2.A.13 The C4-Dicarboxylate Uptake (Dcu) Family
Several proteins of the Dcu family have been sequenced as of 1/1998, but all are from Gram-negative bacteria. Two are from E. coli, one is from Haemophilus influenzae, one is from Serratia marscesens and one (a large, N-terminally truncated fragment) is from Wolenella succinogens. The fully sequenced proteins are of fairly uniform size (434-446 residues). They possess 12 putative transmembrane α-helical spanners, but DcuA has 10 experimentally determined TMSs with both the N- and C-termini localized to the periplasm. For DcuA, the 'positive inside' rule is obeyed, and two putative TMSs are localized to a cytoplasmic loop between TMSs 5 and 6 and in the C-terminal periplasmic region.
The two E. coli proteins, DcuA and DcuB, transport aspartate, malate, fumarate and succinate and function as antiporters with any two of these substrates. They exhibit 36% identity with 63% similarity, and both transport fumarate in exchange for succinate with the same affinity (30 μM). Since DcuA is encoded in an operon with the gene for aspartase, and DcuB is encoded in an operon with the gene for fumarase, their physiological functions may be to catalyze aspartate:fumarate and fumarate:malate exchange during the anaerobic utilization of aspartate and fumarate, respectively. However, the electroneutral antiport of fumarate for succinate during anaerobic fumarate respiration has been demonstrated, and both permeases are induced under anaerobic conditions and are subject to catabolite repression. The two transporters can apparently substitute for each other under certain physiological conditions (Engel et al., 1994; Six et al., 1994; Unden and Bongaerts, 1997).
The generalized transport reaction catalyzed by the proteins of the Dcu family is:
Dicarboxylate1 (out) + Dicarboxylate2 (in) ⇌ Dicarboxylate1 (in) + Dicarboxylate2 (out).