Parkin-dependent regulation of the MCU complex component MICU1

The mitochondrial Ca²⁺ uniporter machinery is a multiprotein complex composed by the Ca²⁺ selective pore-forming subunit, the mitochondrial uniporter (MCU), and accessory proteins, including MICU1, MICU2 and EMRE. Their concerted action is required to fine-tune the uptake of Ca²⁺ into the mitochondrial matrix which both sustains cell bioenergetics and regulates the apoptotic response. To adequately fulfil such requirements and avoid impairment in mitochondrial Ca²⁺ handling, the intracellular turnover of all the MCU components must be tightly regulated. Here we show that the MCU complex regulator MICU1, but not MCU and MICU2, is rapidly and selectively degraded by the Ubiquitin Proteasome System (UPS). Moreover, we show that the multifunctional E3 ubiquitin ligase Parkin (PARK2), whose mutations cause autosomal recessive early-onset Parkinson's disease (PD), is a potential candidate involved in this process since its upregulation strongly decreases the basal level of MICU1. Parkin was found to interact with MICU1 and, interestingly, Parkin Ubl-domain, but not its E3-ubquitin ligase activity, is required for the degradation of MICU1, suggesting that in addition to the well documented role in the control of Parkin basal auto-inhibition, the Ubl-domain might exert important regulatory functions by acting as scaffold for the proteasome-mediated degradation of selected substrates under basal conditions, i.e. to guarantee their turnover. We have found that also MICU2 stability was affected upon Parkin overexpression, probably as a consequence of increased MICU1 degradation. Our findings support a model in which the PD-related E3 ubiquitin ligase Parkin directly participates in the selective regulation of the MCU complex regulator MICU1 and, indirectly, also of the MICU2 gatekeeper, thus indicating that Parkin loss of function could contribute to the impairment of the ability of mitochondria to handle Ca²⁺ and consequently to the pathogenesis of PD.