Targeting the expression of functional murine CMP-sialic acid transporter to the E. coli inner membrane

The heterologous expression of functional mammalian integral membrane proteins still represents a significant hurdle towards their crystallization and structure elucidation. We have therefore explored the use of the OmpA signal sequence to deliberately target the expression of the murine CMP-sialic acid transporter, a Golgi-resident protein with 10 putative transmembrane domains, to the Escherichia coli inner membrane. Here, we show that the expression of an OmpA signal sequence-FLAG-CMP-sialic acid transporter fusion protein in E. coli results in the targeting and insertion of recombinant protein within the inner membrane. Significantly, functionality was confirmed by the ability of spheroplasted E. coli and mixed phosphatidylcholine–E. coli inner membrane proteoliposomes incorporating recombinant CMP-sialic acid transporter to accumulate CMP-sialic acid in vitro.