New Insights into the Mechanism of Chloroplast Protein Import and Its Integration with Protein Quality Control, Organelle Biogenesis and Development

Abstract

The translocons at the outer (TOC) and inner (TIC) envelope membranes of chloroplasts mediate the targeting and import of several thousand nuclear encoded preproteins that are required for organelle biogenesis and homeostasis. The cytosolic events in preprotein targeting remain largely unknown, although cytoplasmic chaperones have been proposed to facilitate delivery to the TOC complex. Preprotein recognition is mediated by the TOC GTPase receptors, Toc159 and Toc34. The receptors constitute a GTP-regulated switch, which initiates membrane translocation via Toc75, a member of the OMP85 (Outer Membrane Protein 85)/TpsB (two partner secretion system B) family of bacterial, plastid and mitochondrial β-barrel outer membrane proteins. The TOC receptor systems have diversified to recognize distinct sets of preproteins, thereby maximizing the efficiency of targeting in response to changes in gene expression during developmental and physiological events that impact organelle function. The TOC complex interacts with the TIC translocon to allow simultaneous translocation of preproteins across the envelope. Two inner membrane complexes, the Tic110 and 1 MDa complexes, have both been implicated as constituents of the TIC translocon, and it remains to be determined how they interact to form the TIC channel and assemble the import-associated chaperone network in the stroma that drives import across the envelope membranes. This review will focus on recent developments in our understanding of the mechanisms and diversity of the TOC-TIC systems. Our goal is to incorporate these recent studies with previous work and present updated or revised models for the function of TOC-TIC in protein import.

The POTRA domains of Toc75 exhibit chaperone-like function to facilitate import into chloroplasts

“…Protein import into chloroplasts is mediated by the TOC complex, with Toc75 serving as the central channel of the core translocon (5,8). Although structures have been reported for the orthologs of the Omp85 family, including FhaC, TamA, and BamA, in Gram-negative bacteria (35,36,(43)(44)(45)(46), no structure has been reported for either Sam50 in mitochondria or Toc75 or OEP80 in chloroplasts.…”

“…In plants, the majority of chloroplast proteins that are encoded in the nucleus are translated in the cytosol with an N-terminal transit peptide, which facilitates their translocation into chloroplasts. Protein import is mediated by the translocons at the outer chloroplast (TOC) and inner chloroplast (TIC) envelopes (5). The major components of the TOC machinery include the preprotein receptors Toc159 and Toc33 (Toc34 in pea), which are membrane-bound GTPases, and Toc75, a β-barrel protein that forms a cation-selective channel through which preproteins cross the outer membrane (6)(7)(8).…”

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

“…TOC159 and TOC33 are exposed to the cytoplasm and function as preprotein co-receptors (6)(7)(8). TOC75 embedded in the outer membrane forms a proteinconducting channel (9).…”

“…In Arabidopsis TOC33 has one homolog (TOC34) and TOC159 has three, TOC90, -120 and -132 (20, 21). In addition, to the GTP-binding (G-) domain the TOC159 homologs have a C-terminal membrane anchoring (M-) domain and a N-terminal acidic (A-) domain (7,22).…”

YCF1: A Green TIC: Response to the de Vries et al. Commentary

“…Our response is that many things have been missed over the last 30 years. Several "old" Tic proteins, such as Tic40 and Tic110, were mistakenly identified very early (probably because of their high abundance in the inner envelope membrane and/or of the use of inadequate methodology) and have long been believed to be true translocon components without further concrete evidence for their direct involvement in protein transport Paila et al, 2015). It has become clear that it is time to reevaluate the functions of these historic Tic proteins as well as of the so-called redox regulators, namely, Tic32, Tic55, and Tic62 (Nakai, 2015).…”