title: The HAC1 gene from Pichia pastoris: characterization and effect of its overexpression on the production of secreted, surface displayed and membrane proteins.
abstract: BACKGROUND
  The unfolded protein response (UPR) in eukaryotes upregulates factors that restore ER homeostasis upon protein folding stress and in yeast is activated by a non-conventional splicing of the HAC1 mRNA. The spliced HAC1 mRNA encodes an active transcription factor that binds to UPR-responsive elements in the promoter of UPR target genes. Overexpression of the HAC1 gene of S. cerevisiae can reportedly lead to increased production of heterologous proteins. To further such studies in the biotechnology favored yeast Pichia pastoris, we cloned and characterized the P. pastoris HAC1 gene and the splice event.
  
  
  RESULTS
  We identified the HAC1 homologue of P. pastoris and its splice sites. Surprisingly, we could not find evidence for the non-spliced HAC1 mRNA when P. pastoris was cultivated in a standard growth medium without any endoplasmic reticulum stress inducers, indicating that the UPR is constitutively active to some extent in this organism. After identification of the sequence encoding active Hac1p we evaluated the effect of its overexpression in Pichia. The KAR2 UPR-responsive gene was strongly upregulated. Electron microscopy revealed an expansion of the intracellular membranes in Hac1p-overexpressing strains. We then evaluated the effect of inducible and constitutive UPR induction on the production of secreted, surface displayed and membrane proteins. Wherever Hac1p overexpression affected heterologous protein expression levels, this effect was always stronger when Hac1p expression was inducible rather than constitutive. Depending on the heterologous protein, co-expression of Hac1p increased, decreased or had no effect on expression level. Moreover, alpha-mating factor prepro signal processing of a G-protein coupled receptor was more efficient with Hac1p overexpression; resulting in a significantly improved homogeneity.
  
  
  CONCLUSIONS
  Overexpression of P. pastoris Hac1p can be used to increase the production of heterologous proteins but needs to be evaluated on a case by case basis. Inducible Hac1p expression is more effective than constitutive expression. Correct processing and thus homogeneity of proteins that are difficult to express, such as GPCRs, can be increased by co-expression with Hac1p.
authors.affiliation: Department of Biochemistry and Microbiology, Ghent University, Ghent, Belgium.
authors.forename: Mouna
authors.initials: M
authors.lastname: Guerfal
authors.forename: Stefan
authors.initials: S
authors.lastname: Ryckaert
authors.forename: Pieter P
authors.initials: PP
authors.lastname: Jacobs
authors.forename: Paul
authors.initials: P
authors.lastname: Ameloot
authors.forename: Kathleen
authors.initials: K
authors.lastname: Van Craenenbroeck
authors.forename: Riet
authors.initials: R
authors.lastname: Derycke
authors.forename: Nico
authors.initials: N
authors.lastname: Callewaert
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D005656:Fungal Proteins / Q000235:genetics* / Q000378:metabolism; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D016753:Interleukin-10 / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D010843:Pichia / Q000235:genetics* / Q000378:metabolism; D012326:RNA Splicing; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism
chemical_list: D005656:Fungal Proteins; D018840:HSP70 Heat-Shock Proteins; C060153:KAR2 protein, yeast; D008565:Membrane Proteins; D012333:RNA, Messenger; D011994:Recombinant Proteins; D014157:Transcription Factors; D016753:Interleukin-10