The standard pathway of membrane protein insertion involves a recognition and docking step, followed by insertion through a pore. The transfer is terminated by a hydrophobic stop-transfer sequence leading to the release of the partially translocated protein. On page , Schleiff et al. report that a pore is not needed for the correct integration of the voltage-dependent anion-selective channel, VDAC, into the mitochondrial outer membrane.
The recognition and pore complexes in the mitochondrial outer membrane are based around the proteins Tom20 and Tom40, respectively. Blockade of Tom40 function with unfoldable proteins or a temperature-sensitive Tom40 mutant does not prevent VDAC import, and Tom40 is not necessary for integration of functional VDAC into synthetic vesicles. Tom20 is necessary for VDAC integration into vesicles, however. Tom20 may function simply by increasing the local concentration of VDAC at the membrane, or it may have a more active role in releasing bound chaperones...
