We have determined the binding site on agitoxin2 (AgTx2) to the KcsA K(+) channel by a transferred cross-saturation (TCS) experiment. The residues significantly affected in the TCS experiments formed a contiguous surface on AgTx2, and substitutions of the surface residues decreased the binding affinity to the KcsA K(+) channel. Based on properties of the AgTx2 binding site with the KcsA K(+) channel, we present a surface motif that is observed in pore-blocking toxins affecting the K(+) channel. Furthermore, we also explain the structural basis of the specificity of the K(+) channel to the toxins. The TCS method utilized here is applicable not only for the channels, which are complexed with other inhibitors, but also with a variety of regulatory molecules, and provides important information about their interface in solution.

CC-chemokine receptor 5 (CCR5) belongs to the G protein-coupled receptor (GPCR) family and plays important roles in the inflammatory response. In addition, its ligands inhibit the HIV infection. Structural analyses of CCR5 have been hampered by its instability in the detergent-solubilized form. Here, CCR5 was reconstituted into high density lipoprotein (rHDL), which enabled CCR5 to maintain its functions for >24 h and to be suitable for structural analyses. By applying the methyl-directed transferred cross-saturation (TCS) method to the complex between rHDL-reconstituted CCR5 and its ligand MIP-1alpha, we demonstrated that valine 59 and valine 63 of MIP-1alpha are in close proximity to CCR5 in the complex. Furthermore, these results suggest that the protective influence on HIV-1 infection of a SNP of MIP-1alpha is due to its change of affinity for CCR5. This method will be useful for investigating the various and complex signaling mediated by GPCR, and will also provide structural information about the interactions of other GPCRs with lipids, ligands, G-proteins, and effector molecules.

Regnase-1 is an RNase that directly cleaves mRNAs of inflammatory genes such as IL-6 and IL-12p40, and negatively regulates cellular inflammatory responses. Here, we report the structures of four domains of Regnase-1 from Mus musculus—the N-terminal domain (NTD), PilT N-terminus like (PIN) domain, zinc finger (ZF) domain and C-terminal domain (CTD). The PIN domain harbors the RNase catalytic center; however, it is insufficient for enzymatic activity. We found that the NTD associates with the PIN domain and significantly enhances its RNase activity. The PIN domain forms a head-to-tail oligomer and the dimer interface overlaps with the NTD binding site. Interestingly, mutations blocking PIN oligomerization had no RNase activity, indicating that both oligomerization and NTD binding are crucial for RNase activity in vitro. These results suggest that Regnase-1 RNase activity is tightly controlled by both intramolecular (NTD-PIN) and intermolecular (PIN-PIN) interactions.

G protein-activated inwardly rectifying potassium channel (GIRK) plays crucial roles in regulating heart rate and neuronal excitability in eukaryotic cells. GIRK is activated by the direct binding of heterotrimeric G protein ␤␥ subunits (G␤␥) upon stimulation of G protein-coupled receptors, such as M2 acetylcholine receptor. The binding of G␤␥ to the cytoplasmic pore (CP) region of GIRK causes structural rearrangements, which are assumed to open the transmembrane ion gate. However, the crucial residues involved in the G␤␥ binding and the structural mechanism of GIRK gating have not been fully elucidated. Here, we have characterized the interaction between the CP region of GIRK and G␤␥, by ITC and NMR. The ITC analyses indicated that four G␤␥ molecules bind to a tetramer of the CP region of GIRK with a dissociation constant of 250 M. The NMR analyses revealed that the G␤␥ binding site spans two neighboring subunits of the GIRK tetramer, which causes conformational rearrangements between subunits. A possible binding mode and mechanism of GIRK gating are proposed.G protein-activated inwardly rectifying potassium channel (GIRK) 3 is a member of the inwardly rectifying potassium channel (Kir) family, which regulates heart rate and neuronal excitability (1, 2). The Kir proteins function as tetramers, consisting of a transmembrane (TM) region and a cytoplasmic pore (CP) region. The helix bundle at the cytoplasmic side of the TM region is assumed to be a K ϩ -ion gate. The opening and closing in the gate (gating) of Kirs are regulated by a variety of cytoplasmic factors. The gating of GIRK is triggered by the binding of its CP region with the heterotrimeric G-protein ␤␥ subunits (G␤␥), which are released from the pertussis toxin-sensitive G protein ␣ subunit (G␣ i/o ), subsequent to the stimulation of a G protein-coupled receptor, such as M2 acetylcholine receptor.Extensive mutational analyses to identify the GIRK residues that are critical for the G␤␥-induced activation revealed several critical residues such as His 57 , Leu 262 , Leu 333, and Gly 336 of GIRK1 (3-5). However, when these residues were mapped on the recently reported crystal structures of Kirs, they did not form a cluster on the protein surface (6 -9). Therefore, no clear consensus has been obtained regarding the region of GIRK that is essential for G␤␥ binding and/or GIRK activation. One of the reasons might be a structural alteration introduced by the mutagenesis (10), which could change the gating property of the channel. Although the various crystal structures have provided little information about the conformational change involved in the gating of the channel, FRET analyses have clearly demonstrated the conformational rearrangements in the CP region of GIRK upon G␤␥ binding (11). However, the resolution of the structural information obtained by the FRET analyses is low, primarily due to the large size of the fluorescent probe proteins.To reveal the structural mechanism by which G␤␥ binding activates GIRK, we have investigated the direct interacti...