OPA1 processing controls mitochondrial fusion and is regulated by mRNA splicing, membrane potential, and Yme1L

Mitochondrial fusion plays an important role in controlling the shape and function of mitochondria (Okamoto and Shaw, 2005; Chan, 2006). In mammalian cells, the dynamin-related GTPase OPA1 is essential for mitochondrial fusion (Cipolat et al., 2004; Chen et al., 2005). The yeast OPA1 orthologue Mgm1 is also essential for fusion (Wong et al., 2000, 2003; Sesaki et al., 2003b) and has been shown to form oligomers important for tethering and fusion of the inner membranes (Meeusen et al., 2006). OPA1 is associated with the inner membrane and protects cells from apoptosis by regulating inner membrane dynamics (Olichon et al., 2003; Frezza et al., 2006). Mutation of OPA1 causes the disease dominant optic atrophy, a degeneration of the retinal ganglion cells (Alexander et al., 2000; Delettre et al., 2000). Both OPA1 and Mgm1 undergo proteolytic processing, and, in the case of Mgm1, such processing has been shown to be essential for mitochondrial fusion activity (Herlan et al., 2003; McQuibban et al., 2003; Sesaki et al., 2003a; Ishihara et al., 2006). Yeast Mgm1 is produced as a precursor with a mitochondrial leader sequence that is cleaved by the mitochondrial processing peptidase (MPP). Mgm1 processed by only MPP leads to the long isoform, l-Mgm1. The mitochondrial rhomboid Pcp1/Rbd1 further cleaves a subset of Mgm1 to form the short isoform, s-Mgm1 (Herlan et al., 2003; McQuibban et al., 2003; Sesaki et al., 2003a). Loss of Pcp1 greatly reduces mitochondrial fusion because a mixture of both the long and short Mgm1 isoforms is essential for normal activity (Herlan et al., 2003). Several issues concerning OPA1 processing remain enigmatic. In contrast to the two isoforms produced by Mgm1, OPA1 produces many more isoforms. OPA1 is encoded by a complicated set of at least eight mRNA splice forms that are produced by differential splicing (Delettre et al., 2001). In addition to the MPP processing site, the polypeptides encoded by each mRNA splice form contain an S1 cleavage site, and some also contain a more C-terminal S2 cleavage site (Fig. 1 A; Ishihara et al., 2006). In principle, therefore, each mRNA splice form can produce a long isoform (produced by cleavage with MPP alone) and one or more short isoforms (produced by cleavage at S1 or S2). The proteases acting at sites S1 and S2 are poorly understood. There is evidence for the involvement of both the rhomboid protease presenilin-associated rhomboid-like (PARL) and the m-AAA protease paraplegin in OPA1 processing (Cipolat et al., 2006; Ishihara et al., 2006). However, cells lacking PARL or paraplegin have normal OPA1 processing (Duvezin-Caubet et al., 2007), suggesting that other proteases remain to be identifi ed. OPA1 processing controls mitochondrial fusion and is regulated by mRNA splicing, membrane potential, and Yme1L