Involved in sterol-regulated ubiquitination and degradation of HMG-CoA reductase HMGCR (PubMed:21343306). Involved in positive regulation of AMPA-selective glutamate receptor GRIA2 recycling to the cell surface (By similarity). Acts as negative regulator of hepatocyte growth during regeneration (By similarity). May contribute to the regulation of translation during cell-cycle progression. May contribute to the regulation of cell proliferation (By similarity). May be involved in centrosome assembly. Modulates stabilization and nucleolar localization of tumor suppressor CDKN2A and enhances association between CDKN2A and NPM1 (By similarity). Interacts with EEF1A1, GRIA2, GRIP1, CAMLG, TUBG1 (By similarity). Interacts with NPM1 and CDKN2A; TMUB1 can enhance interaction between NPM1 and CDKN2A and is proposed to bridge the proteins; proposed to be mediated by iHOPS (By similarity). Interacts with ERLIN2 and AMFR; TMUB1 promotes the interaction of ERLIN2 with AMFR (PubMed:21343306). iHOPS is proposed to be the shuttling form across different cellular compartments. XPO1-dependent exported from the nucleus in dividing cells. Predominantly nuclear during growth arrest. Ubiquitously expressed with highest levels in mammary and thyroid glands, bone marrow and spleen; limited expression in cardiac, pancreatic and ovarian tissues. Processed by regulated intramembrane proteolysis (RIP)in the N-terminus to release iHOPS from membranes (By similarity).