Abstract
It is unclear how opioid selectivity and activation are regulated within the receptor core. In previous studies, the OFQ receptor was converted into a functional opioid receptor by mutating five amino acids at three sites to the corresponding residues conserved across the μ-, κ-, and δ-opioid receptors, suggesting that these sites comprise an opioid binding pocket. To examine this hypothesis, the present study examines whether these conserved residues represent an opioid binding pocket in the context of the opioid receptors, i.e., does their removal from opioid receptors destroy opioid ligand binding? The reciprocal mutations K227A (transmembrane [TM]5), IHI290-292VQV (TM6), and I316T (TM7) were evaluated in the κ-opioid receptor. In terms of alkaloid binding, there were no changes in affinity for mutants K227A and IHI290-292VQV. At mutant I316T, antagonist binding was unaltered, but there was a trend toward slightly decreased agonist affinity. In contrast, the binding of peptides had a more complex pattern. Again, K227A and IHI290-292VQV did not decrease the binding affinity of dynorphin-related peptides. Mutant I316T had 10- to 20-fold decreased affinity for dynorphin-related peptides, suggesting that I316 is part of a critical dynorphin recognition site. In response to alkaloid stimulation, I316T activated more G-protein(s) than wild type, and similar levels were observed in response to dynorphin stimulation. Overall, these results suggest that ligands are capable of achieving high-affinity binding through interaction with multiple sites/conformations of the receptor. These different modes of interaction have different down-stream results in terms of receptor activation and signal transduction.
Footnotes
-
This work was supported by National Institute on Drug Abuse Grants DA 08920 and DA 07268.
- Abbreviations:
- EKC
- ethylketocyclazocine
- EL
- extracellular loop
- G-protein
- guanine nucleotide-binding protein
- GTPγS
- guanosine-5′-O-(3-thio)triphosphate
- OFQ
- orphanin FQ/nociceptin
- TM
- transmembrane
- U-50,488
- (−)-(1S,2R)-cis-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide
- U-69,593
- (+)-(5α,7α,8β)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzenacetamide
- Received July 25, 2001.
- Accepted November 19, 2001.
- The American Society for Pharmacology and Experimental Therapeutics
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|