Lon protease: a novel mitochondrial matrix protein in the interconnection between drug-induced mitochondrial dysfunction and ER stress

Background and Purpose Mitochondria-associated membranes (MAMs) are specific ER-domains that enable direct interaction with mitochondria and mediate metabolic flow and Ca2+ transfer. A growing list of proteins have been identified as MAMs components, but how they are recruited and function during complex cell stress situations is still not understood while the participation of mitochondrial matrix proteins is largely undervalued. Experimental Approach This work compares mitochondrial/ER contact during combined ER stress/mitochondrial dysfunction using a model of human hepatoma cells (Hep3B cell line) treated for 24h with classic pharmacological inducers of ER stress (thapsigargin), mitochondrial dysfunction (CCCP or rotenone) or both (the antiretroviral drug Efavirenz used at clinically relevant concentrations). Key Results Markers of mitochondrial dynamics (Drp1, OPA1, Mfn2) were expressed differentially with these stimuli, pointing to a specificity of combined ER/mitochondrial stress. Lon, a matrix protease involved in protein and mtDNA quality control, was upregulated at mRNA and protein levels under all conditions. However, only with efavirenz, the mitochondrial content of Lon was diminished while its extramitochondrial presence was increased, as did its localization to MAMs, where it enhanced mitochondria/ER interaction as shown by co-immunoprecipitation experiments of MAM protein partners and confocal microscopy imaging. Conclusion and Implications A specific dual drug-induced mitochondria-ER effect enhances MAMs content and extramitochondrial Lon expression. This is the first report of this phenomenon and suggests a novel MAM-linked function of Lon protease.