The Acidic A-Domain of Arabidopsis Toc159 Occurs as a Hyperphosphorylated Protein    

Abstract

The translocon at the outer membrane of the chloroplast assists the import of a large class of preproteins with amino-terminal transit sequences. The preprotein receptors Toc159 and Toc33 in Arabidopsis (Arabidopsis thaliana) are specific for the accumulation of abundant photosynthetic proteins. The receptors are homologous GTPases known to be regulated by phosphorylation within their GTP-binding domains. In addition to the central GTP-binding domain, Toc159 has an acidic N-terminal domain (A-domain) and a C-terminal membrane-anchoring domain (M-domain). The A-domain of Toc159 is dispensable for its in vivo activity in Arabidopsis and prone to degradation in pea (Pisum sativum). Therefore, it has been suggested to have a regulatory function. Here, we show that in Arabidopsis, the A-domain is not simply degraded but that it accumulates as a soluble, phosphorylated protein separated from Toc159. However, the physiological relevance of this process is unclear. The data show that the A-domain of Toc159 as well as those of its homologs Toc132 and Toc120 are targets of a casein kinase 2-like activity.

Plastid Proteome Assembly without Toc159: Photosynthetic Protein Import and Accumulation of N-Acetylated Plastid Precursor Proteins

“…It seems that the A-domain confers precursor selectivity to a constitutive import process, whose efficiency depends mainly on the abundance of the different Toc receptors. Initial results suggest that the A-domain may be cleaved upon assembly of the Toc complex (Agne et al, 2010). The conclusion that the A-domain mediates import specificity could explain why our data did not identify sequence specificity determinants of Toc159-mediated protein import (see Supplemental Figure 5 online).…”

A Comprehensive Analysis of Chromoplast Differentiation Reveals Complex Protein Changes Associated with Plastoglobule Biogenesis and Remodeling of Protein Systems in Sweet Orange Flesh

“…Here, we identified nearly all the elements of the protein import machinery and classified them into protein targeting, which showed a constant abundance during the amyloplast-to-chromoplast transition (Supplemental Table S5). Among these elements, translocon at the outer envelope membrane of chloroplasts159 (TOC159), which is tightly bound to the transit peptides, serves as a receptor for chloroplastdestined preproteins (Aronsson and Jarvis, 2008) and is thought to interact with CASEIN KINASE2 to affect chromoplast biogenesis (Agne et al, 2010;Zeng et al, 2014). TOC75 is the main pore of the TOC complex deeply embedded within the outer membrane, forming a translocation channel (Jarvis, 2008).…”

The novel chloroplast outer membrane kinase KOC1 is a required component of the plastid protein import machinery

“…The A-domain has many predicted and experimentally identified casein kinase II (CKII) sites. CKII efficiently phosphorylates recombinant A-domain in vitro (37). However, some of the phosphorylation sites within the A-domain do not resemble CKII sites suggesting that additional kinases may be involved.…”

“…Wang and colleagues proposed that chloroplast protein import activity may be regulated by ABA via SnRK2 dependent phosphorylation of the A-domain (38). Additional A-domain kinases other than CKII and SnRK2 have been predicted but not identified (37).…”