Abstract
Opioid receptors (ORs) have been observed as homo- and heterodimers, but it is unclear if the dimers are stable under physiological conditions, and whether monomers or dimers comprise the predominant fraction in a cell. Here, we use three live-cell imaging approaches to assess dimerization of ORs at expression levels that are 10-100 x smaller than in classical biochemical assays.
At membrane densities around 25/mu m(2), a split-GFP assay reveals that kappa OR dimerizes, while mu OR and delta OR stay monomeric. At receptor densities < 5/mu m(2), single-molecule imaging showed no kappa OR dimers, supporting the concept that dimer formation depends on receptor membrane density.
To directly observe the transition from monomers to dimers, we used a single-molecule assay to assess membrane protein interactions at densities up to 100 x higher than conventional single-molecule imaging. We observe that kappa OR is monomeric at densities < 10/mu m(2) and forms dimers at densities that are considered physiological.
In contrast, mu OR and delta OR stay monomeric even at the highest densities covered by our approach. The observation of long-lasting co-localization of red and green kappa OR spots suggests that it is a specific effect based on OR dimerization and not an artefact of coincidental encounters.