S T R U C T U R A L B I O L O G Y

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of kinase activity. Using hydrogen-deuterium exchange mass spectrometry (HDX-MS) to map the conformational changes upon PIP3 binding, it was possible to demonstrate that stoichiometric phosphorylation is insufficient to overcome this dependency on PIP3 (Figure 30).

The primary mutation found in Proteus Syndrome, as well as rare mutations observed in cases of megalencephaly, map to the autoinhibitory interface. Mutation of the interface drives Akt hyperphosphorylation, PIP3-independent activity and substrate engagement in the cytosol. Conversely, wild- type Akt is maintained in an inactive conformation in the absence of PIP3 or PI(3,4)P2 [2].

This work forces a re-evaluation of where and when Akt is active in the cell. Quantitative imaging of PIP3 and PI(3,4)P2 has revealed actively signalling pools of Akt throughout the endolyosomal membrane system; nuclear substrates that shuttle between the nucleus and cytoplasm may, therefore, be phosphorylated by endomembrane- bound Akt in the cytoplasm [4]. Activation of Akt is likely to be strictly dependent on the co-targeting of PDK1 and mTORC2 to PIP3- or PI(3,4)P2-containing membranes. Recent work now demonstrates that PDK1 is also allosterically activated by PIP3 and PI(3,4)P2 [5], but how mTORC2 is targeted to such membranes and Akt is still a mystery.

PRINCIPAL PUBLICATION AND AUTHORS

Structure of autoinhibited Akt1 reveals mechanism of PIP3-mediated activation, L. Truebestein (a,b), H. Hornegger (a,b), D. Anrather (c), M. Hartl (c), K.D. Fleming (d), J.T.B. Stariha (d), E. Pardon (e,f), J. Steyaert (e,f), J.E. Burke (d,g), T.A. Leonard (a,b), PNAS 118(33), e2101496118 (2021); https:/doi.org/10.1073/pnas.2101496118 (a) Department of Structural and Computational Biology, Max Perutz Labs, Vienna BioCenter (Austria) (b) Department of Medical Biochemistry, Medical University of Vienna (Austria) (c) Mass Spectrometry Core Facility, Max Perutz Labs, Vienna BioCenter (Austria) (d) Department of Biochemistry and Microbiology, University of Victoria (Canada) (e) Structural Biology Brussels, Vrije Universiteit Brussel (VUB), Brussels (Belgium) (f) VIB-VUB Center for Structural Biology, Vlaams Instituut voor Biotechnologie (VIB), Brussels (Belgium) (g) Department of Biochemistry and Molecular Biology, The University of British Columbia, Vancouver (Canada)

REFERENCES

[1] D.A. Fruman et al., Cell 170(4), 605-635 (2017). [2] M. Ebner et al., Mol. Cell 65(3), 416-431 (2017). [3] I. Lučić et al., PNAS 115(17), E3940-E3949 (2018). [4] T.A. Leonard, PNAS 115(27), E6101-E6102 (2018). [5] A. Levina et al., bioRxiv (2021); https:/ doi.org/10.1101/2021.10.08.463254.

Integrative biology unveils a unique molecular mechanism that regulates true macromolecular batteries

Mitochondria contain protein supercomplexes responsible for energy production within the cell. These enzymatic complexes are true macromolecular batteries that generate ATP. By using an integrative structural biology approach, a unique mechanism of enzyme regulation was discovered, providing insights into the coordination of these metabolic pathways for efficient energy production.

Fatty acid b-oxidation (FAO) and oxidative phosphorylation (OXPHOS) are mitochondrial redox processes that generate energy for the cell. Respiratory protein complexes are part of the OXPHOS system, which couples the flow of electrons through clusters of metals and cofactors with a transfer of protons to create a gradient that ultimately leads to ATP synthesis (Figure 32). In turn, these redox reactions generate reactive oxygen species (ROS), which have important roles in cell signalling and homeostasis, but also cause oxidative damage.

The first complex in the respiratory chain, Complex I (CI), is one of the largest membrane protein complexes made up of 45 subunits and is both the main source and target of ROS. While the molecular structure of the CI subunits is known in atomic detail, much less is known about the biogenesis of CI, a complicated multistep process involving transiently associated assembly factors that integrate subunits and insert cofactors into the final holoenzyme. A key player in CI assembly is the mitochondrial CI assembly (MCIA) complex. This complex consists of three core proteins, NDUFAF1, ACAD9 and ECSIT, which appear to further associate with two peripheral membrane proteins. The organisation and the role of the MCIA complex in CI assembly are still unclear, partly because the individual MCIA complex components are versatile and mediate other cell functions. In particular, ACAD9 is a FAO enzyme, while ECSIT participates in cytoplasmic and nuclear signalling pathways. Recently, ECSIT was suggested to be involved in amyloid toxicity, a hallmark signature of Alzheimer s disease [1,2]. Interestingly, defects in the CI assembly process seem to be implicated in the onset of neurodegeneration [3].