Because of their surface localization, G protein-coupled receptors (GPCRs) are often pharmaceutical targets as they respond to a variety of extracellular stimuli (e.g., light, hormones, small molecules) that may activate or inhibit a downstream signaling response. The adenosine A
2A receptor (A
2AR) is a well-characterized GPCR that is expressed widely throughout the human body, with over 10 crystal structures determined. Truncation of the A
2AR C-terminus is necessary for crystallization as this portion of the receptor is long and unstructured; however, previous work suggests shortening of the A
2AR C-terminus from 412 to 316 amino acids (A
2AΔ316R) ablates downstream signaling, as measured by cAMP production, to below that of constitutive full-length A
2AR levels. As cAMP production is downstream of the first activation event—coupling of G protein to its receptor—investigating that first step in activation is important in understanding how the truncation effects native GPCR function. Here, using purified receptor and G
αs proteins, we characterize the association of A
2AR and A
2AΔ316R to G
αs with and without GDP or GTP
γs using surface plasmon resonance (SPR). G
αs affinity for A
2AR was greatest for apo-G
αs, moderately affected in the presence of GDP and nearly completely ablated by the addition of GTP
γs. Truncation of the A
2AR C-terminus (A
2AΔ316R) decreased the affinity of the unliganded receptor for G
αs by ~20%, suggesting small changes to binding can greatly impact downstream signaling.
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