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G-protein-coupled receptors (GPCRs) are key players in cell signaling, and their cell surface expression is tightly regulated. For many GPCRs such as ß2-AR (ß2-adrenergic receptor), receptor activation leads to downregulation of receptor surface expression, a phenomenon that has been extensively characterized. By contrast, some other GPCRs, such as GABAB receptor, remain relatively stable at the cell surface even after prolonged agonist treatment; however, the underlying mechanisms are unclear. Here, we identify the small GTPase Rap1 as a key regulator for promoting GABAB receptor surface expression. Agonist stimulation of GABAB receptor signals through Gai/o to inhibit Rap1GAPII (also known as Rap1GAP1b, an isoform of Rap1GAP1), thereby activating Rap1 (which has two isoforms, Rap1a and Rap1b) in cultured cerebellar granule neurons (CGNs). The active form of Rap1 is then recruited to GABAB receptor through physical interactions in CGNs. This Rap1-dependent signaling cascade promotes GABAB receptor surface expression by stimulating receptor recycling. Our results uncover a new mechanism regulating GPCR surface expression and also provide a potential explanation for the slow, long-lasting inhibitory action of GABA neurotransmitter. [ABSTRACT FROM AUTHOR]