Each candidate site was initially tested PFT�� purchase whether it was permissive for

UAG suppression by the orthogonal tRNACUALeu/leucyl-tRNA synthetase (LeuRS) pair, which incorporates the natural amino acid leucine (Leu). Each Kir2.1  TAG gene was transfected into HEK293T cells along with the tRNACUALeu/LeuRS   ( Figure 2B). The gene for green fluorescent protein (GFP) engineered with an amber stop codon at Tyr182 (GFP_Y182  TAG) was cotransfected ( Wang et al., 2007). GFP fluorescence would indicate the successful suppression of the UAG stop codon by the orthogonal tRNA/synthetase. The function of individual Kir2.1TAG channels was then determined by whole-cell patch-clamp recordings from GFP-positive cells. For example, a green-positive HEK293T cell transfected with Kir2.1_C169  TAG and the tRNACUALeu/LeuRS produced a basally active inwardly rectifying current that was inhibited by extracellular Ba2+ (IKir), similar to wild-type Kir2.1 channels ( Figure 2C). Of the eight candidate sites, IKir currents measured at −100 mV from HEK293T cells expressing Kir2.1_I143TAG, Kir2.1_C149TAG, Kir2.1_C169TAG, or Kir2.1_I176TAG were significantly larger than those from untransfected cells ( Figure 2D), indicating successful suppression and incorporation of Leu. If a functional Kir2.1 channel could be generated through Leu incorporation at the TAG site, then it seemed likely that the

same site would be compatible for the larger Uaa Cmn. We therefore tested Kir2.1_I143  TAG, Kir2.1_C149  TAG, Kir2.1_C169  TAG, and Kir2.1_I176  TAG for functional incorporation of Cmn ( Figures 2E–2H; Figure S1C). HEK293T cells were transfected with cDNAs for the Kir2.1TAG channel, tRNACUALeu/CmnRS GSK 3 inhibitor and the GFP_Y182TAG reporter ( Figure 2B), and incubated in Cmn (1 mM) aminophylline for 12–24 hr. Functional incorporation of Cmn was expected to lead to either a basally active IKir or an IKir that is revealed upon brief (1 s) light illumination (385 nm at 40 mW/cm2). For HEK293T cells expressing Kir2.1_I143TAG or Kir2.1_I176TAG,

we could detect no IKir before or after light illumination, indicating either no amber suppression or a nonfunctional channel after Cmn incorporation ( Figure 2E; Figure S1C). By contrast, HEK293T cells expressing Kir2.1_C149TAG displayed a large IKir that was unchanged by light illumination ( Figure 2F), suggesting that incorporation of Cmn at C149 did not significantly occlude the pore. It is striking that HEK293T cells expressing Kir2.1_C169TAG displayed little IKir at negative membrane potentials that increased significantly upon light illumination ( Figures 2G and 2H). These results suggested that incorporation of Cmn at C169 largely occludes the channel pore and that the blocking particle is released following brief light stimulation, indicating the successful construction of a photoactivatable Kir2.1 channel. We next examined the light sensitivity features of Kir2.1_C169TAGCmn (referred to as PIRK) channels expressed in HEK293T cells.