Loa loa: structural diversity of a 15-kDa repetitive antigen

Abstract

A ladder antigen of Loa loa was identified on Western blots of all life cycle stages probed with loaisis sera. The smallest subunit has a relative M(r) of about 15 kDa and larger subunits represent size increments of 15.0 kDa. An 1800-bp genomic clone encoding this antigen was characterized further by restriction mapping. Southern blot analysis, and nucleotide sequencing. The antigen is encoded by multiple copies of a gene, linked in tandem repeats of 396 bp, each of which encodes 132 amino acids. These repeats have diverged sufficiently to produce distinct restriction enzyme sites and Southern blot hybridization patterns. The 1764-bp insert contains no introns and encodes 588 amino acids, representing one incomplete and four complete repeats. At the 3' end of three repeats, there are consensus proteolytic cleavage sites; one repeat has no cleavage site. Two perfect repeats show a 93.9% amino acid identity with one another; the rest of the repeats, despite being adjacent to one another, show only 31-42% identical amino acids. Putative asparagine N-linked glycosylation sites are expressed by only two of the repeats. Despite this structural diversity, each L. loa repeat showed homology to Ascaris suum allergen and the homologue protein described in Dirofilaria immitis and Brugia pahangi.