Regulates mitochondrial morphology and mitochondrial DNA (mtDNA) homeostasis (PubMed:41398051). Promotes the formation of specialized mitochondrial subdomains called small MTFP1-enriched mitochondria (SMEM) (PubMed:41398051). Negatively regulates mitochondrial fusion by excluding fusion coordinator OPA1 from SMEM (PubMed:41398051). Recruits mtDNA, potentially through interaction with SSBP1, and facilitates mtDNA transport to the mitochondrial periphery (PubMed:41398051). Also recruits other proteins, including PHB1, PHB2 and MTFP1, into SMEM which promotes peripheral mitochondrial fission, facilitating lysosome-mediated clearance of damaged mtDNA via mitophagy (PubMed:41398051). . [Isoform 2]: Secreted protein which may play a role in transcription regulation via the MAPK3/MAPK1 pathway through an unidentified receptor on the plasma membrane. . This is the function of MISO1 (mitochondrial inner membrane subdomain organizer 1, ENSG00000174928).