New isoforms and assembly of glutamine synthetase in the leaf of wheat (Triticum aestivum L.)

Glutamine synthetase (GS; EC 6.3.1.2) plays a crucial role in the assimilation and re-assimilation of ammonia derived from a wide variety of metabolic processes during plant growth and development. Here, three developmentally regulated isoforms of GS holoenzyme in the leaf of wheat (Triticum aestivum L.) seedlings are described using native-PAGE with a transferase activity assay. The isoforms showed different mobilities in gels, with GS(II)>GS(III)>GS(I). The cytosolic GS(I) was composed of three subunits, GS1, GSr1, and GSr2, with the same molecular weight (39.2kDa), but different pI values. GS(I) appeared at leaf emergence and was active throughout the leaf lifespan. GS(II) and GS(III), both located in the chloroplast, were each composed of a single 42.1kDa subunit with different pI values. GS(II) was active mainly in green leaves, while GS(III) showed brief but higher activity in green leaves grown under field conditions. LC-MS/MS experiments revealed that GS(II) and GS(III) have the same amino acid sequence, but GS(II) has more modification sites. With a modified blue native electrophoresis (BNE) technique and in-gel catalytic activity analysis, only two GS isoforms were observed: one cytosolic and one chloroplastic. Mass calibrations on BNE gels showed that the cytosolic GS1 holoenzyme was ~490kDa and likely a dodecamer, and the chloroplastic GS2 holoenzyme was ~240kDa and likely a hexamer. Our experimental data suggest that the activity of GS isoforms in wheat is regulated by subcellular localization, assembly, and modification to achieve their roles during plant development.