title: An intein with genetically selectable markers provides a new approach to internally label proteins with GFP.
abstract: BACKGROUND
  Inteins are proteins that catalyze their own removal from within larger precursor proteins. In the process they splice the flanking protein sequences, termed the N-and C-terminal exteins. Large inteins frequently have a homing endonuclease that is involved in maintaining the intein in the host. Splicing and nuclease activity are independent and distinct domains in the folded structure. We show here that other biochemical activities can be incorporated into an intein in place of the endonuclease without affecting splicing and that these activities can provide genetic selection for the intein. We have coupled such a genetically marked intein with GFP as the N-terminal extein to create a cassette to introduce GFP within the interior of a targeted protein.
  
  
  RESULTS
  The Pch PRP8 mini-intein of Penicillium chrysogenum was modified to include: 1) aminoglycoside phosphotransferase; 2) imidazoleglycerol-phosphate dehydratase, His5 from S. pombe ; 3) hygromycin B phosphotransferase; and 4) the transcriptional activator LexA-VP16. The proteins were inserted at the site of the lost endonuclease. When expressed in E. coli, all of the modified inteins spliced at high efficiency. Splicing efficiency was also greater than 96% when expressed from a plasmid in S. cerevisiae. In addition the inteins conferred either G418 or hygromycin resistance, or histidine or leucine prototropy, depending on the inserted marker and the yeast genetic background. DNA encoding the marked inteins coupled to GFP as the N-terminal extein was PCR amplified with ends homologous to an internal site in the yeast calmodulin gene CMD1. The DNA was transformed into yeast and integrants obtained by direct selection for the intein's marker. The His5-marked intein yielded a fully functional calmodulin that was tagged with GFP within its central linker.
  
  
  CONCLUSIONS
  Inteins continue to show their flexibility as tools in molecular biology. The Pch PRP8 intein can successfully tolerate a variety of genetic markers and still retain high splicing efficiency. We have shown that a genetically marked intein can be used to insert GFP in one-step within a target protein in vivo.
authors.affiliation: Department of Biochemistry, University of Washington, Seattle, WA, USA.
authors.forename: Richard
authors.initials: R
authors.lastname: Ramsden
authors.forename: Luther
authors.initials: L
authors.lastname: Arms
authors.forename: Trisha N
authors.initials: TN
authors.lastname: Davis
authors.forename: Eric G D
authors.initials: EG
authors.lastname: Muller
mesh_terms: D015153:Blotting, Western; D002147:Calmodulin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D003001:Cloning, Molecular / Q000379:methods*; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000235:genetics; D005819:Genetic Markers / Q000235:genetics*; D049452:Green Fluorescent Proteins / Q000737:chemistry / Q000235:genetics*; D006639:Histidine / Q000737:chemistry / Q000235:genetics; D006836:Hydro-Lyases / Q000235:genetics; D047668:Inteins / Q000235:genetics*; D019868:Kanamycin Kinase / Q000235:genetics; D008957:Models, Genetic; D009842:Oligopeptides / Q000737:chemistry / Q000235:genetics; D010408:Penicillium chrysogenum / Q000235:genetics; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D011995:Recombination, Genetic; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D029702:Schizosaccharomyces pombe Proteins / Q000737:chemistry / Q000235:genetics
chemical_list: C505601:CMD1 protein, S cerevisiae; D002147:Calmodulin; D005819:Genetic Markers; D009842:Oligopeptides; D011993:Recombinant Fusion Proteins; D029701:Saccharomyces cerevisiae Proteins; D029702:Schizosaccharomyces pombe Proteins; D049452:Green Fluorescent Proteins; D006639:Histidine; D017853:Phosphotransferases (Alcohol Group Acceptor); C063242:hygromycin-B kinase; D019868:Kanamycin Kinase; D006836:Hydro-Lyases; C021434:imidazoleglycerolphosphate dehydratase