Characterization of a Giardia lamblia variant-specific surface protein (VSP) gene from isolate GS/M and estimation of the VSP gene repertoire size.

Giardia lamblia undergoes surface antigenic variation. The variant-specific surface proteins (VSPs) of isolate WB are cysteine-rich, can vary dramatically in size, contain Cys-X-X-Cys motifs, and are differentially expressed. GS/M(H7) is a Giardia clone from a different isolate which expresses a VSP epitope not found in WB. The VSP gene encoding this epitope was selected by differential hybridization using radiolabeled cDNA from H7 and variant sibling trophozoite lines from the same isolate that express other VSPs. The VSPH7 gene probe detects an 1800 nucleotide transcript abundant in H7 but undetectable in variant siblings. Primer extension directly from RNA was used to complete the gene sequence which predicted a protein with a molecular weight of 56,832. The protein showed many of the characteristics of 2 previously sequenced WB VSPs including many Cys-X-X-Cys tetrapeptides and a conserved carboxy-terminal region. Genomic Southern analysis indicated the presence of 2 distinct VSPH7 genes in H7. An oligonucleotide from the conserved region was used in combination with one specific for the VSPH7 gene to estimate the VSP repertoire size at between 133 and 151. VSPs, even from isolates expressing unique epitopes, constitute a family of related proteins.