OPA1 is a cause gene for autosomal dominant optic atrophy and possesses eight 
alternative splicing variants. Here, we identified two isoforms of OPA1 proteins 
in HeLa cells and examined their submitochondrial localization and complex 
formations. RT-PCR shows that HeLa cells mainly express isoforms 7 and 1 of 
OPA1. Since the third cleavage site is mainly utilized in HeLa cells, the 
predicted molecular masses of their processed proteins are consistent with the 
93- and 88-kDa proteins. Biochemical examinations indicate that both of the OPA1 
isoforms are present in the intermembrane space. Submitochondrial fractionation 
by sucrose density-gradient centrifugation shows that the 88-kDa protein 
predominantly associates with the mitochondrial outer membrane, on the contrary, 
the 93-kDa protein associates with the inner membrane. Gel filtration analysis 
indicates that they compose the different molecular mass complexes in 
mitochondria. These differences between two isoforms of OPA1 would suggest their 
crucial role involved in the mitochondrial membrane formation.