Extended glycoprotein structure of the seven domains in human carcinoembryonic antigen by X-ray and neutron solution scattering and an automated curve fitting procedure: implications for cellular adhesion.
Carcinoembryonic antigen (CEA) is one of the most widely used cell-surface tumour markers for tumour monitoring and for targeting by antibodies. It is heavily glycosylated (50% carbohydrate) and a monomer is constructed from one V-type and six C2-type fold domains of the immunoglobulin superfamily. The solution arrangement at low resolution of the seven domains in CEA cleaved from its membrane anchor was determined by X-ray and neutron scattering. Guinier analyses showed that the X-ray radius of gyration RG of CEA was 8.0 nm. The length of CEA was 27 to 33 nm, and is consistent with an extended arrangement of seven domains. The X-ray cross-sectional radius of gyration RXS was 2.1 nm, and is consistent with extended carbohydrate structures in CEA. The neutron data gave CEA a relative molecular mass of 150,000, in agreement with a value of 152,500 from composition data, and validated the X-ray analyses. The CEA scattering curves were analysed using an automated computer modelling procedure based on the crystal structure of CD2. The V-type and C2-type domains in CD2 were separated, and the C2-type domain was duplicated five times to create a linear seven-domain starting model for CEA. A total of 28 complex-type oligosaccharide chains in extended conformations were added to this model. By fixing the six interdomain orientations to be the same, three-parameter searches of the rotational orientations between the seven domains gave 4056 possible CEA models. The best curve fits from these corresponded to a family of zig-zag models. The long axis of each domain was set at 160(+/-25) degrees relative to its neighbour, and the two perpendicular axes were orientated at 10(+/-30) degrees and -5(+/-35) degrees. Interestingly, the curve fit from this model is within error of that calculated from a CEA model generated directly from the CD2 crystal structure by the superposition of adjacent domains. Zig-zag models of this type imply that the protein face of the GFCC' beta-sheet in neighbouring CEA domains lie on alternate sides of the CEA structure. Such a model has implications for the adhesion interactions between CEA molecules on adjacent cells or for the antibody targeting of CEA.
Boehm
M K
MK
Department of Biochemistry and Molecular Biology, Royal Free Hospital of Medicine, London, UK.
Mayans
M O
MO
Thornton
J D
JD
Begent
R H
RH
Keep
P A
PA
Perkins
S J
SJ
D000595:Amino Acid Sequence; D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D002272:Carcinoembryonic Antigen / Q000737:chemistry*; D006801:Humans; D007136:Immunoglobulins / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009502:Neutrons; D009844:Oligosaccharides / Q000737:chemistry*; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012542:Scattering, Radiation; D016415:Sequence Alignment; D014461:Ultracentrifugation; D014965:X-Rays
D002272:Carcinoembryonic Antigen; D007136:Immunoglobulins; D009844:Oligosaccharides