Purification of a recombinant membrane protein tagged with a calmodulin-binding domain: properties of chimeras of the Escherichia coli nicotinamide nucleotide transhydrogenase and the C-terminus of human plasma membrane Ca 2+-ATPase

Abstract

A Ca 2+-dependent calmodulin-binding peptide (CBP) is an attractive tag for affinity purification of recombinant proteins, especially membrane proteins, since elution is simply accomplished by removing/chelating Ca 2+. To develop a single-step calmodulin/CBP-dependent purification procedure for Escherichia coli nicotinamide nucleotide transhydrogenase, a 49 amino acid large CBP or a larger 149 amino acid C-terminal fragment of human plasma membrane Ca 2+-ATPase (hPMCA) was fused C-terminally to the β subunit of transhydrogenase. Fusion using the 49 amino acid fragment resulted in a dramatic loss of transhydrogenase expression while fusion with the 149 aminio acid fragment gave a satisfactory expression. This chimeric protein was purified by affinity chromatography on calmodulin–Sepharose with mild elution with EDTA. The purity and activity were comparable to those obtained with His-tagged transhydrogenase and showed an increased stability. CBP-tagged transhydrogenase contained a 4- to 10-fold higher amount of the α subunit relative to the β subunit as compared to wild-type transhydrogenase. To determine whether the latter was due to the CBP tag, a double-tagged transhydrogenase with both an N-terminal 6× His-tag and a CBP-tag, purified by using either tag, gave no significant increase in purity as compared to the single-tagged protein. The reasons for the altered subunit composition are discussed. The results suggest that, depending on the construct, the CBP-tag may be a suitable affinity purification tag for membrane proteins in general.