Cardiolipin, a critical determinant of mitochondrial carrier protein assembly and function

Abstract

The ability of phospholipids to act as determinants of membrane protein structure and function is probably best exemplified by cardiolipin (CL), the signature phospholipid of mitochondria. Early efforts to reconstitute individual respiratory complexes and members of the mitochondrial carrier family, most notably the ADP/ATP carrier (AAC), often demonstrated the importance of CL. Over the past decade, the significance of CL in the organization of components of the electron transport chain into higher order assemblies, termed respiratory supercomplexes, has been established. Another protein required for oxidative phosphorylation, AAC, has received comparatively little attention likely stemming from the fact that AACs were thought to function in isolation as either homodimers or monomers. Recently however, AACs have been demonstrated to interact with the respiratory supercomplex, other members of the mitochondrial carrier family, and the TIM23 translocon. Interestingly, many if not all of these interactions depend on CL. As the paradigm for the mitochondrial carrier family, these discoveries with AAC suggest that other members of this large group of important proteins may be more gregarious than anticipated. Moreover, it is proposed that AAC and perhaps additional members of the mitochondrial carrier family might represent downstream targets of pathological states involving alterations in CL.

Figure 1. Assembly of ATP synthase oligomers, the respiratory supercomplex, and AAC2 complexes in the presence and absence of CL

100 μg of 1.5% (wt/vol) digitonin extracts from mitochondria derived from wild type (+CL) or Δcrd1 (-CL) yeast were resolved by 2D BN/SDS–PAGE and immunoblots performed for complex V (F1α/β), complex IV (Cox2p), and AAC2. * highlights crossreaction with porin of the AAC antiserum. The migrations of the V_dimer, V_monomer, III₂IV₂, III₂IV, and IV supercomplexes are indicated schematically above the appropriate set of panels. The composition of the different AAC2 complexes is indicated above the AAC immunoblots. AAC1, ADP/ATP carrier isoform 1; Pic, phosphate carrier; Dic, dicarboxylate carrier; Ggc, GTP/GDP carrier. The interaction of AAC2 with the indicated mitochondrial carriers occurred in complexes ranging in size from ∼400-160 kDa. The exact size of each complex has not been determined; thus, the order of the depicted AAC2-carrier protein interactions is for illustrative purposes only. Details provided in text.

Figure 2. The contribution of CL to energy efficiency

(A) CL, the “green’ phospholipid, facilitates cyt. c (blue squares) transport between complexes III (cherrywood ovals) and IV (purple ovals) by stabilizing the III₂IV₂-AAC2 supercomplex and stimulates AAC (gray squares) activity by placing it in an electrochemical bath provided by the proton–coupled electron transport activity of complexes III and IV. (B) In the absence of CL, the absolute activity of the electron transport chain is reduced due to partial destabilization of the respiratory supercomplexes, the proton pumping capacity of the electron transport chain is decentralized, and AAC2 no longer resides immediately adjacent to the electron transport chain. © Claypool et al., 2008. Adapted from Figure 7 originally published in The Journal of Cell Biology. doi:10.1083/jcb.200801152.

Figure 3. Are AACs and/or other carrier proteins targets of pathological situations associated with alterations to CL?

Pathologies in which alterations in CL have been implicated and the nature of the alterations in CL are indicated.