The Topology, Regulation, and Function of Cardiolipin Remodeling in Saccharomyces cerevisiae
Baile, Matthew G.
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Cardiolipin (CL) is a unique phospholipid that resides exclusively within mitochondria, and is required for the optimal function of numerous mitochondrial processes. After synthesis, CL undergoes an evolutionarily conserved acyl chain remodeling process where acyl chains are removed by a lipase to form monolyso-CL (MLCL), and replaced by the transacylase tafazzin (Taz1p), reforming CL and establishing a final molecular form that is distinct from newly synthesized CL and highly homogeneous in an organism or cell type. Additionally, mutations in TAZ1 cause Barth syndrome, resulting in cardiac and skeletal myopathy, cyclic neutropenia, and respiratory chain dysfunction. The topology of CL biology is complex; synthesis occurs on the matrix-facing leaflet of the mitochondrial inner membrane (IM), but remodeling is completed on the intermembrane space-facing leaflets of both the IM and outer membrane (OM). We found that Cld1p, the lipase that initiates CL remodeling, functions between Crd1p and Taz1p, and resides in the matrix-facing leaflet of the IM but does not traverse the membrane. Thus, after MLCL is generated by Cld1p, it must translocate either across the IM or to the OM, identifying a previously unappreciated trafficking step required for CL remodeling. Similar to other cellular processes which exhibit spatial separation of enzymatic steps in a pathway, the translocation of MLCL potentially represents a regulation point within CL remodeling. We found that the activity of MLCL translocation was not regulated in any condition we tested. Instead, Cld1p regulates CL remodeling. Its activity is modulated by the available carbon source or by changes in the mitochondrial membrane potential; two separate mechanisms which allow CL remodeling to be regulated either coordinately with or independently from CL biosynthesis. By analyzing Δcld1 yeast containing only unremodeled CL, we analyzed the functional differences between unremodeled and remodeled CL. We found that unremodeled and remodeled CL is equally able to support oxidative phosphorylation assembly and function, and mitochondrial morphology. This is in contrast to the widely accepted hypothesis that CL remodeling establishes a molecular form of CL optimized to support oxidative phosphorylation. Thus CL remodeling may have other physiological roles besides the establishment of a specific molecular form.