Abstract
Cells depend on the continuous renewal of their proteome composition during the cell cycle and in order to replace aberrant proteins or to react to changing environmental conditions. In higher eukaryotes, protein synthesis is achieved by up to five million ribosomes per cell. With the fast kinetics of translation, the large number of newly made proteins generates a substantial burden for protein homeostasis and requires a highly orchestrated cascade of factors promoting folding, sorting and final maturation. Several of the involved factors directly bind to translating ribosomes for the early processing of emerging nascent polypeptides and the translocation of ribosome nascent chain complexes to target membranes. In plant cells, protein synthesis also occurs in chloroplasts serving the expression of a relatively small set of 60–100 protein-coding genes. However, most of these proteins, together with nucleus-derived subunits, form central complexes majorly involved in the essential processes of photosynthetic light reaction, carbon fixation, metabolism and gene expression. Biogenesis of these heterogenic complexes adds an additional level of complexity for protein biogenesis. In this review, we summarize the current knowledge about co-translationally binding factors in chloroplasts and discuss their role in protein folding and ribosome translocation to thylakoid membranes.
Highlights
During protein synthesis, the linear genetic information is decoded into proteins, the versatile macromolecules that contribute to most biological pathways
An elegant recent ribosome profiling study comparing thylakoid association of translating ribosomes in A. thaliana wild-type and srp54 mutant lines greatly extending the pool of putative cpSRP54 substrates [107]. This ribosome profiling approach showed that the loss of cpSRP54 does not generally result in different footprint yields or altered translational output of all plastid open reading frames (ORFs), but there was a clear decrease in membrane protein footprint yield of putative substrate proteins, showing the sativum, binding to the ribosome is proposed to occur via the M domain interacting with the surface‐
Ribosome biology, the process of translation and subsequent nascent chain maturation gained major attention in multiple disciplines of biology. This is mainly caused by the fact that ribosomes have been recognized as dynamic hubs for quickly integrating environmental stimuli and for serving as platforms for protein folding and quality control
Summary
The linear genetic information is decoded into proteins, the versatile macromolecules that contribute to most biological pathways. At the ribosomal tunnel exit site, a number of biogenesis factors directly associate with ribosomes in order to bind emerging nascent chains, which assist in folding, translocation, maturation and early quality control, reviewed in [3,4,9] This event of nascent polypeptide binding and release is highly orchestrated and follows a well-ordered cascade. Co-translational translocation of nascent polypeptides requires targeting of translating ribosomes to subcellular membranes such as the endoplasmic reticulum, reviewed in [12], the mitochondrial outer membrane [13] and the thylakoid membranes of chloroplast (see below) in eukaryotic cells or the inner membrane of bacterial cells for protein secretion and membrane integration. For the parallel biogenesis of imported proteins and further downstream processes, such as biogenesis of thylakoid membranes and the assembly of the major complexes involved in photosynthesis, we refer to excellent reviews in the field, e.g., [26,27,28,29,30,31]
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