The atomistic Mechanism Underlying Regulation of the GPA1 G Protein Signaling Pathway Mediated by Abscisic Acid (ABA) Phytohormone Binding to the GCR1 Plant G Protein Coupled Receptor.

We propose an atomistic mechanism by which key plant processes, including seed dormancy, root elongation, secondary root proliferation, and flower and fruit produc-tion, are regulated. This regulation occurs through binding of the phytohormone abscisic acid (ABA) to the plant G protein-coupled receptor (GPCR) GCR1. This mirrors the central role of GPCRs in animal systems, where they mediate vision, taste, olfaction, pain perception, and neurotransmission. Establishing GCR1 as a bona fide GPCR in plants would represent a transformative advance in plant biology and agriculture. In particular, GCR1 would be shown to transduce ABA signals through interaction with the Gα subunit (GPA1). However, direct experimental evidence for this interaction and conformation that ABA binding to GCR1 modulates GPA1 inactivation, remains elusive. A major obstacle in testing these hypotheses is the lack of structural data on GPA1 interactions within the ABA-GCR1 complex. To address this gap, we employ molecular dynamics (MD) and metadynamics simulations based on the AMBER and CHARM31 force fields to characterize atomistically the ABA-GCR1-GPA1 ternary complex. Our MD simulations reveal an allosteric mechanism whereby GCR1-ABA binding induces a rigid-body closure of the GPA1 Ras and α-helical domains, creating a steric blockade that traps GDP in the nucleotide-binding pocket. This con-formation prevents GTP exchange and maintains GPA1 in an inactive state, effectively terminating the signaling cascade. Free energy landscape analysis further demonstrates that this closed state represents a deep energy minimum, suggesting biological relevance as a regulatory mechanism. We propose specific mutations in the ABA-binding site of GCR1 and at the GCR1-GPA1 interface that could experimentally validate (or refute) our proposed mechanism. Confirmation of this model would pave the way for designing novel agonists and inverse agonists to precisely manipulate critical plant processes.