The structures of cytosolic and plastid-located glutamine synthetases fromMedicago truncatulareveal a common and dynamic architecture

Abstract

The experimental models of dicotyledonous cytoplasmic and plastid-located glutamine synthetases unveil a conserved eukaryotic-type decameric architecture, with subtle structural differences in M. truncatula isoenzymes that account for their distinct herbicide resistance.

A Tale of Two Families: Whole Genome and Segmental Duplications Underlie Glutamine Synthetase and Phosphoenolpyruvate Carboxylase Diversity in Narrow-Leafed Lupin (Lupinus angustifolius L.)

“…Amino acid positions altered by relaxed selection constraints did not include known ligand interacting sites (ATP, glutamate, ammonia, and metal coordination sites were evaluated according to [86]). However, few sequences were considerably truncated and lacked several ligand binding sites, namely: GS1_cs1, Lj0g3v0335159 and GLYMA02G41106; GS1cs2, Lj2g3v0658180; and GS2, Lj6g3v1953860.…”

Glutamine synthetase in Medicago truncatula, unveiling new secrets of a very old enzyme

“…Determining the three-dimensional structures of closely related GS isoenzymes and their catalytic properties is critical to establish structure-function relationships. However, and in spite of the importance of GS in plants, the structure of the plant enzyme has only recently been elucidated ( Unno et al, 2006 ; Torreira et al, 2014 ). For a long time it was believed that eukaryotic GS had a octameric architecture, based on extrapolation from the best-studied prokaryotic enzymes ( Eisenberg et al, 1987 ).…”

“…For a long time it was believed that eukaryotic GS had a octameric architecture, based on extrapolation from the best-studied prokaryotic enzymes ( Eisenberg et al, 1987 ). The determination of the atomic structure of the cytosolic GS1a from maize (ZmGS1a) and M. truncatula (MtGS1a) by X-ray crystallography revealed a decameric organization ( Unno et al, 2006 ; Seabra et al, 2009 ; Torreira et al, 2014 ) that is common to the human and dog ( Krajewski et al, 2008 ) and also to the yeast enzymes ( He et al, 2009 ). Eukaryotic GS is arranged in two stacked (face-to-face) pentameric rings with extensive contacts established between subunits of the same ring and limited interactions with the subunits of the opposite ring.…”

“…Here, we highlight some interesting features related to the dynamics of enzyme catalysis revealed by the structural analysis of the cytosolic (MtGS1a) and plastid-located GSs (MtGS2a) from M. truncatula ( Torreira et al, 2014 ). The three-dimensional structure of unliganded MtGS1a was determined at 2.35 Ǻ resolution by X-ray crystallography (Figure 2 ; Seabra et al, 2009 ), and a comparison with the maize ortholog, whose structure was determined in complex with GS inhibitors ( Unno et al, 2006 ) revealed some critical differences: The Trp141–Gly152 surface segment displays a different arrangement in the two enzymes, resulting in a 15° rotation in the relative position of the two rings in the free MtGS1a enzyme, when compared to the inhibited maize enzyme, suggesting that substrate binding and release could be related to the rotation of the pentameric rings ( Torreira et al, 2014 ). The plastid-located MtGS2a is also a decamer composed of two superposed pentameric rings, as revealed by cryo-electron microscopy, but the association between the double-stacked rings appears to be more flexible than in MtGS1a ( Torreira et al, 2014 ; Figure 2 ).…”

Multiple Complexes of Nitrogen Assimilatory Enzymes in Spinach Chloroplasts: Possible Mechanisms for the Regulation of Enzyme Function

“…GS in higher plants also exists as two distinct types, cytosolic GS (GS1) and plastidic GS (GS2), with different polypeptide sizes of 38–40 kDa and 42–45 kDa, respectively [15] . Plant GS1 and GS2 polypeptides assemble into decameric complexes to be active holoenzymes [16] , [17] . In this study, we found each of NiR, GS and GOGAT assembled into discrete protein complexes in the chloroplasts of spinach leaves, and the dissociation profile of these complexes varied in response to the changes in the stromal conditions.…”