Six of the Htrs were predicted to contain no transmembrane domain and are assumed to recognize intracellular signals. The other Htrs contain two or more transmembrane helices and recognize signals at the membrane or extracellularly. The function of only eight Htrs has been assigned to-date (Table 2). Table 2 The halobacterial transducers as preys Htr Gene Name Signal TM A Y W1 W2 R 1 OE3347F HtrI Orange light (A), Batimastat UV light (R) [35–37] 2 ∙ ∙ ∙ ∙   2 OE3481R HtrII Blue light (R), Ser (A) [38, 39] 2 ∙ ∙ ∙ ∙   3 OE3611R BasT Leu, Ile, Val, Met, Cys (A) [33] 2 ∙ ∙ ∙ ∙   4 OE2189R Htr4   2 ∙ ∙ ∙ ∙   5 OE3474R CosT Compatible

osmolytes (A) [34] 2 ∙ ∙ ∙ ∙   6 OE2168R Htr6   2 ∙ ∙ ∙ ∙   8 OE3167F HtrVIII O 2 (A) [40] 6 ∙ ∙ ∙ ∙   14 OE1536R MpcT ΔΨ (A) [41] 2 ∙ ∙ ∙     17 OE3436R Htr17   3 ∙ ∙       18 OE2195F Htr18   2 (∙) ∙       16 OE1929R Htr16   2 ∙         15 OE2392R Htr15   0   ∙ ∙ ∙   11 OE5243F Car Arg (A) [42] 0       ∙   13 OE2474R Htr13   0     ∙ ∙   12 OE3070R Htr12   0         ∙ 7 OE3473F Htr7   3           9 OE2996R Htr9   0           10 OE3150R HemAT O 2 (R) [43] 0           Transducers were grouped according to their interaction patterns.

Signal indicates attractant (A) or EPZ015666 order repellent (R) signal for the respective transducer where known. TM is the number of predicted transmembrane helices. The columns A, Y, W1, W2 and R indicate whether SBI-0206965 cell line the transducer was identified as interaction partner CheA, CheY, CheW1, CheW2 or CheR, respectively. () Htr18 was not identified with the bait CheA but its putatively associated protein OE2196F. While the confirmed processes in Hbt.salinarum taxis signaling have already led to modeling of motor switching and signal processing [44–47], the understanding on a molecular level is still far from complete. For example, it is still unknown why Hbt.salinarum possesses more than one homologue of CheW, CheC and CheF. The function of CheD and the CheC proteins, which build one of the three adaptation systems in B.subtilis[48], is unclear in Hbt.salinarum. The mechanism of action of the switch factor fumarate, which was discovered in Hbt.salinarum 20 years before ago [49, 50], is also unresolved. Because classical

approaches to define function, for example deletion mutant analysis, are not always conclusive, we set out to investigate the taxis signal transduction system of Hbt.salinarum by protein interaction analysis. In the course of this study, we identified and characterized the archaeal chemotaxis protein family CheF that connects the bacterial-like taxis signaling system to the archaeal flagellar apparatus [10]. Here we report the interaction network of the Hbt.salinarum taxis signaling proteins which presents new knowledge about established Che proteins and identifies connections to proteins that were not known to be linked to taxis signal transduction. Results and Discussion Protein-protein interaction analysis in Hbt.salinarum Like all halophilic archaea, Hbt.