Myophilin is a muscle-associated antigen of the taeniid cestode
Echinococcus granulosus. This protein shows a high amino
acid sequence homology with calponins and calponin-like proteins, which
are
proposed to be associated with the regulation
of smooth muscle contraction. In order to provide supportive evidence for
a
relationship between these proteins, we
characterized myophilin using electrophoretic, biochemical and molecular
biological
approaches. Two-dimensional protein
electrophoretic separation of E. granulosus larval proteins defined
4 isoelectric isoforms of myophilin (α, β, γ and δ),
which
appeared to be a consequence of post-translational modification of a single
gene product. It was also demonstrated
biochemically that E. granulosus myophilin undergoes specific
phosphorylation in vitro by protein kinase C (PKC). Finally,
myophilin homologues were identified in extracts of Taenia hydatigena
and T. ovis by immunoblot. A partial cDNA of
the closely related species, E. multilocularis, was isolated by
cloning procedures and showed 99% homology with the E.
granulosus myophilin gene. The similarities of E. granulosus
myophilin with calponins in their tissue localization, protein
isoform patterns, ability to be phosphorylated in vitro by PKC,
and
the relatively conserved nature of the protein among
related parasites suggest that myophilin may be associated with smooth
muscle contraction.