Final answer:
The first active GPCR to have its X-ray crystal structure determined with its bound G protein was the β₂-adrenergic receptor complex. This milestone advancement in cell signaling research has allowed for a better understanding of GPCRs and their role in various biological processes.
Step-by-step explanation:
The first active G-protein-coupled receptor (GPCR) to show an X-ray crystal structure with its bound G protein was the β2-adrenergic receptor (β2AR) complex.
The structure of the GPCR bound to its G protein provided critical insights into how these receptors activate G proteins, an important process in cell signaling. The challenge of crystallizing membrane-bound receptors like GPCRs has historically been significant due to their flexible nature and the complexities involved in their function and interactions with other cellular components. As techniques have improved, an increasing number of GPCR structures have been determined, allowing for enhanced understanding of their role in cellular communication and potential therapeutic targets.