Efficient folding of the FcepsilonRI alpha-chain membrane-proximal domain D2 depends on the presence of the N-terminal domain D1.
Human high affinity receptor for IgE is a membrane glycoprotein multichain complex presenting two extracellular Ig modules in its alpha-chain (D1D2). The receptor IgE binding region is located within the membrane-proximal module D2, while the N-terminal module D1 appears to promote an optimal receptor conformation for IgE binding. To understand the structural relationship between the two modules, we dissected FcepsilonRI alpha-chain into its discrete Ig units and expressed them in mammalian cells. Unexpectedly, D2 was secreted as a disulphide-linked dimer, while D1 was monomeric. Active secretion and full glycosylation of dimeric D2 suggest a native-like conformation of the protein, justifying the escape from the endoplasmic reticulum/Golgi quality control systems. We then propose a domain-swapping model for D2, in which two interdigitated polypeptide chains assume the overall conformation of two Ig modules, as observed for rat CD2 N-terminal domain. Fusion of an unrelated Ig fold moiety at the N terminus of D2 did not interfere with its dimerisation. While D1D2 assumes a correct fold, co-expression of both isolated domains in the same cell did not restore monomeric folding of D2. Thus, D1 appears to assist the appropriate folding of FcepsilonRI alpha-chain, acting as an uncleavable intramolecular chaperone-like block towards D2.
Vangelista
Luca
L
International Centre for Genetic Engineering and Biotechnology, Padriciano 99, I-34012 Trieste, Italy.
Cesco-Gaspere
Michela
M
Lamba
Doriano
D
Burrone
Oscar
O
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D002462:Cell Membrane / Q000378:metabolism; D006224:Cricetinae; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D017455:Receptors, IgE / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
D017455:Receptors, IgE; D011993:Recombinant Fusion Proteins