Calcium-mediated Association of a Putative Vacuolar Sorting Receptor PV72 with a Propeptide of 2S Albumin

PV72, a type I membrane protein with three epidermal-growth factor (EGF)-like motifs, was found to be localized on the membranes of the precursor-accumulating (PAC) vesicles that accumulated precursors of various seed storage proteins. To clarify the function of PV72 as a sorting receptor, we expressed four modified PV72s and analyzed their ability to bind the internal propeptide (the 2S-I peptide) of pro2S albumin by affinity chromatography and surface plasmon resonance. The recombinant PV72 specifically bound to the 2S-I peptide with a KD value of 0.2 μm, which was low enough for it to function as a receptor. The EGF-like motifs modulated the Ca2+-dependent conformational change of PV72 to form a functional pocket for the ligand binding. The binding of Ca2+ stabilizes the receptor-ligand complex even at pH 4.0. The association and dissociation of PV72 with the ligand is modulated by the Ca2+ concentration (EC50 value = 40 μm) rather than the environmental pH. Overall results suggest that Ca2+ regulates the vacuolar sorting mechanism in higher plants.