A histidine-rich protein from the vitellaria of the liver fluke Fasciola hepatica

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Abstract
The vitellaria are an extensive network of glandular cells and ducts distributed throughout the peripheral tissues of the liver fluke Fasciola hepatica. Eggshell precursor proteins are produced and stockpiled in the vitelline cells of mature flukes. Vitelline protein C has an extraordinary composition: the amino acid 3,4-dihydroxyphenyl-L-alanine (DOPA) and histidine each comprise about 20% of the residues, while glycine represented 41-42% in all variants of what appears to be a microheterogeneous protein family. Protein C has an apparent molecular weight of 16 000-17 000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Although the protein appears homogeneous following polyacrylamide gel electrophoresis in Tris-glycine with SDS and in acetic acid-urea, electrophoresis in borate, however, suggests that the vitelline protein consists of four or more closely related proteins weighing from 16 000 to 18 500. Isoelectric focusing of the protein family in the presence of 8 M urea resolves only two species having pI values of 6.89 and 6.99. A single N-terminus having the sequence H-H-W-D-G-DOPA-G-DOPA-G was detected. The primary structure of vitelline protein C is characterized by a repeated motif consisting of (G-X)n, where X is Ser, DOPA, or His. Most of the His occurs as G-H repeats in a pepsin-resistant fragment of the protein. Previously, a 31-kDa protein, representing up to 6% of the total protein in the fluke, was reported [Waite, J. H., and Rice-Ficht, A. (1987) Biochemistry 26, 7819-7825] to contain significant levels of DOPA. In other respects, however, it is distinct from vitelline protein C. Present studies suggest the existence of at least one other distinct DOPA-containing protein in the vitellaria.

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