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 prote in (QuC3) from serum of the Western Australian macropod Setonix brachy urus (quokka) using the classical method previously used to produce an tiserum against C3 from other mammalian species. Sodium dodecyl sulpha te-polyacrylamide gel electrophoretic analysis of reduced immunoprecip itated QuC3 revealed two polypeptide chains with an estimated M(r) of 128,000 and 82,000, respectively - presumably reflecting subunit molec ules similar to the C3 alpha and beta subunits found in other mammalia n species. No variation in the size of either of the QuC3 alpha or bet a subunits was found in different quokka serum samples. Furthermore, n o variation in the electrophoretic mobility of QuC3 was detected among st the same samples using a standard immunofixation electrophoretic te chnique. The addition of zymosan or immune complexes to fresh quokka s erum resulted in activation of QuC3. Immune complex mediated activatio n of QuC3 was inhibited by the addition of EDTA and Mg2+-EGTA. By cont rast, EDTA only inhibited activation of QuC3 following incubation of f resh quokka serum with zymosan, suggesting that the quokka has a class ical 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 euth erian mammals. However, unlike C3 in other mammalian species, genetic variability of the structure of the quokka C3-like protein may be rest ricted.