Defense against Reactive Carbonyl Species Involves at Least Three Subcellular Compartments Where Individual Components of the System Respond to Cellular Sugar Status

Abstract

Methylglyoxal (MGO) and glyoxal (GO) are toxic reactive carbonyl species generated as by-products of glycolysis. The pre-emption pathway for detoxification of these products, the glyoxalase (GLX) system, involves two consecutive reactions catalyzed by GLXI and GLXII. In , the GLX system is encoded by three homologs of and three homologs of , from which several predicted GLXI and GLXII isoforms can be derived through alternative splicing. We identified the physiologically relevant splice forms using sequencing data and demonstrated that the resulting isoforms have different subcellular localizations. All three GLXI homologs are functional in vivo, as they complemented a yeast GLXI loss-of-function mutant. Efficient MGO and GO detoxification can be controlled by a switch in metal cofactor usage. MGO formation is closely connected to the flux through glycolysis and through the Calvin Benson cycle; accordingly, expression analysis indicated that GLXI is transcriptionally regulated by endogenous sugar levels. Analyses of Arabidopsis loss-of-function lines revealed that the elimination of toxic reactive carbonyl species during germination and seedling establishment depends on the activity of the cytosolic GLXI;3 isoform. The Arabidopsis GLX system involves the cytosol, chloroplasts, and mitochondria, which harbor individual components that might be used at specific developmental stages and respond differentially to cellular sugar status.

From methylglyoxal to pyruvate: a genome-wide study for the identification of glyoxalases and D-lactate dehydrogenases in Sorghum bicolor

“…The longer AtGLYI-2.4 protein form was more similar to rice OsGLYI-8, both in length as well as in nuclear localization [44]. The AtGLYI-2.4 protein, however, also localizes to the chloroplast as reported by Schmitz et al [45]. Likewise, SbGLYI-8/8.1 proteins were also found to harbour putative nuclear localization signals (NLS) and therefore, may get localized in the nucleus as well.…”

“…Interestingly, SbGLYI-8 possessed two spliced forms coding for almost similar length proteins (214 and 227 aa long), and both were predicted to be similarly localised in the mitochondria and/or chloroplast. This was unlike AtGLYI-2 from Arabidopsis, where three out of the four spliced forms coded for the same protein (AtGLYI-2.1/ 2/3, 187 aa) and only one was different (AtGLYI-2.4) being 236 aa long [45]. The longer AtGLYI-2.4 protein form was more similar to rice OsGLYI-8, both in length as well as in nuclear localization [44].…”

“…Four SbGLYI proteins namely, SbGLYI-7, SbGLYI-10, SbGLYI-11 and SbGLYI-14, were found to be Ni 2+ -dependent showing greater homology to the previously characterised Ni 2+dependent GLYI proteins from rice and Arabidopsis [42,43] and having a similar domain length, of approximately 120 aa. Similarly, only one GLYI protein namely, SbGLYI-8 was found to be Zn 2+ -dependent, having a domain length of 140 aa, much like rice OsGLYI-8 [44] and Arabidopsis AtGLYI-2 [43,45] proteins. Interestingly, SbGLYI-8 possessed two spliced forms coding for almost similar length proteins (214 and 227 aa long), and both were predicted to be similarly localised in the mitochondria and/or chloroplast.…”

Responses of the chloroplast glyoxalase system to high CO2 concentrations

“…There are five GLX2 isozymes (AtGLX2-1, -2, -3, -4, and -5) that are annotated based on the primary structure in A. thaliana ( Table 2). All isozymes, except GLX2-2, have putative transit peptides [18]. It has previously been reported that recombinant AtGLX2-2 and -5 proteins show GLX2 activity (Table 2) [13,19,20], and that AtGLX2-1 and -3 possess no SLG hydrolysis activity and do not function in the GLX system [20,21].…”

“…S1) that are predicted to be localised in chloroplasts [17]. Moreover, these AtGLX1s show isomerisation activity for HA to SLG (Table 2) [18].…”