The Clp protease system is required for copper ion‐dependent turnover of the PAA2/HMA8 copper transporter in chloroplasts

Abstract

SummaryThe distribution of essential metal ions over subcellular compartments for use as cofactors requires control of membrane transporters. PAA2/HMA8 is a copper-transporting P 1B -type ATPase in the thylakoid membrane, required for the maturation of plastocyanin. When copper is highly available to the plant this transporter is degraded, which implies the action of a protease.In order to identify the proteolytic machinery responsible for PAA2/HMA8 turnover in Arabidopsis, mutant lines defective in five different chloroplast protease systems were analyzed.Plants defective in the chloroplast caseinolytic protease (Clp) system were specifically impaired in PAA2/HMA8 protein turnover on media containing elevated copper concentrations. However, the abundance of a core Clp component was not directly affected by copper. Furthermore, the expression and activity of both cytosolic and chloroplast-localized superoxide dismutases (SODs), which are known to be dependent on copper, were not altered in the clp mutants, indicating that the loss of PAA2/HMA8 turnover in these lines was not caused by a lack of stromal copper.The results suggest that copper excess in the stroma triggers selection of the thylakoidlocalized PAA2 transporter for degradation by the Clp protease, but not several other chloroplast proteases, and support a novel role for this proteolytic system in cellular copper homeostasis.

Chloroplast Proteases: Updates on Proteolysis within and across Suborganellar Compartments

“…PAA2/HMA8 abundance is down-regulated in response to high copper concentrations (Tapken et al, 2012). This copper-induced down-regulation involves proteolysis by the stromal Clp system rather than thylakoid-located FtsH protease (Tapken et al, 2015).…”

“…The PC degradation activity must be metal independent but is dependent on stromal proteins, the proton motive force across the thylakoid, and ATP hydrolysis. Thylakoid-retained PC cleavage intermediates are exposed in part to the stroma (Midorikawa and Inoue, 2013), potentially allowing the access of stromal proteases, as in the case of PAA2/HMA8 for Clp (Tapken et al, 2015). The PsbP intermediate is detected as a monomer in the stroma when Plsp1 is missing (Shipman-Roston et al, 2010;Midorikawa and Inoue, 2013) but exists within the membrane, presumably through an association with the TAT machinery, when its TPP processing site is abolished (Frielingsdorf and Klösgen, 2007;Midorikawa and Inoue, 2013).…”

Posttranslational Control of ALA Synthesis Includes GluTR Degradation by Clp Protease and Stabilization by GluTR-Binding Protein

“…Recently, the thylakoid copper transporter PPA2 was shown to be degraded by the Clp system under copperreplete conditions. However, this degradation did not involve ClpS1, but was dependent on the ClpC1 chaperone and the ClpPR protease core complex (Tapken et al, 2015). Bacterial ClpS homologs function in the selection and delivery of substrates with an N-terminal degradation signal (an N-degron) for degradation by the Clp system.…”

Discovery of a Unique Clp Component, ClpF, in Chloroplasts: A Proposed Binary ClpF-ClpS1 Adaptor Complex Functions in Substrate Recognition and Delivery

“…van Wijk, and B. Grimm, unpublished data). On the other hand, the chloroplast copper transporter PAA2 was recently shown to be an in vivo substrate for the chloroplast Clp system involving both ClpC and the ClpPR core, but this degradation is independent of ClpS1 (Tapken et al, 2015).…”