OPA1 is a 120kDa large GTPase belonging to the dynamin superfamily. It is the only known mitochondrial inner membrane fusion protein, mediating fusion of the mitochondrial inner membranes following outer membrane fusion. Additionally, OPA1 also regulates cristae morphology and maintains respiratory chain function.
OPA1 has two forms—inner-membrane-anchored long forms (l-OPA1) and cleaved inter-membrane-space only short forms (s-OPA1). L-OPA1 are proteolytically processed by two mitochondrial proteases—OMA1 and YME1L, acting at cleavage sites S1 and S2 respectively, to produce s-OPA1.
In both mice and human, half of the mRNA splice forms of Opa1 are constitutively processed post translation to yield exclusively s-OPA1. However, the specific function of s-OPA1 in mitochondrial fusion has been debated—under basal conditions, s-OPA1 are needed to maintain optimal fusion activity, but in certain stress conditions, s-OPA1 is dispensable for fusion. By constructing cells in which the Opa1 locus no longer produces transcripts with S2 cleavage sites using CRISPR-Cas9, we generated a simplified system to identify the novel YME1L-dependent site S3 that mediates constitutive and complete cleavage of OPA1. We found that S3 site locates within the C-terminal leucine string of Opa1 exon4b, slightly upstream of the well-established S1. We show that mitochondrial morphology is highly sensitive to the ratio of l-OPA1 to s-OPA1, indicating that s-OPA1 fine tunes mitochondrial fusion.</p
