The S-antigen of Plasmodium falciparum: repertoire and origin of diversity.

S-antigens are heat-stable, highly polymorphic proteins released by Plasmodium falciparum at the time of schizont rupture. Previously determined S-antigen sequences allowed the proposal of a general gene structure consisting of 5 sequence blocks. The sequence of the central block of tandem repeats provides a useful means of distinguishing the S-antigen allele and also its serotype, whereas the amino and carboxy terminal sequences defined the S-antigen family, 4 of which have been described. We present the sequence of 3 new S-antigen alleles, for the isolates HB3, KF1916 and KF1917. The allele-defining repeat sequence is ETGPGKAGEQG for HB3, GDQTEGS(S/A)GGK for KF1917 and AGSNE(E/K) for KF1916. The sequences of these newly described S-antigens are consistent with the proposed general gene structure and all belong to defined families, although carboxy-terminal sequences appear to be much more variable within a family than previously realised.