Interactions of endothelin receptor subtypes A and B with Gi, Go, and Gq in reconstituted phospholipid vesicles

To understand the biochemical basis for the functional divergence of the human endothelin receptor subtypes A (ETAR) and B (ETBR), they were expressed, purified from insect Sf9 cells, and reconstituted into phospholipid vesicles with the Go, Gq, and Gi proteins. For each G protein, a unique pattern of reactivity was observed with the different receptor subtypes. Both ETAR and ETBR activated Go to a similar maximal extent, and both subtypes activated Gq with similar EC50 values; however, the ETAR displayed a 2-3-fold higher maximal extent of activation. In contrast, both subtypes activated Gi to a similar maximal extent, but the ETAR displayed a 4-fold higher EC50 value as compared to the ETBR. To test whether these coupling specificities are influenced by C-terminal palmitoylation of the receptor, we mutated a cluster of cysteine residues near the end of the seventh transmembrane helix in both receptors. While the cysteine mutations in the ETBR resulted in a partially palmitoylated receptor, the replacement of these cysteine residues in the ETAR yielded a mostly palmitoyl-deficient receptor and had no effect on Go activation, but caused a reduction in the extents of Gi and Gq stimulation. Together, these studies provide important insights into the specificity of G protein coupling in the endothelin receptors. The ability to discriminate between the different G proteins under various physiological conditions may be a key element in the selection of distinct signal transduction pathways by the two receptor subtypes.

Influence of various nucleotides on the in situ crystallization of Ca2+-ATPase

A reproducible in situ crystallization of the Ca2+-ATPase in isolated sarcoplasmic reticulum (SR) membranes was studied. The addition of various nucleotides to the washing buffer allowed the formation of tubular crystals, which is induced by vanadate. SR membranes washed with nucleotide-free buffer could not form tubular crystals upon subsequent incubation with vanadate.

Characterization of human endothelin B receptor and mutant receptors expressed in insect cells

Endothelin type-B receptor (ET(B)R) forms a stable complex with its ligand, endothelin-1. To facilitate biochemical and biophysical studies of human ET(B)R, several ET(B)R mutants carrying a hexahistidine tag sequence at the N or C terminus were expressed in Sf9 cells and were purified by a combination of biotinylated endothelin-1-ligand-affinity and nickel-affinity chromatographies. The ligand-free receptor was purified by dissociating the ligand x receptor complex with 2 M NaSCN, whereas the ligand-bound ET(B)R was purified by the use of thiol-sensitive biotinylated endothelin-1. While the wild-type ET(B)R was expressed at about 100 pmol 125I-endothelin-1-binding activity/mg membrane protein, the deletion of 36 residues from the N-terminus reduced the expressed activity to about 30%. On the other hand, the lack of glycosylation and the replacement of 2-9 residues in the N-terminal tail resulted in a 20-40% reduction in the expressed activity. Among the mutant proteins, [H57-H62, G63-G65]ET(B)R, carrying six His residues in the N-terminal tail, was studied extensively because it was purified most effectively. Ligand-free [H57-H62, G63-G65]ET(B)R, purified in digitonin, retained full ligand-binding activity, while other detergents led to partial denaturation of the receptor after solubilization or after elution with NaSCN. On the other hand, ligand-bound [H57-H62, G63-G65]ET(B)R could be purified in various detergents, such as n-octyl-beta-D-glucopyranoside or n-decyl-beta-D-maltopyranoside. Ligand-free [H57-H62, G63-G65]ET(B)R reconstituted in phospholipid vesicles stimulated the binding of guanosine 5'-3-O-(thio)triphosphate by Gq in the presence of endothelin-1. Ligand-bound [H57-H62, G63-G65]ET(B)R showed similar catalytic activity in nucleotide exchange by Gq. These results indicate that the ligand x receptor complex in a detergent-micellar solution retained the biologically active structure, and that the presence of ligand, endothelin-1, in the receptor molecule reinforces the stable assembly of a helical bundle and therefore the active structure.