The Role of Plastidic Trigger Factor Serving Protein Biogenesis in Green Algae and Land Plants

Abstract

Biochemical processes in chloroplasts are important for virtually all life forms. Tight regulation of protein homeostasis and the coordinated assembly of protein complexes, composed of both imported and locally synthesized subunits, are vital to plastid functionality. Protein biogenesis requires the action of cotranslationally acting molecular chaperones. One such chaperone is trigger factor (TF), which is known to cotranslationally bind most newly synthesized proteins in bacteria, thereby assisting their correct folding and maturation. However, how these processes are regulated in chloroplasts remains poorly understood. We report here functional investigation of chloroplast-localized TF (TIG1) in the green alga (Chlamydomonas reinhardtii) and the vascular land plant Arabidopsis (Arabidopsis thaliana). We show that chloroplastic TIG1 evolved as a specialized chaperone. Unlike other plastidic chaperones that are functionally interchangeable with their prokaryotic counterpart, TIG1 was not able to complement the broadly acting ortholog in Escherichia coli. Whereas general chaperone properties such as the prevention of aggregates or substrate recognition seems to be conserved between bacterial and plastidic TFs, plant TIG1s differed by associating with only a relatively small population of translating ribosomes. Furthermore, a reduction of plastidic TIG1 levels leads to deregulated protein biogenesis at the expense of increased translation, thereby disrupting the chloroplast energy household. This suggests a central role of TIG1 in protein biogenesis in the chloroplast.

Co-Translational Protein Folding and Sorting in Chloroplasts

“…One explanation for this effect is, that chloroplast translation seems upregulated in C. reinhardtii tig1 mutants. This could be an energy costly reaction to compensate misfolding and degradation of newly synthesized proteins upon lack of the chaperone in plastids [46,50]. All these findings point to an important role of TIG1 during chloroplast biogenesis.…”

“…Deletion or reduction of chloroplast TIG1 results in subtle phenotypes with growth defects occurring upon prolonged dark exposure of C. reinhardtii tig1 mutants and reduced linear electron flow rates of photosynthesis in C. reinhardtii and A. thaliana tig1 mutants. This points toward an altered energy budgeting in the absence of the chaperone [46]. One explanation for this effect is, that chloroplast translation seems upregulated in C. reinhardtii tig1 mutants.…”

“…Unlike TF of E. coli, chloroplast TIG1 is not present in molar excess to ribosomes but rather comprises just~10% of the molar concentration of chloroplast ribosomes. This, together with the finding that only 2%-5% of chloroplast-localized TIG1 molecules bind to ribosomes, indicates that the chaperone is not required for folding of all chloroplast-encoded proteins [46]. Direct ribosome binding of TIG1 and TIG2 still awaits proof but there is evidence that the interaction might be similar in chloroplasts and bacteria ( Figure 1).…”

“…Interestingly, TIG1 seemed to have evolved a more specific function in chloroplasts, since TIG1 of C. reinhardtii and A. thaliana are both not able to substitute their counterpart in bacteria, unlike other plastidic chaperones such as co-chaperones of HSP70B or CPN60 which are able to complement the respective bacterial mutants [46][47][48][49]. This might be the consequence of a lower ribosome-binding affinity or their narrower substrate specificity compared with the broad affinity of bacterial trigger factor [46]. However, chloroplast TIG1s share a certain substrate binding specificity with E. coli TF, also binding to peptides with short hydrophobic segments [46].…”

“…This might be the consequence of a lower ribosome-binding affinity or their narrower substrate specificity compared with the broad affinity of bacterial trigger factor [46]. However, chloroplast TIG1s share a certain substrate binding specificity with E. coli TF, also binding to peptides with short hydrophobic segments [46]. Deletion or reduction of chloroplast TIG1 results in subtle phenotypes with growth defects occurring upon prolonged dark exposure of C. reinhardtii tig1 mutants and reduced linear electron flow rates of photosynthesis in C. reinhardtii and A. thaliana tig1 mutants.…”