Protein processing in chloroplasts
Instead of degrading proteins, some highly specialized proteases
such as methionine aminopeptidases (MAPs), which are located in
the chloroplast stroma and stromules, remove the N-terminal
methionine during protein processing, which is crucial for
maturation of chloroplast-encoded proteins (Nishimura et al.,
2017). With regard to nuclear-encoded chloroplastic proteins,
maturation requires removal of N-terminal chloroplast import
sequence, which is carried out by an SPP metalloprotease, yielding
mature proteins and liberated 20–100 aa transit peptides (Su et al.,
1999). In rice, SPP is expressed in all tissues, and mutations in the
SPP gene can lead to leaf chlorosis and growth inhibition (Yueet al.,
2010). Therefore, chloroplast proteases are involved in processing
of both nuclear-encoded and chloroplast-encoded proteins and are
required for functional chloroplasts.
Degradation of peptides in chloroplasts
The free peptides and transit signals generated by chloroplastic
proteases (Choi & Licht, 2005; Krojer et al., 2008; Ghifari
et al., 2019) are subsequently degraded into amino acids via the
chloroplastic peptidolytic machinery (Teixeira et al., 2017),
which comprises organellar oligopeptidase OOP, and presequence protease PreP. Both these oligopeptidases are metalloproteases which are located in the chloroplast stroma and
mitochondrial matrix. Peptides of 8–23 and 10–65 aa are
known to be respectively targeted by OOP and PreP proteases
(Kmiec & Glaser, 2012; Nishimura et al., 2017) for conversion
into small 2–5 aa fragments. Knocking out OOP and PreP
leads to accumulation of peptides derived from both Nterminal and internal parts of chloroplast proteins (Kmiec
et al., 2018). A double Arabidopsis knockout of prep1prep2
resulted in chlorosis, decreased growth rate and mitochondrial
respiration (Kmiec & Glaser, 2012). Thus, oligopeptidases
PreP and OOP are pivotal for degradation of free transit
peptides and peptides derived from degraded proteins.
These small peptides may be subsequently degraded by
aminopeptidases (M1, M17-10, M17-20 and M18) to free
amino acids (Teixeira et al., 2017). In Arabidopsis, double
mutations in the M1 and M17-20 genes did not affect growth
rate but reduced Chl content, seed germination and free
amino acid abundance, particularly serine (Teixeira et al.,
2017). Thus, these proteases are involved in peptide utilization
and recycling of amino acids in chloroplasts, and are crucial
for maintaining normal function
感謝 俄羅斯科學院謝米亞金-奧夫奇尼科夫生物有機化學研究所植物功能基因組學與蛋白質組學實驗室引用文獻