Molecular and functional characterization of vacuolar-ATPase from the American dog tick Dermacentor variabilis

Vacuolar (V)‐ATPase is a proton‐translocating enzyme that acidifies cellular compartments for various functions such as receptor‐mediated endocytosis, intracellular trafficking and protein degradation. Previous studies in Dermacentor variabilis chronically infected with Rickettsia montanensis have identified V‐ATPase as one of the tick‐derived molecules transcribed in response to rickettsial infection. To examine the role of the tick V‐ATPase in tick–Rickettsia interactions, a full‐length 2887‐bp cDNA (2532‐bp open reading frame) clone corresponding to the transcript of the V0 domain subunit a of D. variabilis V‐ATPase (DvVATPaseV0a) gene encoding an 843 amino acid protein with an estimated molecular weight of ∼96 kDa was isolated from D. variabilis. Amino acid sequence analysis of DvVATPaseV0a showed the highest similarity to VATPaseV0a from Ixodes scapularis. A potential N‐glycosylation site and eight putative transmembrane segments were identified in the sequence. Western blot analysis of tick tissues probed with polyclonal antibody raised against recombinant DvVATPaseV0a revealed the expression of V‐ATPase in the tick ovary. Transcriptional profiles of DvVATPaseV0a demonstrated a greater mRNA expression in the tick ovary, compared with the midgut and salivary glands; however, the mRNA level in each of these tick tissues remained unchanged after infection with R. montanensis for 1 h. V‐ATPase inhibition bioassays resulted in a significant decrease in the ability of R. montanensis to invade tick cells in vitro, suggesting a role of V‐ATPase in rickettsial infection of tick cells. Characterization of tick‐derived molecules involved in rickettsial infection is essential for a thorough understanding of rickettsial transmission within tick populations and the ecology of tick‐borne rickettsial diseases.