Expression ofAnaplasma marginaleMajor Surface Protein 2 Operon-Associated Proteins during Mammalian and Arthropod Infection

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Abstract
The antigenically variant major surface protein 2 (MSP2) ofAnaplasma marginaleis expressed from a 3.5-kb operon that contains, in a 5′-to-3′ direction, four open reading frames,opag3,opag2,opag1, andmsp2. This operon structure was shown to be conserved among genotypically and phenotypically distinctA. marginale,A. ovis, andA. centralestrains.The individual OpAG amino acid sequences are highly conserved amongA. marginalestrains, with identities ranging from 95 to 99%. OpAG2 and OpAG3 were expressed by all examinedA. marginalestrains during the acute rickettsemia in the mammalian host and, like MSP2, localize to the bacterial surface. OpAG2 and OpAG3 were also expressed in an infectedIxodes scapularistick cell line. In contrast, the sameA. marginalestrains expressed only OpAG2 in two differentDermacentorspp. during transmission feeding. OpAG1 expression was not detected in the infected mammalian host, the infected tick cell line, or within infectedDermacentorticks. The differential expression of outer membrane proteins from within an operon is a novel finding in tick-transmitted bacteria, and the regulation of expression may be broadly applicable to understanding how the pathogen adapts to the mammalian host-tick vector transition.