Dissecting the thermodynamic contributions of the charged residues in the membrane anchoring of Bcl-xl C-terminal domain

Bcl-2 Family Members and the Mitochondrial Import Machineries: The Roads to Death

The localization of Bcl-2 family members at the mitochondrial outer membrane (MOM) is a crucial step in the implementation of apoptosis. We review evidence showing the role of the components of the mitochondrial import machineries (translocase of the outer membrane (TOM) and the sorting and assembly machinery (SAM)) in the mitochondrial localization of Bcl-2 family members and how these machineries regulate the function of pro- and anti-apoptotic proteins in resting cells and in cells committed into apoptosis.

Shifting Polar Residues Across Primary Sequence Frames of Transmembrane Domains Calibrates Membrane Permeation Thermodynamics

Permeation of the mitochondrial outer membrane (MOM) using the transmembrane domains (TMDs) is the key step of the Bcl-2 family of proteins to control apoptosis. The primary sequences of the TMDs of the family members like Bcl-xL, Bcl-2, Bak, etc. indicate the presence of charged residues at the C-terminal tip to be essential for drilling the membrane. However, Bax, a variant of the same family, is an exception, as the charged residues are shifted away from the tip by two positional frames in the primary sequence, but does it matter really? The free energy landscapes of membrane permeation, computed from a total of ∼13.3 μs of conformational sampling, show how such shifting of the amino acid frames in the primary sequence is correlated with the energy landscape that ensures the balance between membrane permeation and cytosolic population. Shifting the charged residues back to the terminal, in suitable mutants of Bax, proves the necessity of terminal charged residues by improving the insertion free energy but adds a high energy barrier unless some other polar residues are adjusted further. The difference in the TMDs of Bcl-xL and Bax is also reflected in their mechanism to drill the MOM-like anionic membrane; only Bax-TMD requires surface crowding to favorably shape the permeation landscape by weakening the bilayer integrity. So, this investigation suggests that such proteins can calibrate the free energy landscape of membrane permeation by adjusting the positions of the charged or polar residues in the primary sequence frames, a strategy analogous to the game of the "sliding tile puzzle" but played with primary sequence frames.