Stoichiometry of two plant glycine decarboxylase complexes and comparison with a cyanobacterial glycine cleavage system

Abstract

<jats:title>ABSTRACT</jats:title><jats:p>The multienzyme glycine cleavage system (GCS) converts glycine and tetrahydrofolate to the one-carbon compound 5,10-methylenetetrahydrofolate, which is of vital importance for most if not all organisms. Photorespiring plant mitochondria contain very high levels of GCS proteins organised as a fragile glycine decarboxylase complex (GDC). The aim of this study is to provide mass spectrometry-based stoichiometric data for the plant leaf GDC and examine whether complex formation could be a general property of the GCS in photosynthesizing organisms. The molar ratios of the leaf GDC component proteins are 1L<jats:sub>2</jats:sub>-4P<jats:sub>2</jats:sub>-8T-26H and 1L<jats:sub>2</jats:sub>-4P<jats:sub>2</jats:sub>-8T-20H for pea and Arabidopsis, respectively, as determined by mass spectrometry. The minimum mass of the plant leaf GDC ranges from 1,550-1,650 kDa, which is larger than previously assumed. The Arabidopsis GDC contains four times more of the isoforms GCS-P1 and GCS-L1 in comparison with GCS-P2 and GCS-L2, respectively, whereas the H-isoproteins GCS-H1 and GCS-H3 are fully redundant as indicated by their about equal amounts. Isoform GCS-H2 is not present in leaf mitochondria. In the cyanobacterium <jats:italic>Synechocystis</jats:italic> sp. PCC 6803, GCS proteins are present at low concentration but above the complex formation threshold reported for pea leaf GDC. Indeed, formation of a cyanobacterial GDC from the individual recombinant GCS proteins <jats:italic>in vitro</jats:italic> could be demonstrated. Presence and metabolic significance of a <jats:italic>Synechocystis</jats:italic> GDC <jats:italic>in vivo</jats:italic> remain to be examined but could involve multimers of the GCS H-protein that dynamically crosslink the three GCS enzyme proteins, facilitating glycine metabolism by the formation of multienzyme metabolic complexes.</jats:p>

Arabidopsis lysin motif/F‐box‐containing protein InLYP1 fine‐tunes glycine metabolism by degrading glycine decarboxylase GLDP2

“…In addition, we identified two glycine decarboxylases, GLDP1 and GLDP2, as InLYP1‐interacting proteins (Figure 4), and obtained several findings that well resonated with earlier studies on GLDPs: (i) we showed that InLYP1 could promote the ubiquitination and degradation of GLDP2 but not GLDP1 (Figure 6). Consistently, although GLDP1 and GLDP2 are co‐expressed at comparable levels (Engel et al ., 2007; Schulze et al ., 2013), the protein abundance of GLDP1 is four times as much as that of GLDP2 (Wittmiβ et al ., 2020). It is an interesting question why InLYP1 only mediates the ubiquitination of GLDP2 (Figure 6e).…”

“…In line with these observations, when compared with the gldp2 null plants, the gldp1 null plants could accumulate more glycine under intensified photorespiration (Engel et al ., 2007) and exhibit more severe germination defect in the presence of exogenous glycine (Figure 7c); (iii) both GLDP1 and GLDP2 contain predictable coiled coils (Figure S6), and can form homodimers or heterodimers in plant cells (Figure 7d–f). In accordance with our finding, recombinant algal GLDP proteins have been shown to form homodimers by X‐ray crystallography or size‐exclusion chromatography (Hasse et al, 2013; Wittmiβ et al ., 2020).…”