Mfn2 is an outer-mitochondrial-membrane mitofusin involved in GTP-dependent mitochondrial fusion. Its activity helps join the outer boundaries of neighbouring mitochondria, enabling later mixing of their contents through the coordinated fusion machinery. Mfn2 can associate with itself or with Mfn1. [ref]
In the mouse study, loss of Mfn2 produced fragmented mitochondria in embryonic fibroblasts and a placental defect distinct from the phenotype of Mfn1 deficiency. These comparisons show why two related proteins need not be interchangeable in every cell type. Network fragmentation alone does not identify whether reduced fusion or increased fission caused the change.
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