Nanoscale organization of β2-adrenergic receptor-Venus fusion protein domains on the surface of mammalian cells

Adrenergic receptors are a key component of nanoscale multiprotein complexes that are responsible for controlling the beat rate in a mammalian heart. We demonstrate the ability of near-field scanning optical microscopy (NSOM) to visualize β2-adrenergic receptors (β2AR) fused to the GFP analogue Venus at the nanoscale on HEK293 cells. The expression of the β2AR-Venus fusion protein was tightly controlled using a tetracycline-induced promoter. Both the size and density of the observed nanoscale domains are dependent on the level of induction and thus the level of protein expression. At concentrations between 100 and 700 ng/ml of inducer doxycycline, the size of domains containing the β2AR-Venus fusion protein appears to remain roughly constant, but the number of domains per cell increase. At 700 ng/ml doxycycline the functional receptors are organized into domains with an average diameter of 150 nm with a density similar to that observed for the native protein on primary murine cells. By contrast, larger micron-sized domains of β2AR are observed in the membrane of the HEK293 cells that stably overexpress β2AR-GFP and β2AR-eYFP. We conclude that precise chemical control of gene expression is highly advantageous for the use β2AR-Venus fusion proteins as models for β2AR function. These observations are critical for designing future cell models and assays based on β2AR, since the receptor biology is consistent with a relatively low density of nanoscale receptor domains.