Phosphate transport in membrane vesicles of Paracoccus denitrificans

Abstract

The mitochondrial Pi carrier mediates the electroneutral, reversible exchange of Pi for OHacross the inner mitochondrial membrane [ 1,2] . Thus Pi uptake into the mitochondrion occurs primarily in response to the pH gradient (alkaline inside) imposed across the mitochondrial membrane [3] ; this is effectively the proton symport of Mitchell [4]. In bacteria the mechanism of Pi uptake is less well understood. In Staphylococcus aureus and escherichia coli [4] and in Streptococcus faecalis [5] it has been proposed that Pi uptake occurs by an electroneutral exchange of Pi for OHacross the bacterial plasma membrane, but in none of the bacteria so far investigated has Pi uptake in response to a pH gradient been demonstrated. The plasma membrane of Paracoccus denitrificans, previously Micrococcus denitrificans [6] , is already known to possess many features of the mitochondrial inner membrane [7], and this prompted us to determine whether P. denitrificans also possesses the same type of Pi carrier as the mitochondrion. In the present paper we show that Pi uptake into membrane vesicles prepared from the plasma membrane of P. denitrificans can be driven by a pH gradient (alkaline inside) applied across the vesicle membrane. We also show that the Pi carrier present in these vesicles (again like the mitochondrial Pi carrier) is reversible and sensitive to SH-group reagents.