The function of RCC1L (RCC1 like, Ensembl gene identifier ENSG00000274523) is as follows. 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 (PubMed:27667664). . [Isoform 1]: Plays an essential role in mitochondrial ribosome biogenesis via its association with GTPases that play a role in the assembly of the large ribosome subunit. . [Isoform 2]: Plays an essential role in mitochondrial ribosome biogenesis via its association with GTPases that play a role in the assembly of the small ribosome subunit. .