Involvement of PHOSPHATE TRANSPORTER TRAFFIC FACILITATOR1 in COPII assembly by interacting with SAR1 GTPase

Inorganic phosphate (Pi) uptake and translocation are crucial for plant growth and development, relying on plasma membrane targeting of PHOSPHATE TRANSPORTER1 (PHT1) transporters. The plant-specific endoplasmic reticulum (ER)-resident PHOSPHATE TRANSPORTER TRAFFIC FACILITATOR1 (PHF1) is structurally related to SEC12, which initiates the coat protein complex II (COPII) assembly as a guanine nucleotide exchange factor (GEF) by activating the small GTPase SAR1. In contrast, PHF1 loses the conserved catalytic residues critical for GEF activity and specifically assists the ER exit of the PHT1 transporters. However, the underlying molecular mechanism remains unknown. In this study, we showed that overexpression of Arabidopsis thaliana PHT1;1 (AtPHT1;1) in the tobacco transient expression system caused a portion of AtPHF1 distribution into AtSAR1b- and AtSEC24a-labeled ER exit sites. We demonstrated that AtPHF1 interacts with AtSAR1b and AtSAR1c based on the tripartite split-GFP association in agro-infiltrated tobacco leaves and verified this interaction using miniTurbo-based proximity labeling. We also confirmed its physiological relevance by co-immunoprecipitation of the endogenous AtPHF1 with AtSAR1c-GFP in Arabidopsis transgenic lines. Importantly, AtPHF1 preferentially interacts with the GDP-locked AtSAR1. Therefore, we propose that AtPHF1 or the AtPHT1;1-AtPHF1 complex interacts with the SAR1 GTPase to participate in the early step of COPII recruitment for the ER export of PHT1 transporters.