Abstract
Copper ion homeostasis involves a finely tuned and complex multi-level response system. This study expands on various aspects of the system in the model filamentous fungus Aspergillus nidulans. An RNA-seq screen in standard growth and copper toxicity conditions revealed expression changes in key copper response elements, providing an insight into their coordinated functions. The same study allowed for the deeper characterization of the two high-affinity copper transporters: AnCtrA and AnCtrC. In mild copper deficiency conditions, the null mutant of AnctrC resulted in secondary level copper limitation effects, while deletion of AnctrA resulted in primary level copper limitation effects under extreme copper scarcity conditions. Each transporter followed a characteristic expression and cellular localization pattern. Although both proteins partially localized at the plasma membrane, AnCtrC was visible at membranes that resembled the ER, whilst a substantial pool of AnCtrA accumulated in vesicular structures resembling endosomes. Altogether, our results support the view that AnCtrC plays a major role in covering the nutritional copper requirements and AnCtrA acts as a specific transporter for extreme copper deficiency scenarios.
Highlights
Copper (Cu) is an indispensable trace element for most living organisms
All colonies were grown in pH 6.8 buffered solid Aspergillus minimal medium (AMM) containing Käfer’s trace elements (Kafer, 1965) 1% (w/v) D-glucose, 71 μM sodium nitrate and appropriately supplemented according to the procedure described in Pontecorvo et al (1953)
A very important element for proper copper uptake is the plasma membrane oxidoreductase that reduces copper prior to internalization. In this table we show a characterized metalloreductase, AnfreA, and two other predicted plasma membrane oxidoreductases AN0773 and AnfreC that are co-regulated with the copper transporter proteins (Ctr) proteins and could be possible metalloreductase candidates
Summary
Copper (Cu) is an indispensable trace element for most living organisms. Its capacity to adopt an oxidized (Cu2+) and a reduced (Cu+) state is exploited by many enzymes to act as redox cofactor in enzyme catalyzed processes (Nevitt et al, 2012); cytochrome c, a key component of mitochondrial cellular respiration process; superoxide dismutase, for ROS neutralization; laccases, a protein family with great biotechnology implications which are involved in fungal pigment synthesis; and lysil oxidase for collagen maturation, for example (Scherer and Fischer, 2001; Lutsenko, 2010; Ding et al, 2011; Smith et al, 2017). Free intracellular copper can interfere with red-ox processes generating reactive oxygen species (ROS) or cause metalloprotein dysfunction by displacement of other bound metal ions (Fridovich, 1983; Macomber and Imlay, 2009; Besold et al, 2016). They include copper uptake, intracellular traffic, storage, and detoxification processes (Balamurugan and Schaffner, 2006; Nevitt et al, 2012). The expression of the high affinity Cu uptake system is transcriptionally regulated by a copper metalloregulatory transcription factor (CuMRTF) named ScMac (Labbe et al, 1997; Keller et al, 2005). In the absence of copper, ScMac is able to bind DNA, thereby activating the expression of the high affinity Cu uptake system. High Cu concentrations, in turn, result in ScMac inactivation (Zhu et al, 1998)
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