Determination of the disulfide bonds within a B domain variant surface glycoprotein from Trypanosoma congolense

The disulfide bonds within a variant surface glycoprotein from Trypanosoma congolense have been determined. L-[S-35]Cysteine metabolically labeled pro tein was digested with trypsin, and radiolabeled peptides were separated by reversed-phase high performance liquid chromatography, and putative cystin e-containing peptides mere subdigested with other proteases and analyzed af ter further purification by amino acid sequencing and mass spectrometry. Al l eight cysteine residues of the protein, located within the N-terminal dom ain, are covalently linked. The four disulfide bonds are between cysteines 16/236, 171/193, 195/206, and 286/298. This is, for the first time, the det ermination of disulfide bonds within a variant surface glycoprotein belongi ng to the B-type, As all the eight cysteines of BENat 1.3 variant surface g lycoprotein are positionally conserved, the cystine pattern of this protein can be regarded as a prototype of disulfide bonding within B-type variant surface glycoproteins. Although the cysteine residues of B-type variant sur face glycoproteins are located at completely different positions in the pro tein chain compared with A-type variant surface glycoproteins, the position s of the disulfide bonds can easily be integrated into the A-type tertiary structure. This result implies that, despite their enormous amino acid sequ ence variability, variant surface glycoproteins, regardless of their subtyp e, can fold into a similar tertiary structure.