Liquid droplets function as membraneless organelles that compartmentalize and facilitate efficient biological reactions. They are formed by proteins with an intrinsically disordered region(s) (IDR) via liquid-liquid phase separation. Mieap/SPATA18, a p53-inducible protein, plays a critical role in the suppression of human and murine colorectal tumors via mitochondrial quality control. However, the regulatory mechanism underlying this process remains unclear. Here, we report that Mieap is an IDR-containing protein that drives the formation of liquid droplets in the mitochondria. Mieap liquid droplets (MLDs) specifically phase separate the mitochondrial phospholipid cardiolipin. Lipidomic analysis of cardiolipin suggested that Mieap promotes enzymatic reactions involved in cardiolipin metabolism, including biosynthesis and remodeling. Accordingly, four cardiolipin biosynthesis enzymes, TAMM41, PGS1, PTPMT1, and CRLS1, and two remodeling enzymes, PLA2G6 and TAZ, are phase separated by MLDs. Mieap-deficient mice exhibited altered cristae structure in the liver and kidney mitochondria and a trend of obesity. These results suggest that Mieap drives the formation of membraneless organelles to compartmentalize and promotes cardiolipin metabolism at the inner mitochondrial membrane, thus playing a possible role in mitochondrial quality control.

Liquid droplets function as membraneless organelles that compartmentalize and facilitate efficient biological reactions. They are formed by proteins with intrinsically disordered regions (IDRs) via liquid–liquid phase separation. Mieap/SPATA18, a p53-inducible protein, participates in suppression of colorectal tumors by promoting mitochondrial quality control. However, the regulatory mechanism involved remains unclear. Here, we report that Mieap is an IDR-containing protein that drives formation of liquid droplets in mitochondria. Mieap liquid droplets specifically phase separate the mitochondrial phospholipid, cardiolipin. Lipidomic analysis of cardiolipin suggests that Mieap promotes enzymatic reactions involved in cardiolipin metabolism, including biosynthesis and remodeling. Accordingly, four cardiolipin biosynthetic enzymes, TAMM41, PGS1, PTPMT1, and CRLS1, and two remodeling enzymes, PLA2G6 and TAZ, are phase-separated by Mieap liquid droplets. Mieap-deficient mice exhibit altered crista structure in mitochondria of various tissues, including brown fat, and tend to become obese. These results suggest that Mieap drives formation of membraneless organelles to compartmentalize and promote cardiolipin metabolism at the inner mitochondrial membrane, thus potentially contributing to mitochondrial quality control.

Biomolecular condensates function as membraneless organelles that compartmentalize and facilitate efficient biological reactions. They are formed by proteins with intrinsically disordered regions (IDRs) via liquid–liquid phase separation. Mieap/SPATA18, a p53-inducible protein, participates in suppression of colorectal tumors by promoting mitochondrial quality control. However, the regulatory mechanism involved remains unclear. Here, we report that Mieap is an IDR-containing protein that drives formation of biomolecular condensates in mitochondria. Mieap biomolecular condensates specifically phase separate the mitochondrial phospholipid, cardiolipin. Lipidomic analysis of cardiolipin suggests that Mieap promotes enzymatic reactions involved in cardiolipin metabolism, including biosynthesis and remodeling. Accordingly, four cardiolipin biosynthetic enzymes, TAMM41, PGS1, PTPMT1, and CRLS1, and two remodeling enzymes, PLA2G6 and TAZ, are phase-separated by Mieap biomolecular condensates. Mieap-deficient mice exhibit altered crista structure in mitochondria of various tissues, including brown fat, and tend to become obese. These results suggest that Mieap drives formation of membraneless organelles to compartmentalize and promote cardiolipin metabolism at the inner mitochondrial membrane, thus potentially contributing to mitochondrial quality control.