This sort of collaboration between chaperones and proteases appears to be a widespread mechanism that ensures protein homeostasis in plant cells. The synergistic interaction between chaperones and proteases provides what is called ‘protein quality control’ (Bukau et al., 2006). Furthermore, chaperones play a role in the degradation of unfoldable proteins, working in concert with proteases to avoid cellular damage. These chaperones are involved in processes such as the folding of nascent proteins on ribosomes, transport across membranes and refolding after exposure to various stressful situations. Most cellular proteins require the attentions of molecular chaperones at different stages of their lifetime. Protein quality control and the HSP70 chaperone system in plants Here, we provide an overview of the entire repertoire of these proteins in A. thaliana with respect to their physiological function and possible evolutionary origin. The DNAJD type comprises proteins with a J-like domain only, and has 15 members in Arabidopsis thaliana, whereas proteins of the DNAJE (33 Arabidopsis members) and DNAJF (three Arabidopsis members) types contain a DNAJA-like zinc-finger domain and DNAJA/B-like C-terminal domain, respectively. Therefore, we propose to extend the classification of DNAJ-related proteins into three different groups. Indeed, the so-called DNAJ-like proteins with a degenerate J-domain have been previously coined as DNAJD proteins, and also proteins containing only a DNAJ-like zinc-finger motif appear to be involved in protein homeostasis. There is increasing evidence that some of the activities of DNAJ proteins do not require the J-domain, highlighting the functional significance of the other two domains. DNAJA proteins contain, in addition, both the zinc-finger motif and the C-terminal domain which are involved in substrate binding, while DNAJB retains only the latter and DNAJC comprises only the J-domain. All three classical types of DNAJ proteins (DNAJA, DNAJB and DNAJC types) possess the J-domain for interaction with HSP70. Classical DNAJ proteins are co-chaperones that together with HSP70s control protein homeostasis.
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