Interaction of N‐acetyl‐l‐glutamate kinase with the PII signal transducer in the non‐photosynthetic alga Polytomella parva: Co‐evolution towards a hetero‐oligomeric enzyme

Abstract

During evolution, several algae and plants became heterotrophic and lost photosynthesis; however, in most cases, a nonphotosynthetic plastid was maintained. Among these organisms, the colourless alga Polytomella parva is a special case, as its plastid is devoid of any DNA, but is maintained for specific metabolic tasks carried out by nuclear encoded enzymes. This makes P. parva attractive to study molecular events underlying the transition from autotrophic to heterotrophic lifestyle. Here we characterize metabolic adaptation strategies of P. parva in comparison to the closely related photosynthetic alga Chlamydomonas reinhardtii with a focus on the role of plastid‐localized PII signalling protein. Polytomella parva accumulates significantly higher amounts of most TCA cycle intermediates as well as glutamate, aspartate and arginine, the latter being specific for the colourless plastid. Correlating with the altered metabolite status, the carbon/nitrogen sensory PII signalling protein and its regulatory target N‐acetyl‐l‐glutamate‐kinase (NAGK; the controlling enzyme of arginine biosynthesis) show unique features: They have co‐evolved into a stable hetero‐oligomeric complex, irrespective of effector molecules. The PII signalling protein, so far known as a transiently interacting signalling protein, appears as a permanent subunit of the enzyme NAGK. NAGK requires PII to properly sense the feedback inhibitor arginine, and moreover, PII tunes arginine‐inhibition in response to glutamine. No other PII effector molecules interfere, indicating that the PII‐NAGK system in P. parva has lost the ability to estimate the cellular energy and carbon status but has specialized to provide an entirely glutamine‐dependent arginine feedback control, highlighting the evolutionary plasticity of PII signalling system.

From cyanobacteria to Archaeplastida: new evolutionary insights into PII signalling in the plant kingdom

“…Moreover, there is no response towards 2-OG. Consistent with the properties of PII-NAGK activation, a metabolomic analysis revealed that P. parva accumulates significantly larger amounts of Arg and Glu than does C. reinhardtii, whereas cellular Gln concentrations are relatively low (Selim et al, 2020). Experiments with heterologous combinations of PII and NAGK proteins indicated that the effector-molecule-independent interaction is not a property of only one of the two partner proteins but instead reflects a mutual adaptation of both proteins that favours the formation of stable PII-NAGK complexes.…”

“…In general, PII forms an activating complex with NAGK and thereby prevents feedback inhibition by Arg (Fig. 4) (Heinrich et al, 2004;Ll acer et al, 2007;Mizuno et al, 2007b;Beez et al, 2009;Chellamuthu et al, 2014;Lapina et al, 2018;Selim et al, 2020). The crystal structures of PII-NAGK complexes from S. elongatus (PDB: 2V5H) and A. thaliana (PDB: 2RD5) and of a heterocomplex of Chlamydomonas reinhardtii PII with A. thaliana NAGK (PDB: 4USJ) have been reported.…”

“…Recognizing the insights to be gained by examining the responses of PII-NAGK to effector molecules (summarized in Table 1), the PII-NAGK system of the colourless alga Polytomella parva was characterized (Selim et al, 2020). Although phylogenetically closely related to Chlamydomonas, P. parva has evolved a heterotrophic lifestyle, with the complete loss of photosynthesis.…”

“…Its plastids, which are devoid of any DNA, have been maintained for specific metabolic tasks carried out by nuclear-encoded enzymes. In P. parva, PII forms a very stable, almost nondissociable complex with NAGK that is independent of any effector molecules (Selim et al, 2020). In the absence of Gln, PII increases the sensitivity of NAGK towards Arg by nearly four-fold compared to the free enzyme (Arg-IC 50 0.18 mM vs Arg-IC 50 0.67 mM, respectively) whereas relatively low concentrations of Gln (Gln-EC 50 1.1-1.8 mM) strongly abrogate Arg feedback inhibition of the enzyme complex.…”

Nitrogen storage regulation by PII protein: lessons learned from taxonomic outliers

“…However, Selim et al . show that this is not the case, by studying a taxonomic outlier: The heterotrophic nonphotosynthetic unicellular alga Polytomella parva , a close relative of Chlamydomonas reinhardtii that although retaining the plastid, lacks photosynthetic pigments and plastidic DNA.…”