Variant-specific surface proteins of Giardia lamblia are zinc-binding proteins.
Giardia lamblia undergoes surface antigenic variation. The variant-specific surface proteins (VSPs) are a distinct family of cysteine-rich proteins. Characteristically, cysteine residues occur mostly as CXXC tetrapeptides. Four of the reported five VSPs contain a putative metal-binding domain that resembles other metal-binding motifs; the fifth is closely related but lacks an essential histidine. Three different native VSPs bound Zn2+. Co2+, Cu2+, and Cd2+ inhibited Zn2+ binding. Analysis of recombinant VSP fusion proteins showed that the putative binding motif bound Zn2+. Surprisingly, peptide fragments from other regions of the VSP contain numerous CXXCXnCXXC motifs that also bound Zn2+. Analysis of deduced amino acid sequences showed well-conserved CXXC spacing in three out of five VSPs, suggesting conservation of structure despite amino acid sequence divergence. The function of VSPs is unknown, but by binding Zn2+ or other metals in the intestine, VSPs may contribute to Zn2+ malnutrition or inhibition of metal-dependent intestinal enzymes, which would lead to malabsorption, a well-known consequence of giardiasis.
Nash
T E
TE
Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892.
Mowatt
M R
MR
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002352:Carrier Proteins / Q000378:metabolism*; D002413:Cations, Divalent / Q000378:metabolism; D016829:Giardia lamblia / Q000378:metabolism*; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000378:metabolism; D015800:Protozoan Proteins / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D015032:Zinc / Q000378:metabolism*
D002352:Carrier Proteins; D002413:Cations, Divalent; D010446:Peptide Fragments; D015800:Protozoan Proteins; D011993:Recombinant Fusion Proteins; D015032:Zinc