Plasmodium i ax merozoite surface proteins-3 and-3 share structural similarities with P. i ax merozoite surface protein-3 and define a new gene family

The genes encoding two merozoite surface proteins of Plasmodium i ax that are related to PvMSP3 [1] are reported. One of these genes was identified within P. i ax gt11 clone 5.4, which was selected by immunoscreening with a Saimiri monkey antiserum. The insert DNA of this clone was used as a probe to isolate the complete gene from a P. i ax DASH genomic (g) DNA library. Antibodies to recombinant 5.4 and subsequent fusion proteins produce a pattern of circumferential surface fluorescence by indirect immunofluorescence assays (IFA) on segmented schizonts and free intact merozoites, and recognize a 125 kDa protein via western immunoblots. The gene, however, encodes a protein with a calculated size of 75 677 Da, and 3 and 5 RACE analyses were employed to confirm the size of the gene and its coding region. The second related P. i ax gene was isolated by hybridization of a fragment of an orthologous P. knowlesi gene. The encoded proteins of all three related P. i ax genes have putative signal peptides, large central domains that contain 20% alanine residues bound by charged regions, are predicted to form -helices with heptad repeat coiled-coil structures, and do not have a hydrophobic region that could anchor them to the surface of the merozoite. Although the overall identity in amino acid alignment among the three encoded proteins is low ( 40%), the shared predicted structural features and motifs indicate that they are members of an intra-species family, which we are designating as the PvMSP-3 family with the reported members being Pvmsp-3 , Pvmsp-3 , and Pvmsp-3 . We further demonstrate that this family also includes related proteins from P. knowlesi and P. falciparum. © 2001 Elsevier Science B.V. All rights reserved.