CFTR folding and maturation (norm and CF)
The cystic fibrosis transmembrane conductance regulator
(CFTR) is a member of the ATP-binding cassette transporter
superfamily. It acts in the apical part of the epithelial cells as a plasma-membrane
cyclic AMP-activated chloride anion, bicarbonate anion and glutathione channel [1], [2], [3]. The most common
CFTR mutation is loss of a Phe residue at position 508
(deltaF508-CFTR) [4].
Folding and maturation of wt-CFTR and
deltaF508-CFTR are somewhat different. In both cases,
folding starts when Oligosaccharyltransferase complex (OST
complex) binds to the newly synthesized
wt-CFTR or
deltaF508-CFTR via Asn-X-Ser/Thr consensus
sequences that are co-translationally inserted into the endoplasmatic reticulum (ER) by a
branched 14-unit oligosaccharide (two N-acetylglucosamines,
nine mannoses, and three glucoses) [5]. This process is known as
N-glycosylation or core-glycosylation (see in detail about N-glycosylation [6] and OST complex [7], [8]).
Then, most of deltaF508-CFTR is degraded via Heat shock
70kDa protein 8 (Hsc70)-dependent ubiquitination. Unlike
deltaF508-CFTR, wt-CFTR
proceeds in the folding pathway through an interaction of its N-glycosyl residues. Glycan
moiety is recognized by any Glucosidases I and
II, which trim two glucose residues sequentially. The exact
Glucosidases which participate in this process are unknown.
Monoglucosylated oligosaccharide structure is then recognized by the lectin
Calnexin [5], [9].
Chaperones that are currently thought to significantly affect
CFTR ER-associated folding pathways, include cytosolic
chaperones DnaJ homolog subfamily B members 1 and 6 (Hsp40
and Hdj-1), Heat shock proteins 70 and 90kDa
(Hsp70 and Hsp90) [10], [11] and others [12]. ATPase
activity of Hsp70 may be regulated by
Hsp70-interacting protein (HspBP1).
HspBP1 can bind Hsp70, changes
the conformation of the ATPase domain, and inhibits
Hsp70-associated protein folding [13]. The exact
role of most chaperones is unknown [12].
Dissociation from Calnexin coincides with trimming of the
third glucose residue by Glucosidases II. If the
CFTR is folded at this stage, it proceeds to the secretory
pathway to Golgi. However, misfolded CFTR is specifically
recognized as such by UDP-glycoprotein glucosyltransferase (e.g.,
UGCGL1), which reglucosylates them. Reglucosylated
CFTR again may interact with Calnexin
[5], [9].
In addition, prolonged presence in the Calnexin cycle may
cause misfolded CFTR to become a substrate of Mannosidase
alpha class 1B member 1 (MA1B1). MA1B1
removes mannose from the middle branch CFTR,
forming a Man8B isomer which in turn is recognized by another lectin ER degradation
enhancer, mannosidase alpha-like 1 (EDEM) that targets it to
proteasomal Glycoprotein endoplasmic reticulum-associated degradation (GERAD) [5], [9]. GERAD is a major degradative pathway for misfolded
CFTR.
References:
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CFTR directly mediates nucleotide-regulated glutathione flux.
The EMBO journal 2003 May 1;22(9):1981-9
- Chan HC, Shi QX, Zhou CX, Wang XF, Xu WM, Chen WY, Chen AJ, Ni Y, Yuan YY
Critical role of CFTR in uterine bicarbonate secretion and the fertilizing capacity of sperm.
Molecular and cellular endocrinology 2006 May 16;250(1-2):106-13
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The ABC protein turned chloride channel whose failure causes cystic fibrosis.
Nature 2006 Mar 23;440(7083):477-83
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Is cystic fibrosis a TH17 disease?
Inflammation research : official journal of the European Histamine Research Society ... [et al.] 2007 Jun;56(6):221-7
- Amaral MD
Processing of CFTR: traversing the cellular maze--how much CFTR needs to go through to avoid cystic fibrosis?
Pediatric pulmonology 2005 Jun;39(6):479-91
- Popov M, Tam LY, Li J, Reithmeier RA
Mapping the ends of transmembrane segments in a polytopic membrane protein. Scanning N-glycosylation mutagenesis of extracytosolic loops in the anion exchanger, band 3.
The Journal of biological chemistry 1997 Jul 18;272(29):18325-32
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Oligosaccharyltransferase isoforms that contain different catalytic STT3 subunits have distinct enzymatic properties.
Molecular cell 2003 Jul;12(1):101-11
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Proteomic analysis of mammalian oligosaccharyltransferase reveals multiple subcomplexes that contain Sec61, TRAP, and two potential new subunits.
Biochemistry 2005 Apr 26;44(16):5982-92
- Farinha CM, Amaral MD
Most F508del-CFTR is targeted to degradation at an early folding checkpoint and independently of calnexin.
Molecular and cellular biology 2005 Jun;25(12):5242-52
- Loo MA, Jensen TJ, Cui L, Hou Y, Chang XB, Riordan JR
Perturbation of Hsp90 interaction with nascent CFTR prevents its maturation and accelerates its degradation by the proteasome.
The EMBO journal 1998 Dec 1;17(23):6879-87
- Farinha CM, Nogueira P, Mendes F, Penque D, Amaral MD
The human DnaJ homologue (Hdj)-1/heat-shock protein (Hsp) 40 co-chaperone is required for the in vivo stabilization of the cystic fibrosis transmembrane conductance regulator by Hsp70.
The Biochemical journal 2002 Sep 15;366(Pt 3):797-806
- Wang X, Venable J, LaPointe P, Hutt DM, Koulov AV, Coppinger J, Gurkan C, Kellner W, Matteson J, Plutner H, Riordan JR, Kelly JW, Yates JR 3rd, Balch WE
Hsp90 cochaperone Aha1 downregulation rescues misfolding of CFTR in cystic fibrosis.
Cell 2006 Nov 17;127(4):803-15
- McLellan CA, Raynes DA, Guerriero V
HspBP1, an Hsp70 cochaperone, has two structural domains and is capable of altering the conformation of the Hsp70 ATPase domain.
The Journal of biological chemistry 2003 May 23;278(21):19017-22