A critical aspect of mitosis is the interaction of the kinetochore with spindle microtubules. Fission yeast Mal3 is a member of the EB1 family of microtubule plus-end binding proteins, which have been implicated in this process. However, the Mal3 interaction partner at the kinetochore had not been identified. Here, we show that the mal3 mutant phenotype can be suppressed by the presence of extra Spc7, an essential kinetochore protein associated with the central centromere region. Mal3 and Spc7 interact physically as both proteins can be coimmunoprecipitated. Overexpression of a Spc7 variant severely compromises kinetochore-microtubule interaction, indicating that the Spc7 protein plays a role in this process. Spc7 function seems to be conserved because, Spc105, a Saccharomyces cerevisiae homolog of Spc7, identified by mass spectrometry as a component of the conserved Ndc80 complex, can rescue mal3 mutant strains. INTRODUCTIONSegregation of chromosomes requires the association of spindle microtubules and chromosomes. Attachment of the mitotic spindle fibers occurs at a multicomponent protein complex, the kinetochore, that is assembled on centromeric DNA. This DNA region differs greatly in structure and size among various organisms (reviewed in Pidoux and Allshire, 2000;Cleveland et al., 2003). The budding yeast centromere DNA is the simplest one described and consists of a very well defined 125-base pair region, whereas in higher eucaryotes, centromeric DNA is made up of highly repetitive sequences encompassing up to millions of base pairs. The centromere DNA of the fission yeast Schizosaccharomyces pombe lies in between these two extremes: it occupies between 40 and 100 kb on each chromosome and is composed of a central region flanked by inner and outer repetitive sequences. To date, proteins found to be associated with these regions either bind to the central core region or to the outer repeats, thus pointing to the existence of two distinct domains in the fission yeast centromere (reviewed in Pidoux and Allshire, 2000). The heterochromatic outer repeats are required for centromere cohesion (reviewed in Bernard and Allshire, 2002), whereas the central region is needed for the assembly of the kinetochore per se (Saitoh et al., 1997;Goshima et al., 1999;Jin et al., 2002;Pidoux et al., 2003). However, in spite of the different cis-acting DNA requirements, a substantial number of kinetochore proteins have been conserved from yeast to humans, among them the four-component Ndc80 complex. This complex is required for kinetochore-microtubule association and spindle checkpoint signaling (He et al., 2001;Janke et al., 2001;Wigge and Kilmartin, 2001;Bharadwaj et al., 2004;McCleland et al., 2004).The spindle microtubules that attach to kinetochores are highly dynamic structures that alternate between phases of growth and shrinkage (Kirschner and Mitchison, 1986). This dynamic behavior is also observed after microtubules are attached to kinetochores and is coregulated by components of the kinetochore complex and microtubule...

bThe data presented here reveal a new facet of the physiological adjustment processes through which Bacillus subtilis can derive osmostress protection. We found that the import of proteogenic (Glu, Gln, Asp, Asn, and Arg) and of nonproteogenic (Orn and Cit) amino acids and their metabolic conversion into proline enhances growth under otherwise osmotically unfavorable conditions. Osmoprotection by amino acids depends on the functioning of the ProJ-ProA-ProH enzymes, but different entry points into this biosynthetic route are used by different amino acids to finally yield the compatible solute proline. Glu, Gln, Asp, and Asn are used to replenish the cellular pool of glutamate, the precursor for proline production, whereas Arg, Orn, and Cit are converted into ␥-glutamic semialdehyde/⌬ 1 -pyrroline-5-carboxylate, an intermediate in proline biosynthesis. The import of Glu, Gln, Asp, Asn, Arg, Orn, and Cit did not lead to a further increase in the size of the proline pool that is already present in osmotically stressed cells. Hence, our data suggest that osmoprotection of B. subtilis by this group of amino acids rests on the savings in biosynthetic building blocks and energy that would otherwise have to be devoted either to the synthesis of the proline precursor glutamate or of proline itself. Since glutamate is the direct biosynthetic precursor for proline, we studied its uptake and found that GltT, an Na ؉ -coupled symporter, is the main uptake system for both glutamate and aspartate in B. subtilis. Collectively, our data show how effectively B. subtilis can exploit environmental resources to derive osmotic-stress protection through physiological means. Bacillus subtilis is a resident of the upper layers of the soil and of the rhizosphere, and it can also efficiently colonize root surfaces (1-3). The blueprint of its genome (4) bears the hallmarks of a bacterium that can exploit many plant-produced compounds for its growth. Accordingly, a considerable portion of the coding capacity of the B. subtilis chromosome (5) is devoted to highaffinity import systems (6) that allow the scavenging of a wide spectrum of nutrients. Reoccurring and persisting high osmolarity in the soil ecosystem (7) is a situation in which B. subtilis can take advantage of the import of plant-produced compounds (8-10) for its physiological adjustment to these unfavorable environmental conditions (7, 11).As in many bacterial species (11-13), cellular adaptation of B. subtilis to both sudden and sustained increases in the external osmolarity involves a two-stage process (7,14). It initially encompasses the uptake of large quantities of potassium as an emergency stress reaction to curb water efflux (14, 15) and, subsequently, the replacement of part of this ion by organic osmolytes, such as proline (Pro) and glycine betaine (GB), to decrease the ionic strength of the cytoplasm and to optimize its solvent properties (14,(16)(17)(18)(19). These organic osmolytes, commonly referred to as compatible solutes, are highly compliant with cellular physiolog...

Spc7, a member of the conserved Spc105/KNL-1 family of kinetochore proteins, was identified as an interaction partner of the EB1 homologue Mal3. Spc7 associates with the central centromere region of the chromosome but does not affect transcriptional silencing. Here, we show that Spc7 is required for the integrity of the spindle as well as for targeting of MIND but not of Ndc80 complex components to the kinetochore. Spindle defects in spc7 mutants were severe ranging from the inability to form a bipolar spindle in early mitosis to broken spindles in midanaphase B. spc7 mutant phenotypes were partially rescued by extra alpha-tubulin or extra Mal2. Thus, Spc7 interacts genetically with the Mal2-containing Sim4 complex.

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Chlamydia pneumoniae is an intracellular Gram-negative bacterium that possesses a type III secretion system (T3SS), which enables the pathogen to deliver, in a single step, effector proteins for modulation of host-cell functions into the human host cell cytosol to establish a unique intracellular niche for replication. The translocon proteins located at the top of the T3SS needle filament are essential for its function, as they form pores in the host-cell membrane. Interestingly, unlike other Gram-negative bacteria, C. pneumoniae has two putative translocon operons, named LcrH_1 and LcrH_2. However, little is known about chlamydial translocon proteins. In this study, we analyzed CPn0809, one of the putative hydrophobic translocators encoded by the LcrH_1 operon, and identified an ‘SseC-like family’ domain characteristic of T3S translocators. Using bright-field and confocal microscopy, we found that CPn0809 is associated with EBs during early and very late phases of a C. pneumoniae infection. Furthermore, CPn0809 forms oligomers, and interacts with the T3SS chaperone LcrH_1, via its N-terminal segment. Moreover, expression of full-length CPn0809 in the heterologous host Escherichia coli causes a grave cytotoxic effect that leads to cell death. Taken together, our data indicate that CPn0809 likely represents one of the translocon proteins of the C. pneumoniae T3SS, and possibly plays a role in the translocation of effector proteins in the early stages of infection.