Lectin AtGAL1 interacts with high‐mannose glycoform of the purple acid phosphatase AtPAP26 secreted by phosphate‐starved Arabidopsis

Among 29 predicted Arabidopsis purple acid phosphatases (PAPs), AtPAP26 functions as the principle extracellular and intracellular PAP isozyme that is upregulated to recycle and scavenge Pi during Pi‐deprivation or leaf senescence. Our companion paper documented the copurification of a secreted, high‐mannose AtPAP26‐S2 glycoform with AtGAL1 (At1g78850), a Pi starvation‐inducible (PSI), and Galanthus nivalis agglutinin‐related (mannose‐binding) and apple domain lectin. This study tests the hypothesis that AtGAL1 binds AtPAP26‐S2 to modify its enzymatic properties. Far‐western immunodot blotting established that AtGAL1 readily associates with AtPAP26‐S2 but not the low mannose AtPAP26‐S1 glycoform nor other secreted PSI PAPs (i.e., AtPAP12 or AtPAP25). Analytical gel filtration indicated that 55‐kDa AtGAL1 and AtPAP26‐S2 polypeptides associate to form a 112‐kDa heterodimer. Microscopic imaging of transiently expressed, fluorescent protein‐tagged AtGAL1, and associated bimolecular fluorescence complementation assays demonstrated that (a) like AtPAP26, AtGAL1 also localizes to lytic vacuoles of Pi‐deprived Arabidopsis and (b) both proteins interact in vivo. AtGAL1 preincubation significantly enhanced the acid phosphatase activity and thermal stability of AtPAP26‐S2 but not AtPAP26‐S1. We hypothesize that AtGAL1 plays an important role during Pi deprivation through its interaction with mannose‐rich glycans of AtPAP26‐S2 and consequent positive impact on AtPAP26‐S2 activity and stability. AtPAP26 is the predominant purple acid phosphatase isozyme that functions in vacuolar Pi recycling and extracellular Pi scavenging from organic‐P sources during nutritional phosphorus deprivation or leaf senescence of Arabidopsis thaliana. Our companion paper documented the copurification of a secreted, high‐mannose AtPAP26‐S2 glycoform with AtGAL1 (At1g78850), a Pi starvation‐inducible, and Galanthus nivalis agglutinin‐related (mannose‐binding) and apple domain lectin. Here, we establish that AtGAL1 associates with AtPAP26‐S2 both in vitro and in vivo and that this interaction enhances AtPAP26‐S2's phosphatase activity and physical stability. Our results indicate that a complete description of plant Pi starvation responses needs to also consider the role of glycobiology and lectins.