Abstract
In the present paper we explored the capacity of yeast Saccharomyces cerevisiae as host for heterologous expression of human Aquaporin-1. Aquaporin-1 cDNA was expressed from a galactose inducible promoter situated on a plasmid with an adjustable copy number. Human Aquaporin-1 was C-terminally tagged with yeast enhanced GFP for quantification of functional expression, determination of sub-cellular localization, estimation of in vivo folding efficiency and establishment of a purification protocol. Aquaporin-1 was found to constitute 8.5 percent of total membrane protein content after expression at 15°C in a yeast host over-producing the Gal4p transcriptional activator and growth in amino acid supplemented minimal medium. In-gel fluorescence combined with western blotting showed that low accumulation of correctly folded recombinant Aquaporin-1 at 30°C was due to in vivo mal-folding. Reduction of the expression temperature to 15°C almost completely prevented Aquaporin-1 mal-folding. Bioimaging of live yeast cells revealed that recombinant Aquaporin-1 accumulated in the yeast plasma membrane. A detergent screen for solubilization revealed that CYMAL-5 was superior in solubilizing recombinant Aquaporin-1 and generated a monodisperse protein preparation. A single Ni-affinity chromatography step was used to obtain almost pure Aquaporin-1. Recombinant Aquaporin-1 produced in S. cerevisiae was not N-glycosylated in contrast to the protein found in human erythrocytes.
Highlights
The nucleotide sequences of the rapidly increasing number of fully sequenced genomes have revealed that approximately one third of all genes in any kingdom encode integral membrane proteins [1]
The expression studies were performed in presence of all amino acids except leucine and isoleucine as we previously described these conditions to be beneficial for recombinant membrane protein accumulation [34]
The present paper describes an expression system and expression conditions that produce a very high membrane density of heterologously expressed human Aquaporin 1
Summary
The nucleotide sequences of the rapidly increasing number of fully sequenced genomes have revealed that approximately one third of all genes in any kingdom encode integral membrane proteins [1]. For essentially all membrane proteins their density in natural tissue is so low that purification on the milligram scale required for crystallization attempts is excluded. Access to purified membrane proteins is further complicated by the observation that the expression systems that successfully delivered more than 90% of proteins used for solving the structures for water soluble proteins, have failed in producing the required densities of recombinant membrane proteins. The reason for this is probably a general failure of cells to cope with high level expression of recombinant membrane embedded proteins [9,10]. Very few examples are found in literature on heterologous expression of eukaryotic membrane proteins to a level where large-scale purification is straight forward or even possible [11,12]
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