Guanine nucleotide exchange factor (GEF) for mitochondrial dynamin-related GTPase OPA1. Activates OPA1, by exchanging bound GDP for free GTP, and drives OPA1 and MFN1-dependent mitochondrial fusion (PubMed:28746876). Plays an essential role in mitochondrial ribosome biogenesis. As a component of a functional protein-RNA module, consisting of RCC1L, NGRN, RPUSD3, RPUSD4, TRUB2, FASTKD2 and 16S mitochondrial ribosomal RNA (16S mt-rRNA), controls 16S mt-rRNA abundance and is required for intra-mitochondrial translation of core subunits of the oxidative phosphorylation system (By similarity). Forms a regulatory protein-RNA complex, consisting of RCC1L, NGRN, RPUSD3, RPUSD4, TRUB2, FASTKD2 and 16S mt-rRNA. Interacts with 16S mt-rRNA; this interaction is direct (By similarity). Interacts with OPA1; this interaction is direct (PubMed:28746876). At E8.5, broadly expressed in yolk sac placenta, decidua, and embryo, with highest levels found in the trophoblast giant cells (TGCs) and ectoplacental cone (at protein level). The RCC1-like repeats assemble into a circular seven-bladed beta propeller structure. Each blade is composed of four antiparallel beta-strands with loops between each strand.