Dynamin-related GTPase that is essential for normal mitochondrial morphology by mediating fusion of the mitochondrial inner membranes, regulating cristae morphology and maintaining respiratory chain function (PubMed:16778770, PubMed:17709429, PubMed:20185555, PubMed:24616225, PubMed:28628083, PubMed:28746876, PubMed:31922487, PubMed:32228866, PubMed:32567732, PubMed:33130824, PubMed:33237841, PubMed:37612504, PubMed:37612506). Exists in two forms: the transmembrane, long form (Dynamin-like GTPase OPA1, long form; L-OPA1), which is tethered to the inner mitochondrial membrane, and the short soluble form (Dynamin-like GTPase OPA1, short form; S-OPA1), which results from proteolytic cleavage and localizes in the intermembrane space (PubMed:31922487, PubMed:32228866, PubMed:33237841, PubMed:37612504, PubMed:37612506). Both forms (L-OPA1 and S-OPA1) cooperate to catalyze the fusion of the mitochondrial inner membrane (PubMed:31922487, PubMed:37612504, PubMed:37612506). The equilibrium between L-OPA1 and S-OPA1 is essential: excess levels of S-OPA1, produced by cleavage by OMA1 following loss of mitochondrial membrane potential, lead to an impaired equilibrium between L-OPA1 and S-OPA1, inhibiting mitochondrial fusion (PubMed:20038677, PubMed:31922487). The balance between L-OPA1 and S-OPA1 also influences cristae shape and morphology (By similarity). Involved in remodeling cristae and the release of cytochrome c during apoptosis (By similarity). Proteolytic processing by PARL in response to intrinsic apoptotic signals may lead to disassembly of OPA1 oligomers and release of the caspase activator cytochrome C (CYCS) into the mitochondrial intermembrane space (By similarity). Acts as a regulator of T-helper Th17 cells, which are characterized by cells with fused mitochondria with tight cristae, by mediating mitochondrial membrane remodeling: OPA1 is required for interleukin-17 (IL-17) production (By similarity). Its role in mitochondrial morphology is required for mitochondrial genome maintenance (PubMed:18158317, PubMed:20974897). {ECO:0000250|UniProtKB:P58281, ECO:0000269|PubMed:16778770, ECO:0000269|PubMed:17709429, ECO:0000269|PubMed:18158317, ECO:0000269|PubMed:20038677, ECO:0000269|PubMed:20185555, ECO:0000269|PubMed:20974897, ECO:0000269|PubMed:24616225, ECO:0000269|PubMed:28628083, ECO:0000269|PubMed:28746876, ECO:0000269|PubMed:31922487, ECO:0000269|PubMed:32228866, ECO:0000269|PubMed:32567732, ECO:0000269|PubMed:33130824, ECO:0000269|PubMed:33237841, ECO:0000269|PubMed:37612504, ECO:0000269|PubMed:37612506}. [Dynamin-like GTPase OPA1, long form]: Constitutes the transmembrane long form (L-OPA1) that plays a central role in mitochondrial inner membrane fusion and cristae morphology (PubMed:31922487, PubMed:32228866, PubMed:37612504, PubMed:37612506). L-OPA1 and the soluble short form (S-OPA1) form higher-order helical assemblies that coordinate the fusion of mitochondrial inner membranes (PubMed:31922487, PubMed:37612504, PubMed:37612506). Inner membrane- anchored L-OPA1 molecules initiate membrane remodeling by recruiting soluble S-OPA1 to rapidly polymerize into a flexible cylindrical scaffold encaging the mitochondrial inner membrane (PubMed:37612504, PubMed:37612506). Once at the membrane surface, the formation of S-OPA1 helices induce bilayer curvature (PubMed:37612504, PubMed:37612506). OPA1 dimerization through the paddle region, which inserts into cardiolipin-containing membrane, promotes GTP hydrolysis and the helical assembly of a flexible OPA1 lattice on the membrane, which drives membrane curvature and mitochondrial fusion (PubMed:28628083, PubMed:37612504, PubMed:37612506). Plays a role in the maintenance and remodeling of mitochondrial cristae, some invaginations of the mitochondrial inner membrane that provide an increase in the surface area (PubMed:32567732, PubMed:33130824). Probably acts by forming helical filaments at the inside of inner membrane tubes with the shape and dimensions of crista junctions (By similarity). The equilibrium between L-OPA1 and S-OPA1 influences cristae shape and morphology: increased L-OPA1 levels promote cristae stacking and elongated mitochondria, while increased S-OPA1 levels correlated with irregular cristae packing and round mitochondria shape (By similarity). {ECO:0000250|UniProtKB:G0SGC7, ECO:0000250|UniProtKB:P58281, ECO:0000269|PubMed:28628083, ECO:0000269|PubMed:31922487, ECO:0000269|PubMed:32228866, ECO:0000269|PubMed:32567732, ECO:0000269|PubMed:33130824, ECO:0000269|PubMed:37612504, ECO:0000269|PubMed:37612506}. [Dynamin-like GTPase OPA1, short form]: Constitutes the soluble short form (S-OPA1) generated by cleavage by OMA1, which plays a central role in mitochondrial inner membrane fusion and cristae morphology (PubMed:31922487, PubMed:32228866, PubMed:32245890, PubMed:37612504, PubMed:37612506). The transmembrane long form (L-OPA1) and the S-OPA1 form higher-order helical assemblies that coordinate the fusion of mitochondrial inner membranes (PubMed:31922487, PubMed:32228866, PubMed:37612504, PubMed:37612506). Inner membrane- anchored L-OPA1 molecules initiate membrane remodeling by recruiting soluble S-OPA1 to rapidly polymerize into a flexible cylindrical scaffold encaging the mitochondrial inner membrane (PubMed:32228866, PubMed:37612504, PubMed:37612506). Once at the membrane surface, the formation of S-OPA1 helices induce bilayer curvature (PubMed:37612504, PubMed:37612506). OPA1 dimerization through the paddle region, which inserts into cardiolipin-containing membrane, promotes GTP hydrolysis and the helical assembly of a flexible OPA1 lattice on the membrane, which drives membrane curvature and mitochondrial fusion (PubMed:28628083, PubMed:37612504, PubMed:37612506). Excess levels of S-OPA1 produced by cleavage by OMA1 following stress conditions that induce loss of mitochondrial membrane potential, lead to an impaired equilibrium between L-OPA1 and S-OPA1, thereby inhibiting mitochondrial fusion (PubMed:20038677). Involved in mitochondrial safeguard in response to transient mitochondrial membrane depolarization by mediating flickering: cleavage by OMA1 leads to excess production of S- OPA1, preventing mitochondrial hyperfusion (By similarity). Plays a role in the maintenance and remodeling of mitochondrial cristae, some invaginations of the mitochondrial inner membrane that provide an increase in the surface area (PubMed:32245890). Probably acts by forming helical filaments at the inside of inner membrane tubes with the shape and dimensions of crista junctions (By similarity). The equilibrium between L-OPA1 and S-OPA1 influences cristae shape and morphology: increased L-OPA1 levels promote cristae stacking and elongated mitochondria, while increased S-OPA1 levels correlated with irregular cristae packing and round mitochondria shape (By similarity). {ECO:0000250|UniProtKB:G0SGC7, ECO:0000250|UniProtKB:P58281, ECO:0000269|PubMed:20038677, ECO:0000269|PubMed:28628083, ECO:0000269|PubMed:31922487, ECO:0000269|PubMed:32228866, ECO:0000269|PubMed:32245890, ECO:0000269|PubMed:37612504, ECO:0000269|PubMed:37612506}. [Isoform 1]: Coexpression of isoform 1 with shorter alternative products is required for optimal activity in promoting mitochondrial fusion. . [Isoform 4]: Isoforms that contain the alternative exon 4b are required for mitochondrial genome maintenance, possibly by anchoring the mitochondrial nucleoids to the inner mitochondrial membrane. . [Isoform 5]: Isoforms that contain the alternative exon 4b are required for mitochondrial genome maintenance, possibly by anchoring the mitochondrial nucleoids to the inner mitochondrial membrane. . This is the function of OPA1 (OPA1 mitochondrial dynamin like GTPase, Ensembl gene identifier ENSG00000198836).