Application of green fluorescent protein-protein A fusion protein to western blotting

Expression and secretion of recombinant ZZ-EGFP fusion protein by the methylotrophic yeast Pichia pastoris

We constructed a fusion protein ZZ-EGFP by fusing the ZZ domains of staphylococcal protein A (SpA) and enhanced green fluorescent protein (EGFP). ZZ-EGFP was secreted in the yeast, Pichia pastoris, with a hexahistidine tag. Its expression level was determined by measuring the fluorescence of EGFP. When the recombinant yeast cells in shake-flasks were induced with 0.5% methanol for 96 h, a maximum yield of 115 mg ZZ-EGFP/l was obtained. The resulting ZZ-EGFP fusion protein retained immunoglobulin G (IgG)-binding capacity and EGFP fluorescence. ZZ-EGFP was then used in immunofluorescence assays for detecting antinuclear antibodies (ANA); it produced a good signal that was comparable in its brightness and fluorescence pattern to that generated with fluorescein isothiocyanate (FITC)-labelled anti-human IgG. Thus, ZZ-EGFP showed great potential in immunological applications due to its ability to bind to various IgG from different animal sources.

A homogeneous assay for relative affinity of binding proteins using a green fluorescent protein tag and membrane disk

When the association between a ligand immobilized on a membrane disk and a fluorescence-labeled analyte was monitored with a fluorescent microplate reader, the time-dependent increase in fluorescence intensity of the reaction mixture was observed. A novel assay system for the specific interaction based on this phenomenon was designated the homogeneous assay for fluorescence concentrated on membrane (HAFCOM). In this study, streptococcal protein G (SpG) and glycogen-binding subunit R5 of protein phosphatase 1 (PPP1R5) tagged by green fluorescent protein (GFP) were used as the fluorescence-labeled analytes, and the affinity change caused by various amino acid substitutions was measured with HAFCOM. From the site-directed mutagenesis of SpG and PPP1R5, it was clarified that (i) the association rate constant of the Lys454Pro/Glu456Gln mutant of SpG to goat immunoglobulin G was almost equivalent to that of the wild-type but its dissociation rate constant was about 2.7 times that of the wild-type and (ii) the amino acid substitutions of Phe180 in PPP1R5 reduced glycogen-binding by 30-50%. Since HAFCOM using the GFP-tagged analyte requires no special chemicals and instruments, this system can easily and economically assay the specific interaction between target protein and ligand.

Site-specific integration of targeted DNA into animal cell genomes

Novel genetically engineered retroviral vectors and targeting plasmids are described that enable the site-specific targeting of exogenous DNA into the genomes of cultured animal cells. The protocol involves the transduction of competent cells by a chimeric retroviral vector containing a transcription unit composed of two linked cassettes: an upstream marker gene under the control of the viral 5' LTR; and a downstream reporter trap containing a strong promoter 5' to a 48bp yeast FRT element. When cells containing such integrated units are co-transfected with a plasmid encoding yeast FLP recombinase and a promoterless targeting plasmid containing a reporter cDNA tract 3' to an homologous FRT element, the targeting plasmid recombines at the chromosomally preconfigured FRT site, and a new hemizygous function is introduced into the downstream cassette. These reagents provide a new portable system for site-specific targeting of chemically modified genes into uniform and unique sites in genomically integrated transcription units.