The Arabidopsis calmodulin-like proteins AtCML30 and AtCML3 are targeted to mitochondria and peroxisomes, respectively

Abstract

Calmodulin (CaM) is a ubiquitous sensor/transducer of calcium signals in eukaryotic organisms. While CaM mediated calcium regulation of cytosolic processes is well established, there is growing evidence for the inclusion of organelles such as chloroplasts, mitochondria and peroxisomes into the calcium/calmodulin regulation network. A number of CaM-binding proteins have been identified in these organelles and processes such as protein import into chloroplasts and mitochondria have been shown to be governed by CaM regulation. What have been missing to date are the mediators of this regulation since no CaM or calmodulin-like protein (CML) has been identified in any of these organelles. Here we show that two Arabidopsis CMLs, AtCML3 and AtCML30, are localized in peroxisomes and mitochondria, respectively. AtCML3 is targeted via an unusual C-terminal PTS1-like tripeptide while AtCML30 utilizes an N-terminal, non-cleavable transit peptide. Both proteins possess the typical structure of CaMs, with two pairs of EF-hand motifs separated by a short linker domain. They furthermore display common characteristics, such as calcium-dependent alteration of gel mobility and calcium-dependent exposure of a hydrophobic surface. This indicates that they can function in a similar manner as canonical CaMs. The presence of close homologues to AtCML3 and AtCML30 in other plants further indicates that organellar targeting of these CMLs is not a specific feature of Arabidopsis. The identification of peroxisomal and mitochondrial CMLs is an important step in the understanding how these organelles are integrated into the cellular calcium/calmodulin signaling pathways.

The peroxisome: an update on mysteries

“…The detection of calmodulin-like proteins in plant peroxisomes as well as the Ca 2? /calmodulin sensitivity of a plant catalase isoform (Chigri et al 2012;Yang and Poovaiah 2002) further points in this direction.…”

CaM and CML emergence in the green lineage

“…Most CaM and/or CML family members are predicted to be cytosolic and nuclear; however, several studies indicate that CaMs and several CMLs are localized to the vacuole, chloroplasts, and mitochondria [22,43,47]. Moreover, CaM53 in petunia and CaM61 in rice illustrate two cases of CaM with an extension in the C-terminal region of 34 and 38 residues, respectively, that contains a prenylation motif.…”

The EF-Hand Ca2+ Binding Protein MICU Choreographs Mitochondrial Ca2+ Dynamics in Arabidopsis

“…In plants, Ca 2+ binding EF-hand proteins, including a glutamate dehydrogenase, a family of MCF ATP/P i transporters (APC1-3), and two alternative NAD(P)H dehydrogenases (NDB1 and NDB2), localize to mitochondria and show Ca 2+ -dependent activity (although the latter are regulated by Ca 2+ from the intermembrane space side of the inner mitochondrial membrane; Møller et al, 1981;Rasmusson and Møller, 1991;Turano et al, 1997;Geisler et al, 2007;Stael et al, 2011;Monné et al, 2015). Furthermore, the EF-hand protein LETM1 and the calmodulin-like protein CML30 have recently been found to be localized in mitochondria, and additional proteins with Ca 2+ binding motifs have been detected in proteomic data sets of plant mitochondria (Chigri et al, 2012;Zhang et al, 2012;Salvato et al, 2014). Moreover, there is evidence for Ca 2+ dependence of mitochondrial permeability transition, protein import, and respiration in plants (Miernyk et al, 1987;Arpagaus et al, 2002;Kuhn et al, 2009;Manzoor et al, 2012).…”