TABLE 2

Relationship between molecular brightness and receptor expression level

Plasma membrane 2D FCS molecular brightness analysis of HEK293 cells expressing different levels of GFP-tagged M₁-muscarinic receptors (M₁/GFP), β₂-adrenergic receptors (β₂-AR/GFP), and monomeric CD-86 labeled with two GFP tags (CD-86/GFP-GFP). The photon count rate (kHz) is a measure of the overall fluorescence intensity of the region of plasma membrane in the observation volume and n is the number of receptor dimers in the observation volume (calculated as in Eq. 4). Molecular brightness values are reported as photon counts per molecule. Data represent the mean ± S.E.M. from three cells with similar photon count rates (kHz). The area of plasma membrane in the observation volume was calculated using πω₀², where ω₀ is the radius of the observation volume (calculated as in the Materials and Methods), and was used to determine the number of dimers per µm² of plasma membrane. An HEK293 cell total surface area equivalent to 2591 μm² (Sommerhage et al., 2008) was used to estimate the total number of dimers per cell.