Constitutive signal bias mediated by the human GHRHR splice variant 1
Alternative splicing of G protein-coupled receptors has been observed, but their functions are largely unknown. Here, we report that a splice variant (SV1) of the human growth hormone releasing hormone receptor (GHRHR) is capable of transducing biased signal. Differing only at the receptor N terminus, GHRHR predominantly activates Gs while SV1 selectively couples to {beta}-arrestins. Based on the cryo-electron microscopy structures of SV1 in the apo state or in complex with the Gs protein, molecular dynamics simulations reveal that the N termini of GHRHR and SV1 differentiate the downstream signaling pathways, Gs vs. {beta}-arrestins. Suggested by mutagenesis and functional studies, it appears that GHRH-elicited signal bias towards {beta}-arrestin recruitment is constitutively mediated by SV1. The level of SV1 expression in prostate cancer cells is also positively correlated with ERK1/2 phosphorylation but negatively correlated with cAMP response. Our findings imply that constitutive signal bias may be a mechanism that ensures cancer cell proliferation. Significance StatementThe mechanism of functional changes induced by alternative splicing of GHRHR is largely unknown. Here, we demonstrate that GHRH-elicited signal bias towards {beta}-arrestin recruitment is constitutively mediated by SV1. The cryo-electron microscopy structures of SV1 and molecular dynamics simulations reveal the different functionalities between GHRHR and SV1 at the near-atomic level, i.e., the N termini of GHRHR and SV1 differentiate the downstream signaling pathways, Gs vs. {beta}-arrestins. Our findings provide valuable insights into functional diversity of class B1 GPCRs which may aid in the design of better therapeutic agents against certain cancers.