The Leishmania promastigote surface antigen 2 complex is differentially expressed during the parasite life cycle

The promastigote surface antigen 2 (PSA-2) complex comprises a family of antigenically similar polypeptides of M(r) 96 000, 80 000 and 50 000, anchored to the membrane with glycosylphosphatidylinositol. Although PSA-2 was initially detected only in promastigotes, Northern blot analysis indicated that mRNA transcripts are also present in amastigotes. Unlike the situation in promastigotes, where at least four major transcripts (2.6-5.3 kb) were detected, only one major (2.6 kb) and two minor transcripts were present in amastigotes. A cDNA clone encoding a member of the PSA-2 family expressed in amastigotes was isolated using DNA probes. The predicted protein sequence of M(r) 40 000 is distinct from promastigote sequences, but shows significant similarity to previously described members of the family from L. major and L. amazonensis. Antibodies to the carboxyl terminal sequence conserved in all L. major PSA-2 studied to date, as well as antibodies affinity purified on the amastigote cDNA-derived polypeptide recognized a major M(r) 50 000 amastigote polypeptide. Immune-electron microscopy localized both promastigote and amastigote PSA-2 to the cell surface. The expression of PSA-2 polypeptides during the transformation of amastigotes into promastigotes was ordered in a time-dependent manner, with the promastigote M(r) 80 000 polypeptide appearing first, followed by the M(r) 96 000 polypeptide. In contrast to the glycosylphosphatidylinositol anchor of promastigote PSA-2, which could be hydrolysed by phosphatidylinositol-specific phospholipase C, the amastigote form was resistant to this enzyme.