATP-actin monomers to promote actin assembly.

The 26S proteasome is responsible for the controlled proteolysis of a vast number of proteins, including crucial cell cycle regulators. Accordingly, in Saccharomyces cerevisiae, 26S proteasome function is mandatory for cell cycle progression. In budding yeast, the 26S proteasome is assembled in the nucleus, where it is localized throughout the cell cycle. We report that upon cell entry into quiescence, proteasome subunits massively relocalize from the nucleus into motile cytoplasmic structures. We further demonstrate that these structures are proteasome cytoplasmic reservoirs that are rapidly mobilized upon exit from quiescence. Therefore, we have named these previously unknown structures proteasome storage granules (PSGs). Finally, we observe conserved formation and mobilization of these PSGs in the evolutionary distant yeast Schizosaccharomyces pombe. This conservation implies a broad significance for these proteasome reserves.

Most eukaryotic cells spend most of their life in a quiescent state, poised to respond to specific signals to proliferate. In Saccharomyces cerevisiae, entry into and exit from quiescence are dependent only on the availability of nutrients in the environment. The transition from quiescence to proliferation requires not only drastic metabolic changes but also a complete remodeling of various cellular structures. Here, we describe an actin cytoskeleton organization specific of the yeast quiescent state. When cells cease to divide, actin is reorganized into structures that we named "actin bodies." We show that actin bodies contain F-actin and several actin-binding proteins such as fimbrin and capping protein. Furthermore, by contrast to actin patches or cables, actin bodies are mostly immobile, and we could not detect any actin filament turnover. Finally, we show that upon cells refeeding, actin bodies rapidly disappear and actin cables and patches can be assembled in the absence of de novo protein synthesis. This led us to propose that actin bodies are a reserve of actin that can be immediately mobilized for actin cables and patches formation upon reentry into a proliferation cycle.

We propose that, rather than first being recruited to the cell cortex, dynein is delivered to the cortex on the plus ends of polymerizing aMTs. Dynein may then undergo Num1p-dependent activation and transfer to the region of cortical contact. Based on the similar effects of loss of Num1p and loss of dynactin on dynein localization, we suggest that Num1p might also enhance dynein motor activity or processivity, perhaps by clustering dynein motors.

Atp6p is an essential subunit of the ATP synthase proton translocating domain, which is encoded by the mitochondrial DNA (mtDNA) in yeast. We have replaced the coding sequence of Atp6p gene with the non-respiratory genetic marker ARG8 m . Due to the presence of ARG8 m , accumulation of ؊ / 0 petites issued from large deletions in mtDNA could be restricted to 20 -30% by growing the atp6 mutant in media lacking arginine. This moderate mtDNA instability created favorable conditions to investigate the consequences of a specific lack in Atp6p. Interestingly, in addition to the expected loss of ATP synthase activity, the cytochrome c oxidase respiratory enzyme steadystate level was found to be extremely low (<5%) in the atp6 mutant. We show that the cytochrome c oxidase-poor accumulation was caused by a failure in the synthesis of one of its mtDNA-encoded subunits, Cox1p, indicating that, in yeast mitochondria, Cox1p synthesis is a key target for cytochrome c oxidase abundance regulation in relation to the ATP synthase activity. We provide direct evidence showing that in the absence of Atp6p the remaining subunits of the ATP synthase can still assemble. Mitochondrial cristae were detected in the atp6 mutant, showing that neither Atp6p nor the ATP synthase activity is critical for their formation. However, the atp6 mutant exhibited unusual mitochondrial structure and distribution anomalies, presumably caused by a strong delay in inner membrane fusion.In the mitochondrial inner membrane, the F 1 F 0 -type ATP synthase produces ATP from ADP and inorganic phosphate by using the energy of the transmembrane electrochemical proton gradient generated by the respiratory chain in the course of electron transfer to oxygen. The ATP synthase harbors two major structural domains, a transmembrane component (F 0 ) containing a proton-permeable pore and a peripheral, matrixlocalized, catalytic component (F 1 ) where the ATP is synthesized (1-4). In the F 0 , the core of the proton channel consists of a ring of c subunits (ten in yeast (4)) and one a subunit (Atp6p). Proton movement through this channel coincides with rotation of the subunit c ring (5-9), which results in conformational changes favoring ATP synthesis in the F 1 (1).Due to its good fermenting capacity the yeast Saccharomyces cerevisiae has been extensively used as a genetic system for the study of the mitochondrial ATP synthase (for reviews see Refs. 10 and 11). As in most eukaryotes, the yeast ATP synthase has a dual genetic origin, nuclear and mitochondrial. The yeast mitochondrial ATP synthase genes (ATP6, ATP9, and ATP8) encode the proton channel subunits a and c (usually referred to in yeast as Atp6p and Atp9p), respectively, and a third F 0 subunit (Atp8p) of unknown function. Dozens of mutations in the nuclear ATP synthase genes have provided much information on their protein products (10, 11). In contrast, only a very few mutants of the mitochondrial ATP synthase genes have been reported. Random generation of respiratory growth-deficient yeast strains issued fro...