Haloarchaeal proteins are adapted to these conditions: they contain an excess of acidic amino acids, especially on the surface of the protein, and the frequency of the basic amino acid lysine is reduced [52, 53]. While maintaining solubility and stability under high-salt conditions, the adapted proteins tend to lose their physiological interactions and even denature in solutions of low ionic strength (see  and references therein). At the beginning of this study we were not aware of any method that had been successfully applied to analyze the interactions
between halophilic proteins on a medium or large scale. Screening a test set learn more of expected interactors from Hbt.salinarum using the yeast two-hybrid system failed for all selleck chemical tested haloarchaeal proteins (data not shown). The reason turned out to be autoactivation by the (acidic) Hbt.salinarum proteins being used as bait and probably also misfolding of the halophilic proteins when expressed in yeast. To circumvent these issues, we established two affinity purification methods for haloarchaeal protein complexes with subsequent identification of the complex components
by mass spectrometry (affinity purification mass spectrometry, AP-MS). As demonstrated earlier, the cellulose-binding domain (CBD) from the CipB protein from Clostridium thermocellum can be used as an affinity tag to purify halophilic proteins under high salt conditions [55–57]. We expressed the proteins under investigation—which were then called bait proteins—fused to this salt-insensitive affinity tag in their native JAK inhibitor review host Hbt.salinarum to ensure correct folding of the halophilic proteins (Additional file 1). We put the bait proteins under control of a relatively strong promoter resulting in bait overproduction. This was necessary to overcome sensitivity problems but came at the cost of losing the cellular stoichiometry between the next bait protein and its interaction partners. In our first method, termed one-step bait fishing (Figure 1A), Hbt.salinarum cells expressing the bait-CBD fusion protein were lysed and the cell lysate was applied to
a cellulose column. This enabled binding of the bait protein along with its endogenous protein interaction partners (the prey proteins) to the column. After careful washing to remove unbound proteins, the bait-prey complexes were eluted from the column and proteins identified by mass spectrometry. Figure 1 Schematic of purification procedures. A One-Step bait fishing. A Hbt.salinarum strain overexpressing the bait protein fused to CBD is cultured in synthetic medium containing 13C6-leucine. The corresponding bait-control strain overexpressing the bait protein without CBD is cultured in synthetic medium containing 12C6-leucine. The lysate from both strains is mixed and purification done on one cellulose column. B Two-Step bait fishing.