Functional characterization of tick glutaminyl cyclase

Ticks are among the most important disease vectors, second only to mosquitoes in terms of impact. They transmit a wide variety of pathogens including viruses, bacteria, protozoan, and helminthes. Our laboratory has focused on identifying sialome protein targets, since many disease agents are present in tick saliva and tick feeding is required in order to complete their lifecycle. During the course of a massive EST sequencing project, we identified a gene with significant sequence homology to the enzyme glutaminyl cyclase. This enzyme catalyzes the cyclization of N-terminal glutamine residues into pyroglutamate, which enhances protein stability and is necessary for several bioactive hormones to bind to their cognate receptors. We have shown that glutaminyl cyclase from Amblyomma maculatum (AmQC) and Ixodes scapularis (IsQC) have considerable sequence homology to type II QC’s from animals and other arthropods. The enzymatic activity was determined in saliva collected from adult female Amblyomma maculatum ticks. Recombinant AmQC and IsQC were cloned and expressed into a Drosophila expression system to test for glutaminyl cyclase enzymatic activity. In this study, we also examined the physiological effect of silencing the glutaminyl cyclase by RNAi. Many of these ticks failed to feed to repletion, which supports our hypothesis that glutaminyl cyclase is important for tick feeding success.