Construction of thermostable cellobiohydrolase I from the fungus Talaromyces cellulolyticus by protein engineering.

Fungus-derived GH-7 family cellobiohydrolase I (CBHI, EC 3.2.1.91) is one of the most important industrial enzymes for cellulosic biomass saccharification. Talaromyces cellulolyticus is well known as a mesophilic fungus producing a high amount of CBHI. Thermostability enhances the economic value of enzymes by making them more robust. However, CBHI has proven difficult to engineer, a fact that stems in part from its low expression in heterozygous hosts and its complex structure. Here, we report the successful improvement of the thermostability of CBHI from T. cellulolyticus using our homologous expression system and protein engineering method. We examined the key structures that seem to contribute to its thermostability using the 3D structural information of CBHI. Some parts of the structure of the Talaromyces emersonii CBHI were grafted into T. cellulolyticus CBHI and thermostable mutant CBHIs were constructed. The thermostability was primarily because of the improvement in the loop structures, and the positive effects of the mutations for thermostability were additive. By combing the mutations, the constructed thermophilic CBHI exhibits high hydrolytic activity toward crystalline cellulose with an optimum temperature at over 70°C. In addition, the strategy can be applied to the construction of the other thermostable CBHIs.

Crystal structure of ß-galactosidase from Bacillus circulans ATCC 31382 (BgaD) and the construction of the thermophilic mutants.

UNLABELLED: ß-Galactosidase (EC 3.2.1.23) from Bacillus circulans ATCC 31382, designated BgaD, exhibits high transglycosylation activity to produce galacto-oligosaccharides. BgaD has been speculated to have a multiple domain architecture including a F5/8-type C domain or a discoidin domain in the C-terminal peptide region from amino acid sequence analysis. Here, we solved the first crystal structure of the C-terminal deletion mutant BgaD-D, consisting of sugar binding, Glyco_hydro, catalytic and bacterial Ig-like domains, at 2.5 Å. In the asymmetric unit, two molecules of BgaD-D were identified and the value of VM was estimated to be 5.0 Å(3) · Da(-1). It has been speculated that BgaD-D consists of four domains. From the structural analysis, however, we clarified that BgaD-D consists of five domains. We identified a new domain structure comprised of ß-sheets in BgaD. The catalytic domain exhibits a TIM barrel structure with a small pocket suited for accommodating the disaccharides. Detailed structural information for the amino acid residues related to activity and substrate specificity was clarified in the catalytic domain. Furthermore, using the structural information, we successfully constructed some thermostable mutants via protein engineering method. DATABASE: Coordinates for the BgaD-D structure have been deposited in the Protein Data Bank under accession code 4YPJ.

Cell death caused by excision of centromeric DNA from a chromosome in Saccharomyces cerevisiae.

If genetically modified organisms (GMOs) are spread through the natural environment, it might affect the natural environment. To help prevent the spread of GMOs, we examined whether it is possible to introduce conditional lethality by excising centromeric DNA from a chromosome by site-specific recombination in Saccharomyces cerevisiae as model organism. First, we constructed haploid cells in which excision of the centromeric DNA from chromosome IV can occur due to recombinase induced by galactose. By this excision, cell death can occur. In diploid cells, cell death can also occur by excision from both homologous chromosomes IV. Furthermore, cell death can occur in the case of chromosome V. A small number of surviving cells appeared with excision of centromeric DNA, and the diploid showed greater viability than the haploid in both chromosomes IV and V. The surviving cells appeared mainly due to deletion of a recombination target site (RS) from the chromosome.

The relationship between chromosomal positioning within the nucleus and the SSD1 gene in Saccharomyces cerevisiae.

Eukaryotic cells are characterized by very large chromosomal DNAs efficiently packed within the nucleus. To identify the mechanism of chromosomal packaging based on the uniqueness of the centromere region in Saccharomyces cerevisiae, we isolated the HCH6 mutant, which shows 2.5-fold higher efficiency of site-specific recombination between the CEN5 and HIS3 loci than the wild-type CH53 strain. This mutant also displayed defects in cell integrity at high temperature. The SSD1 gene was perhaps responsible for this defect. The efficiency of site-specific recombination was decreased by the introduction of SSD1 in HCH6 cells and increased by disruption of SSD1 in the wild-type cells. Furthermore, the distances between the CEN5 and HIS3 loci and between the CEN5 locus and the spindle pole body (SPB) indicated that disrupting SSD1 caused a loss of the anchoring of the CEN5 locus near SPB. These results suggest Ssd1p-dependent cross-talk between chromosomal positioning within the nucleus and the positioning of cellular components within the cell.