Stage-specific isoforms of Ascaris suum complex II: the fumarate reductaseof the parasitic adult and the succinate dehydrogenase of free-living larvae share a common iron-sulfur subunit

Complex II of adult Ascaris suum muscle exhibits high fumarate reductase (F RD) activity and plays a key role in anaerobic electron-transport during ad aptation to their microaerobic habitat. In contrast larval (L2) complex II shows a much lower FRD activity than the adult enzyme, and functions as suc cinate dehydrogenase (SDH) in aerobic respiration. We have reported the sta ge-specific isoforms of complex II in A. suum mitochondria, and showed that at least the flavoprotein subunit (Fp) and the small subunit of cytochrome b (cybS) of the larval complex II differ from those of adult. In the prese nt study, complete cDNAs for the iron-sulfur subunit (Ip) of complex II, wh ich with Fp forms the catalytic portion of complex IT, have been cloned and sequenced from anaerobic adult ii. suum, and the free-living nematode, Cae norhabditis elegans. The amino acid sequences of the Ip subunits of these t wo nematodes are similar, particularly around the three cysteine-lich regio ns that are thought to comprise the iron-sulfur clusters of the enzyme. The Ip from A. suum larvae was also characterized because Northern hybridizati on showed that the adult Ip is also expressed in L2. The Ip of larval compl ex II was recognized by the antibody against adult Ip. and was indistinguis hable from the adult Ip by peptide mapping. The N-terminal 42 amino acid se quence of Ip in the larval complex II purified by DEAE-cellulofine column c hromatography was identical to that of the mature form of the adult Ip. Fur thermore. the amino acid composition of larval Ip determined by micro-analy sis on a PVDF membrane is almost the same as that of adult Ip. These result s, together with the fact, that homology probing by RT-PCR, using degenerat ed primers. failed to find a larval-specific Tp, suggest that the two diffe rent stage-specific forms of the A. suum complex II share a common Ip subun it, even though the adult enzyme functions as a FRD, while larval enzyme ac ts as an SDH. (C) 2000 Elsevier Science B.V. All rights reserved.