N-Degron Pathways in Plastids

• Published in: Trends in plant science Vol. 24; no. 10; pp. 917 - 926
• Main Authors: Bouchnak, Imen, van Wijk, Klaas J.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.10.2019; Elsevier BV
• Subjects: Algae; chloroplast; Chloroplasts; CLPS; Cytosol; Endopeptidase Clp; Eukaryotes; Homology; Hydrophobicity; N-degron pathway; N-end rule; Plastids; Prokaryotes; Protease; protein stability; Proteinase; Proteins; proteostasis; Residues; Substrates; N-end rule; CLPS; chloroplast; proteostasis; N-degron pathway; protein stability
• ISSN: 1360-1385; 1878-4372
• DOI: 10.1016/j.tplants.2019.06.013

Protein amino (N) termini are major determinants of protein stability in the cytosol of eukaryotes and prokaryotes, conceptualized in the N-end rule pathway, lately referred to as N-degron pathways. Here we argue for the existence of N-degron pathways in plastids of apicomplexa, algae, and plants. The prokaryotic N-degron pathway depends on a caseinolytic protease (CLP) S recognin (adaptor) for the recognition and delivery of N-degron-bearing substrates to CLP chaperone-protease systems. Diversified CLP systems are found in chloroplasts and nonphotosynthetic plastids, including CLPS homologs that specifically interact with a subset of N-terminal residues and stromal proteins. Chloroplast N-terminome data show enrichment of classic stabilizing residues [Ala (A), Ser (S), Val (V), Thr (T)] and avoidance of charged and large hydrophobic residues. We outline experimental test strategies for plastid N-degron pathways. Arabidopsis thaliana plastids contain a CLP protease system, including CLPS1, a homolog of the bacterial N-recognin CLPS. CLPS1 interacts with several stromal proteins. This interaction requires the presence of two conserved degron-binding residues. CLPS1 shows specific in vitro affinity for several type 2 N termini, especially F and W.A. thaliana CLPS1 interacts with CLPF, a protein unique to higher plant plastids. CLPF is likely to be a coadaptor of CLPS1, together involved in substrate delivery to the CLPC chaperones.The A. thaliana stromal N terminome is enriched for the small, uncharged, classical stabilizing residues A, V and T [often with N-terminal (Nt) acetylation], and S, whereas other residues at the N terminus of identified proteins are absent or highly underrepresented. Stromal Nt maturation is likely to be a multistep process involving SPP and uncharacterized peptidases and Nt acetylases.Current experimental evidence suggests the existence of a unique plastid N-degron pathway.