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A glycoform of the secreted purple acid phosphatase AtPAP26 co‐purifies with a mannose‐binding lectin (AtGAL1) upregulated by phosphate‐starved Arabidopsis

Ghahremani, Mina, Tran, Hue, Biglou, Sanaz G., O'Gallagher, Bryden, She, Yi‐Min, Plaxton, William C.
Plant, cell and environment 2019 v.42 no.4 pp. 1139-1157
Arabidopsis, Galanthus nivalis, acid phosphatase, apples, cross reaction, disulfide bonds, glycosylation, immunoblotting, lectins, mass spectrometry, mutants, polypeptides, polysaccharides
The purple acid phosphatase AtPAP26 plays a central role in Pi‐scavenging by Pi‐starved (−Pi) Arabidopsis. Mass spectrometry (MS) of AtPAP26‐S1 and AtPAP26‐S2 glycoforms secreted by −Pi suspension cells demonstrated that N‐glycans at Asn³⁶⁵ and Asn⁴²² were modified in AtPAP26‐S2 to form high‐mannose glycans. A 55‐kDa protein that co‐purified with AtPAP26‐S2 was identified as a Galanthus nivalis agglutinin‐related and apple domain lectin‐1 (AtGAL1; At1g78850). MS revealed that AtGAL1 was bisphosphorylated at Tyr³⁸ and Thr³⁹ and glycosylated at four conserved Asn residues. When AtGAL was incubated in the presence of a thiol‐reducing reagent prior to immunoblotting, its cross‐reactivity with anti‐AtGAL1‐IgG was markedly attenuated (consistent with three predicted disulfide bonds in AtGAL1's apple domain). Secreted AtGAL1 polypeptides were upregulated to a far greater extent than AtGAL1 transcripts during Pi deprivation, indicating posttranscriptional control of AtGAL1 expression. Growth of a −Pi atgal1 mutant was unaffected, possibly due to compensation by AtGAL1's closest paralog, AtGAL2 (At1g78860). Nevertheless, AtGAL1's induction by numerous stresses combined with the broad distribution of AtGAL1‐like lectins in diverse species implies an important function for AtGAL1 orthologs within the plant kingdom. We hypothesize that binding of AtPAP26‐S2's high‐mannose glycans by AtGAL1 enhances AtPAP26 function to facilitate Pi‐scavenging by −Pi Arabidopsis.