Affiliation:
1. Program in Vector-Borne Diseases, Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington 99164-7040
2. Animal Disease Research Unit, USDA Agricultural Research Service, Pullman, Washington 99164-6630
Abstract
ABSTRACT
Members of the variable merozoite surface antigen (
vmsa
) gene family of
Babesia bovis
encode membrane proteins involved in erythrocyte invasion. In this study, we have identified and sequenced the complete 8.3-kb genomic locus containing
msa-2
, a member of the
vmsa
family, in the biologically cloned Mexico Mo7 strain. Four tandemly arranged copies of
msa-2
-related genes were found in the locus. The four genes, designated
msa-2a
1
(which corresponds to the originally described
msa-2
gene),
msa-2a
2
,
msa-2b
, and
msa-2c
, were shown to be transcribed and expressed and encode proteins with open reading frames ranging in size from 266 (MSA-2c) to 317 (MSA-2a
1
) amino acids. MSA-2a
1
and -2a
2
are the most closely related of the four proteins (90% identity), differing by (i) the number of 24-amino-acid repeats that comprise a surface-exposed B-cell epitope and (ii) the presence of a 32-amino-acid area of recombination between MSA-2a
2
and -2b. In contrast,
msa-2c
is most closely related to the previously described
babr 0.8
gene in Australia strains of
B. bovis
. Comparison of MSA-2 proteins in the Argentina R1A strain of
B. bovis
with the Mexico Mo7 clone revealed a relatively high degree of conservation (83.6, 69.4, 79.1, and 88.7% amino acid identity for MSA-2a
1
, -2a
2
, -2b, and -2c, respectively), in contrast to the extensive MSA-1 sequence variation (52% identity) between the same two strains. Postinfection bovine immune serum contains antibodies that bound to each of the recombinant MSA-2 proteins. Blocking assays demonstrated the presence of unique B-cell epitopes in MSA-2a
1
, -2b, and -2c. The results support the evolution of the
msa
-
2
locus through at least two gene duplications, with selection for multiple related but antigenically distinct merozoite surface proteins.
Publisher
American Society for Microbiology
Subject
Infectious Diseases,Immunology,Microbiology,Parasitology
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