Purification of two active fusion proteins of the Na+‐dependent citrate carrier of Klebsiella pneumoniae

Abstract
The sodium‐ion‐dependent citrate carrier of Klebsiella pneumoniae (CitS) was purified by means of bioengineerical methods. By fusing the biotin acceptor domain of the α‐subunit of the oxaloacetate decarboxylase of K pneumoniae to the C‐terminus of CitS, purification of the carrier was achieved by use of a monomeric avidin‐Sepharose column. Additionally, we were able to purify a CitS‐protein with an N‐terminal histidine‐tag by immobilized metal chelate affinity chromatography (with Ni2+‐nitrilotriacetic acid‐(NTA‐) resin). Both purified fusion proteins showed citrate transport activity after reconstitution into liposomes by the freeze/thaw/sonication procedure.