Trypanosoma congolense: Surface glycoproteins of two early bloodstream variants : II. Purification and partial chemical characterization

Two sequential variant-specific glycoproteins have been purified from two variants of Trypanosoma congolense expressed during a relapsing infection. Isolation of the two glycoproteins, termed VSG-1 and VSG-2, respectively, employed glycerol lysis followed by purification on concanavalin A, Sephadex G-25, and gradient-eluted DE-52 columns. Partially purified VSG proteins were immunologically cross-reactive, but highly purified VSGs showed no cross-reactivity under the conditions employed. Both VSG-1 and VSG-2 consisted of a triplet of polypeptides. Although each member of a triplet subset could be distinguished by isoelectric focusing, all three gave identical N-terminal amino acid sequences and nearly identical tryptic peptide maps. The members of the VSG-1 polypeptide subset differed from those of the VSG-2 subset both with regard to N-terminal amino acid sequence and in tryptic peptide map patterns. Comparison of N-terminal sequences of VSG-1 and VSG-2 did, however, show that the sequences could be aligned to give a modest degree of amino acid homology (27%). This alignment also produced a minimum in the number of two-base changes, suggesting that the observed homology is not a coincidence and that these two proteins may well have arisen by gene duplication followed by retention of multiple point mutations.