Genetic, structural, and functional studies of a C3-like protein in the marsupial Setonix brachyurus.

A monospecific goat antiserum was prepared against a putative C3 protein (QuC3) from serum of the Western Australian macropod Setonix brachyurus (quokka) using the classical method previously used to produce antiserum against C3 from other mammalian species. Sodium dodecyl sulphate-polyacrylamide gel electrophoretic analysis of reduced immunoprecipitated QuC3 revealed two polypeptide chains with an estimated M(r) of 128,000 and 82,000, respectively--presumably reflecting subunit molecules similar to the C3 alpha and beta subunits found in other mammalian species. No variation in the size of either of the QuC3 alpha or beta subunits was found in different quokka serum samples. Furthermore, no variation in the electrophoretic mobility of QuC3 was detected amongst the same samples using a standard immunofixation electrophoretic technique. The addition of zymosan or immune complexes to fresh quokka serum resulted in activation of QuC3. Immune complex mediated activation of QuC3 was inhibited by the addition of EDTA and Mg(2+)-EGTA. By contrast, EDTA only inhibited activation of QuC3 following incubation of fresh quokka serum with zymosan, suggesting that the quokka has a classical and an alternative C3 activation pathway, similar to those found in eutherian mammals. This study has shown for the first time that one of the macropod marsupials (S. brachyurus) has a C3-like protein with similar structural and functional characteristics to C3 found in eutherian mammals. However, unlike C3 in other mammalian species, genetic variability of the structure of the quokka C3-like protein may be restricted.