Core component of the CTLH E3 ubiquitin-protein ligase complex that selectively accepts ubiquitin from UBE2H and mediates ubiquitination and subsequent proteasomal degradation of the transcription factor HBP1. MAEA and RMND5A are both required for catalytic activity of the CTLH E3 ubiquitin-protein ligase complex. MAEA is required for normal cell proliferation. The CTLH E3 ubiquitin-protein ligase complex is not required for the degradation of enzymes involved in gluconeogenesis, such as FBP1 (By similarity). Plays a role in erythroblast enucleation during erythrocyte maturation and in the development of mature macrophages (PubMed:16707498). Mediates the attachment of erythroid cell to mature macrophages; this MAEA-mediated contact inhibits erythroid cell apoptosis (By similarity). Participates in erythroblastic island formation, which is the functional unit of definitive erythropoiesis (PubMed:16707498, PubMed:17071116). Associates with F-actin to regulate actin distribution in erythroblasts and macrophages (PubMed:16707498). May contribute to nuclear architecture and cells division events (By similarity). S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Identified in the CTLH complex that contains GID4, RANBP9 and/or RANBP10, MKLN1, MAEA, RMND5A (or alternatively its paralog RMND5B), GID8, ARMC8, WDR26 and YPEL5. Within this complex, MAEA, RMND5A (or alternatively its paralog RMND5B), GID8, WDR26, and RANBP9 and/or RANBP10 form the catalytic core, while GID4, MKLN1, ARMC8 and YPEL5 have ancillary roles (By similarity). Interacts with F-actin (PubMed:16707498). Detected in a nuclear, speckled-like pattern (PubMed:17071116). Localized with condensed chromatin at prophase; Detected in nuclear spindle poles at metaphase and in the contractile ring during telophase and cytokinesis (By similarity). Present in cytoplasm, nuclear matrix and at the cell surface in macrophages; predominantly nuclear in immature macrophages and predominantly detected at the cell surface in mature macrophages (PubMed:17071116). Colocalizes with F-actin in macrophages (PubMed:16707498). Detected in embryonic fibroblasts (PubMed:16707498). Detected in macrophages (PubMed:17071116). Detected in heart. liver, spleen and kidney (at protein level) (PubMed:16510120). The expected RING-type zinc finger domain is highly divergent and most of the expected Cys residues are not conserved. Still, the protein is required for CTLH complex E3 ubiquitin-protein transferase activity. In addition, the conserved Cys-314 in this highly divergent region is required for ubiquitination by the yeast GID complex, suggesting a direct role in catalyzing ubiquitination. Autoubiquitinated as component of the CTLH E3 ubiquitin-protein ligase complex (in vitro). Mice die perinatally and exhibit profound alterations in the hematopoietic system. Blood of 14.5-16.5 dpc embryos contained mostly nucleated erythrocytes suggesting a defect in terminal maturation and enucleation of precursor cells. In the fetal liver, large immature erythroblasts predominate, macrophage exhibit an immature morphology and their number is reduced; No erythroblasts are attached to the macrophages suggesting impairment of erythroblastic island formation.