Herkt, C. | Search bioRxiv

bioRxiv2023

Competition co-immunoprecipitation reveals interactors of the chloroplast CPN60 chaperonin machinery

The functionality of essential metabolic processes in chloroplasts depends on a balanced integration of nuclear-and chloroplast-encoded polypeptides into the plastids proteome. The chloroplast chaperonin machinery is an essential player in chloroplast protein folding with a more intricate structure and subunit composition compared to the orthologous GroEL/ES chaperonin of Escherichia coli. However, its exact role in chloroplasts remains obscure, mainly because of a very limited knowledge about the folded substrates. We employed the competition immunoprecipitation method for the identification of the chaperonins substrates in Chlamydomonas reinhardtii. Co-immunoprecipitation of the target complex in the presence of increasing amounts of isotope-labelled competitor epitope and subsequent mass spectrometry analysis specifically allowed to distinguish true interactors from unspecifically co-precipitated proteins. Besides known substrates such as RbcL, we revealed numerous new substrates with high confidence. Identified substrate proteins differ from bulk chloroplast proteins by a higher content of beta-sheets, lower alpha-helical content and increased aggregation propensity. Immunoprecipitations performed with a subunit of the co-chaperonin lid revealed the ClpP protease as a specific partner complex, with altered interactions during heat stress, pointing to a close collaboration of these machineries to maintain protein homeostasis in the chloroplast.

molecular biology↗

bioRxiv2019

The proteomic inventory reveals the chloroplast ribosome as nexus within a diverse protein network

Chloroplast gene expression is tightly regulated and majorly controlled on the level of protein synthesis. Fine-tuning of translation is vital for plant development, acclimation to environmental challenges and for the assembly of major protein complexes such as the photosynthesis machinery. However, many regulatory mediators and the interaction network of chloroplast ribosomes are not known to date. We report here on a deep proteomic analysis of the plastidic ribosome interaction network in Chlamydomonas reinhardtii cells. Affinity-purification of ribosomes was achieved via endogenous affinity tagging of the chloroplast-encoded protein Rpl5, yielding a specific enrichment of >650 chloroplast-localized proteins. The ribosome interaction network was validated for several proteins and provides a new source of mainly conserved factors directly linking translation with central processes such as protein folding, photosystem biogenesis, redox control, RNA maturation, energy and metabolite homeostasis. Our approach provided the first evidence for the existence of a plastidic co-translational acting N-acetyltransferase (cpNAT1). Expression of tagged cpNAT1 confirmed its ribosome-association, and we demonstrated the ability of cpNAT1 to acetylate substrate proteins at their N-terminus. Our dataset establishes that the chloroplast protein synthesis machinery acts as nexus in a highly choreographed, spatially interconnected protein network and underscores its wide-ranging regulatory potential during gene expression. ONE-SENTENCE SUMMARYAffinity purification of Chlamydomonas reinhardtii chloroplast ribosomes and subsequent proteomic analysis revealed a broad spectrum of interactors ranging from global translation control to specific pathways.

biochemistry↗