Metaxin is an outer membrane protein of mammalian mitochondria which is 
suggested to be involved in protein import into the organelle. RNA blot analysis 
showed that distribution of metaxin mRNA in human tissues differs from that of 
mRNA for the translocase component Tom20. Effect of overexpression of human 
metaxin on mitochondrial preprotein import and processing in COS-7 cells was 
studied. Overexpression of metaxin resulted in impaired mitochondrial import of 
natural and chimeric preproteins and in their accumulation. We previously 
reported that overexpression of Tom20 in cultured cells causes inhibition of 
import of mitochondrial preprotein. Coexpression of metaxin with Tom20 had no 
further effect on the preprotein import. Overexpression of the cytosolic domain 
of metaxin also caused inhibition of preprotein import, although less strongly 
than the full-length metaxin. In blue native PAGE, Tom40, Tom22, and a portion 
of Tom20 migrated as a complex of approximately 400 kDa, and the other portion 
of Tom20 migrated in smaller forms of approximately 100 and approximately 40 
kDa. On the other hand, metaxin migrated at a position of approximately 50 kDa. 
These results confirm earlier in vitro results that metaxin participates in 
preprotein import into mammalian mitochondria, and indicates that it does not 
associate with the Tom complex.