Savicalin, a lipocalin from hemocytes of the soft tick, Ornithodoros savignyi

Abstract

Savicalin, is a lipocalin found in the hemocytes of the soft tick, Ornithodoros savignyi. It could be assigned to the tick lipocalin family based on BLAST analysis. Savicalin is the first non-salivary gland lipocalin described in ticks. The mature sequence is composed of 188 amino acids with a molecular mass of 21481.9 Da. A homolog for savicalin was found in a whole body EST-library from a related soft tick O. porcinus, while other tick salivary gland derived lipocalins retrieved from the non-redundant sequence database are more distantly related. Homology modeling supports the inclusion of savicalin into the lipocalin family. The model as well as multiple alignments suggests the presence of five disulphide bonds. Two conserved disulphide bonds are found in hard and soft tick lipocalins. A third disulphide bond is shared with the TSGP4-clade of leukotriene C4 binding soft tick lipocalins and a fourth is shared with a lipocalin from the hard tick Ixodes scapularis. The fifth disulphide bond is unique and links strands D-E. Phylogenetic analysis showed that savicalin is a distant relative of salivary gland derived lipocalins, but groups within a clade that is possibly non-salivary gland derived. It lacks the biogenic amine-binding motif associated with tick histamine and serotonin binding proteins. Expression profiles indicate that savicalin is found in hemocytes, midgut and ovaries, but not in the salivary glands. Up-regulation occurs in hemocytes after bacterial challenge and in midguts and ovaries after feeding. Given its tissue distribution and up-regulation of expression, it is possible that this lipocalin functions in tick development after feeding or in an anti-microbial capacity.