9.A.65 The Membrane Protein Insertion Fusion Partner, P9 (MPI-P9) Family 

Studies of membrane proteins have been hampered by the difficulties associated with obtaining sufficient amounts of protein. Jung et al. 2015 reported a membrane protein expression system that uses the major envelope protein (P9) of phage phi6 as an N-terminal fusion partner. Pseudomonas phage membrane protein P9 facilitated the integration of human proteins into the Escherichia coli cell membrane. This system was used to produce various multi-pass transmembrane proteins, including G-protein-coupled receptors, transporters, and ion channels of human origin. Green fluorescent protein fusions were used to confirm the correct folding of the expressed proteins. Of the 14 membrane proteins tested, eight were highly expressed, three were moderately expressed, and three were poorly expressed in E. coli. Seven of the eight highly expressed proteins could be purified after extraction with the mild detergent lauryldimethylamine-oxide. Although a few proteins have previously been developed as fusion partners to augment membrane protein production, the major envelope protein, P9, may be better suited to the efficient expression of eukaryotic transmembrane proteins in E. coli (Jung et al. 2015).