Molecular characterization of V-type H+-ATPase (B-subunit) in gills of euryhaline crabs and its physiological role in osmoregulatory ion uptake

The vacuolar-type Ht-ATPase (V-ATPase) has been implicated in osmoregulator y ion uptake across external epithelia of a growing variety of species adap ted to life in fresh water. In the present study, we investigated whether t he V-ATPase may also function in a euryhaline species that tolerates bracki sh water (8 parts per thousand salinity) but not fresh water, the shore cra b Carcinus maenas, cDNA coding for the regulatory B-subunit of the V-ATPase was amplified and sequenced from C. maenas gills and partially sequenced f rom four other crab species, Two isoforms differing in the 3'-untranslated region were found in C. maenas, In this species, the abundance of B-subunit mRNA was greater in the respiratory anterior gills than the ion-transporti ng posterior gills and was not increased by acclimation to dilute salinity. Immunocytochemical analysis showed that the B-subunit protein is not targe ted to the apical membrane but is distributed throughout the cytoplasmic co mpartment. Physiological studies of isolated perfused gills indicated that the V-ATPase inhibitor bafilomycin had no effect on transepithelial potenti al difference, Thus, in contrast to the freshwater-tolerant Chinese crab Er iocheir sinensis, in which the V-ATPase appears to play an important osmore gulatory role, the V-ATPase in C. maenas probably functions in acidificatio n of intracellular organelles but not in transbranchial NaCl uptake.