title: Keratin K18 increases cystic fibrosis transmembrane conductance regulator (CFTR) surface expression by binding to its C-terminal hydrophobic patch.
abstract: BACKGROUND
  CFTR function is tightly regulated by many interacting proteins.
  
  
  RESULTS
  Intermediate filament protein keratin 18 increases the cell surface expression of CFTR by interacting with the C-terminal hydrophobic patch of CFTR.
  
  
  CONCLUSIONS
  K18 controls the function of CFTR.
  
  
  CONCLUSIONS
  These findings offer novel insights into the regulation of CFTR and suggest that K18 and its dimerization partner, K8, may be modifier genes in cystic fibrosis. Malfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) leads to cystic fibrosis, but the regulation of CFTR is not fully understood. Here, we identified the intermediate filament protein keratin K18 (K18) as a CFTR-binding protein by various approaches. We mapped a highly conserved "hydrophobic patch" ((1413)FLVI(1416)) in the CFTR C-terminus, known to determine plasmalemmal CFTR stability, as the K18-binding site. On the other hand, the C-terminal tail of K18 was found to be a critical determinant for binding CFTR. Overexpression of K18 in cells robustly increased the surface expression of wild-type CFTR, whereas depletion of K18 through RNA interference specifically diminished it. K18 binding increased the surface expression of CFTR by accelerating its apical recycling rate without altering CFTR biosynthesis, maturation, or internalization. Importantly, CFTR surface expression was markedly reduced in duodenal and gallbladder epithelia of K18(-/-) mice. Taken together, our results suggest that K18 increases the cell surface expression of CFTR by interacting with the CFTR C-terminal hydrophobic patch. These findings offer novel insights into the regulation of CFTR and suggest that K18 and its dimerization partner, K8, may be modifier genes in cystic fibrosis.
authors.affiliation: Division of Life Science, The Hong Kong University of Science and Technology, Hong Kong, China.
authors.forename: Yuanyuan
authors.initials: Y
authors.lastname: Duan
authors.forename: Ying
authors.initials: Y
authors.lastname: Sun
authors.forename: Fan
authors.initials: F
authors.lastname: Zhang
authors.forename: Wei Kevin
authors.initials: WK
authors.lastname: Zhang
authors.forename: Dong
authors.initials: D
authors.lastname: Wang
authors.forename: Yan
authors.initials: Y
authors.lastname: Wang
authors.forename: Xu
authors.initials: X
authors.lastname: Cao
authors.forename: Wenbao
authors.initials: W
authors.lastname: Hu
authors.forename: Changyan
authors.initials: C
authors.lastname: Xie
authors.forename: John
authors.initials: J
authors.lastname: Cuppoletti
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Magin
authors.forename: Haixia
authors.initials: H
authors.lastname: Wang
authors.forename: Zhenguo
authors.initials: Z
authors.lastname: Wu
authors.forename: Ning
authors.initials: N
authors.lastname: Li
authors.forename: Pingbo
authors.initials: P
authors.lastname: Huang
mesh_terms: D000818:Animals; D002460:Cell Line; D003550:Cystic Fibrosis / Q000235:genetics / Q000378:metabolism*; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D053538:Keratin-18 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D020798:Two-Hybrid System Techniques
chemical_list: D053538:Keratin-18; D019005:Cystic Fibrosis Transmembrane Conductance Regulator