Externalization of the phospholipid cardiolipin (CL) to the outer mitochondrial membrane has been proposed to act as a mitophagy trigger. CL would act as a signal for binding the LC3 macroautophagy/autophagy proteins. As yet, the behavior of the LC3-subfamily members has not been directly compared in a detailed way. In the present contribution, an analysis of LC3A, LC3B and LC3C interaction with CL-containing model membranes, and of their ability to translocate to mitochondria, is described. Binding of LC3A to CL was stronger than that of LC3B; both proteins showed a similar ability to colocalize with mitochondria upon induction of CL externalization in SH-SY5Y cells. Besides, the double silencing of LC3A and LC3B proteins was seen to decrease CCCP-induced mitophagy. Residues 14 and 18 located in the N-terminal region of LC3A were shown to be important for its recognition of damaged mitochondria during rotenone- or CCCP-induced mitophagy. Moreover, the in vitro results suggested a possible role of LC3A, but not of LC3B, in oxidized-CL recognition as a counterweight to excessive apoptosis activation. In the case of LC3C, even if this protein showed a stronger CL binding than LC3B or LC3A, the interaction was less specific, and colocalization of LC3C with mitochondria was not rotenone dependent. These results suggest that, at variance with LC3A, LC3C does not participate in cargo recognition during CL-mediated-mitophagy. The data support the notion that the various LC3-subfamily members might play different roles during autophagy initiation, identifying LC3A as a novel stakeholder in CL-mediated mitophagy.

9 pages. -- Figure S1. LC3 binding to liposomes is enhanced by membrane curvature and, in a dose-dependent manner, by CL. Interaction of LC3 proteins with vesicles of different radii and/or CL contents was measured by a vesicle flotation assay. -- Figure S2. Increasing ionic strength of the medium decreases the binding of LC3 to CL. Effect of the increased ionic strength on the interaction of LC3 proteins with CL-containing membranes, measured by a vesicle flotation assay. -- Figure S3. Changing a specific LC3C N-terminal residue has no effect on the protein binding to CL. -- Figure S4. Autophagy quantification with native GFP-LC3/GABARAP proteins and with non-conjugatable mutants confirms that most puncta in cells are autophagic vesicles and not aggregates. -- Figure S5. LC3A puncta and their colocalization with mitochondria increase with rotenone and CCCP treatment. Cells were co-transfected with DsRed2-Mito7 (DsRed) and with GFP-tagged WT or mutant LC3A. Vehicle (Veh) controls were treated with DMSO. -- Figure S6. The LC3B-AK double mutation that increases LC3B binding to CL in vitro does not have a comparable effect in cells. Cells were transfected with GFP-tagged WT or mutant LC3 protein. Mitochondria were labeled using MitoTracker Red, prior to the treatments. Vehicle (Veh) controls were treated with DMSO. -- Figure S7. mRNA and protein expression of LC3A, LC3B and LC3C in SH-SY5Y cells. -- Table S1. Primers used to perform site-directed mutagenesis. -- Table S2. Primers used to perform RT-qPCR. -- Table S3. siRNA synthesized by IDT to silence LC3A and LC3B proteins.

This work was supported in part by the Spanish Ministerio de Ciencia e Innovación (MCI), Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) (grant No. PGC2018-099857-B-I00), by the Basque Government (grants No. IT1625-22 and IT1270-19), by Fundación Ramón Areces (CIVP20A6619), by Fundación Biofísica Bizkaia and by the Basque Excellence Research Centre (BERC) program of the Basque Government. MI and YV were recipients of predoctoral FPU fellowships from the Spanish Ministry of Science Innovation and Universities (FPU16/05873, FPU18/00799), UB thanks the University of the Basque Country for a predoctoral contract, JHH was supported by a Postdoctoral Fellowship from the Basque Government.

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