Has E3 ubiquitin ligase activity towards IGBP1, promoting its monoubiquitination, which results in deprotection of the catalytic subunit of protein phosphatase PP2A, and its subsequent degradation by polyubiquitination. {ECO:0000269|PubMed:10400985, ECO:0000269|PubMed:11685209, ECO:0000269|PubMed:22613722}. This is the function of MID1 (midline 1, Ensembl gene identifier ENSG00000101871).