The transport signal on Sec22 for packaging into COPII-coated vesicles is a conformational epitope.
The mechanism of cargo concentration into ER-derived vesicles involves interactions between the COPII vesicular coat complex and cargo transport signals--peptide sequences of 10-15 residues. The SNARE protein Sec22 contains a signal that binds the COPII subcomplex Sec23/24 and specifies its endoplasmic reticulum (ER) exit as an unassembled SNARE. The 200 kDa crystal structure of Sec22 bound to Sec23/24 reveals that the transport signal is a folded epitope rather than a conventional short peptide sequence. The NIE segment of the SNARE motif folds against the N-terminal longin domain, and this closed form of Sec22 binds at the Sec23/24 interface. Thus, COPII recognizes unassembled Sec22 via a folded epitope, whereas Sec22 assembly into SNARE complexes would mask the NIE segment. The concept of a conformational epitope as a transport signal suggests packaging mechanisms in which a coat is sensitive to the folded state of a cargo protein or the assembled state of a multiprotein complex.
Mancias
Joseph D
JD
Howard Hughes Medical Institute, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021, USA.
Goldberg
Jonathan
J
D000818:Animals; D001692:Biological Transport; D022181:COP-Coated Vesicles / Q000378:metabolism*; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D004721:Endoplasmic Reticulum / Q000378:metabolism; D000939:Epitopes; D006801:Humans; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism; D008954:Models, Biological; D008969:Molecular Sequence Data; D046912:Multiprotein Complexes / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D021382:Protein Sorting Signals; D021381:Protein Transport / Q000502:physiology; D050683:R-SNARE Proteins / Q000737:chemistry / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D050600:SNARE Proteins; D033921:Vesicular Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
D002352:Carrier Proteins; D000939:Epitopes; D026901:Membrane Transport Proteins; D046912:Multiprotein Complexes; D021382:Protein Sorting Signals; D050683:R-SNARE Proteins; D011993:Recombinant Fusion Proteins; C484373:SEC22A protein, human; C512739:SEC23A protein, human; D050600:SNARE Proteins; D033921:Vesicular Transport Proteins