The plastidial protein acetyltransferase GNAT1 forms a complex with GNAT2, yet their interaction is dispensable for state transitions
Résumé
Protein N-acetylation is one of the most abundant co- and post-translational modifications in
eukaryotes, extending its occurrence to chloroplasts within vascular plants. Recently, a novel
plastidial enzyme family comprising eight acetyltransferases that exhibit dual lysine and Nterminus
acetylation activities was unveiled in Arabidopsis. Among these, GNAT1, GNAT2, and
GNAT3 reveal notable phylogenetic proximity, forming a subgroup termed NAA90. Our study
focused on characterizing GNAT1, closely related to the state transition acetyltransferase GNAT2.
In contrast to GNAT2, GNAT1 did not prove essential for state transitions and displayed no
discernible phenotypic difference compared to the wild type under high light conditions, while
gnat2 mutants were severely affected. However, gnat1 mutants exhibited a tighter packing of the
thylakoid membranes akin to gnat2 mutants. In vitro studies with recombinant GNAT1
demonstrated robust N-terminus acetylation activity on synthetic substrate peptides. This activity
was confirmed in vivo through N-terminal acetylome profiling in two independent gnat1 knockout
lines. This attributed several acetylation sites on plastidial proteins to GNAT1, reflecting a subset
of GNAT2’s substrate spectrum. Moreover, co-immunoprecipitation coupled to mass spectrometry
revealed a robust interaction between GNAT1 and GNAT2, as well as a significant association of
GNAT2 with GNAT3 - the third acetyltransferase within the NAA90 subfamily. This study unveils
the existence of at least two acetyltransferase complexes within chloroplasts, whereby complex
formation might have a critical effect on the fine-tuning of the overall acetyltransferase activities.
These findings introduce a novel layer of regulation in acetylation-dependent adjustments in
plastidial metabolism.
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