Voltage-dependent anion-selective channel (VDAC) is a beta-barrel protein in the 
outer mitochondrial membrane that is necessary for metabolite exchange with the 
cytosol and is proposed to be involved in certain forms of apoptosis. We studied 
the biogenesis of VDAC in human mitochondria by depleting the components of the 
mitochondrial import machinery by using RNA interference. Here, we show the 
importance of the translocase of the outer mitochondrial membrane (TOM) complex 
in the import of the VDAC precursor. The deletion of Sam50, the central 
component of the sorting and assembly machinery (SAM), led to both a strong 
defect in the assembly of VDAC and a reduction in the steady-state level of 
VDAC. Metaxin 2-depleted mitochondria had reduced levels of metaxin 1 and were 
deficient in import and assembly of VDAC and Tom40, but not of three 
matrix-targeted precursors. We also observed a reduction in the levels of 
metaxin 1 and metaxin 2 in Sam50-depleted mitochondria, implying a connection 
between these three proteins, although Sam50 and metaxins seemed to be in 
different complexes. We conclude that the pathway of VDAC biogenesis in human 
mitochondria involves the TOM complex, Sam50 and metaxins, and that it is 
evolutionarily conserved.