Abstract
Proteins are the main machinery for all living processes in a cell; they provide structural elements, regulate biochemical reactions as enzymes, and are the interface to the outside as receptors and transporters. Like any other machinery proteins have to be assembled correctly and need maintenance after damage, e.g., caused by changes in environmental conditions, genetic mutations, and limitations in the availability of cofactors. Proteases and chaperones help in repair, assembly, and folding of damaged and misfolded protein complexes cost-effective, with low energy investment compared with neo-synthesis. Despite their importance for viability, the specific biological role of most proteases in vivo is largely unknown. Deg/HtrA proteases, a family of serine-type ATP-independent proteases, have been shown in higher plants to be involved in the degradation of the Photosystem II reaction center protein D1. The objective of this review is to highlight the structure and function of their cyanobacterial orthologs. Homology modeling was used to find specific features of the SynDeg/HtrA proteases of Synechocystis sp. PCC 6803. Based on the available data concerning their location and their physiological substrates we conclude that these Deg proteases not only have important housekeeping and chaperone functions within the cell, but also are needed for remodeling the cell exterior.
Highlights
Proteolysis, associated with nutrient uptake, processing and activation of proteins, or removal of damaged proteins, is vital for every cell
Proteases belonging to the Deg or HtrA family are known in all kingdoms, in Bacteria, Eukarya, and even some Archaea (Koonin and Aravind, 2002; Page and Di Cera, 2008)
PDZ domains mediate protein–protein interactions and are important for substrate recognition and/or for the regulation of proteolytic activity (Krojer et al, 2002; Wilken et al, 2004), the PDZ domains of Deg/HtrA proteases even have been shown to act as enzymatic co-factors
Summary
Proteolysis, associated with nutrient uptake, processing and activation of proteins, or removal of damaged proteins, is vital for every cell. The first element of PDZ domains in Deg peptidases is the carboxylate-binding loop, which interacts with the C-termini of substrate proteins and presents those to the proteolytic active site. Pyruvate kinase (Pyk2) and fructose biphosphate aldolase (FbaII) are upregulated in single mutants, while FbaII has been found to be down-regulated in the triple Syn-deg mutant, both under normal growth and under stress conditions (Miranda et al, 2013; Cheregi et al, 2015; Lam et al, 2015) pointing to intricate regulatory mechanisms that sense the absence of one or more SynDeg proteases. AtDeg is the TABLE 1 | Differentially regulated proteins identified by gel based assay (DIGE; Cheregi et al, 2015; Lam et al, 2015) and gel free assay (COFRADIC; Lam et al, 2015) comparing WT Synechocystis 6803 with the single mutants hhoA, hhoB, and htrA or the triple Deg mutant ( deg), grown at normal conditions
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