E3 ubiquitin (Ub) ligases play critical roles in target selection for degradation and can trigger signal transduction, membrane trafficking, DNA damage repair, and membrane repair1,2. They are classified into three different types depending on the mechanism of Ub transfer: the Really Interesting New Gene (RING), Homolgous to E6-AP Carboxyl Terminus (HECT), and RING-Between-RING (RBR) Ub ligases3,4. Among these classes, RING is dominant, and one of the most abundant families of RING-type Ub ligases comprise the TRIpartite Motif (TRIM)-containing proteins, which contains more than 80 members among the 600 Ub ligases found in humans5,6.
Members of the TRIM superfamily contain multimodular domains7, including relatively conserved N-terminal RBCC (RING, B-box, and coiled-coil) and variable C-terminal domains (Fig. 1a). Although they have a similar architecture, their functions are diverse and involve many cellular processes, such as retroviral restriction (TRIM5, TRIM22, and TRIM25)8,9,10,11, DNA damage response (TRIM24)12, gene silencing (TRIM28 and TRIM71)13,14, autophagy (TRIM32)15, and membrane repair (TRIM72)2. As diverse as their roles, mutations in TRIM genes cause various genetic disorders, including Mulibrey Nanism (TRIM37)16, Sjögren’s syndrome (TRIM21/Ro52)17,18,19,20, Opitz G/BBB syndrome (TRIM18/MID1)21,22, familial Mediterranean fever (TRIM20/pyrin)23, acute promyelocytic leukemia (TRIM19/PML)24, and muscular dystrophy (TRIM32)25. Despite the increasing number of genetic and cellular studies, biochemical and structural evidence are still limited, and only domain structures are available26,27,28,29,30,31,32. We recently presented dimeric and oligomeric structures of TRIM72, in which oligomerization was found to be coupled to ubiquitylation and phospholipid membrane recognition33.
TRIM72, also known as mitsugumin 53 (MG53), is expressed mainly in muscles34,35,36 and is a key initiator of the plasma membrane repair machinery following acute membrane damage, such as that induced by ischemia/reperfusion injury2,37,38. Furthermore, recombinant TRIM72 has shown therapeutic potential for a variety of muscle and non-muscle tissue injuries36. In previous studies, the oligomerization of TRIM72 was shown to be a key process for membrane repair and activation31,33. TRIM72 facilitates negatively charged small repair vesicles to injury sites on the plasma membrane. The TRIM72-bound repair vesicles are assumed to accumulate at the injury site where they form a membrane patch to seal holes in the plasma membrane33,39. In addition to membrane repair, TRIM72 participates in diverse cellular processes that occur in the membrane environment, such as filopodia-like structures, exocytosis, caveolar structures, and lipid rafts34,40,41. It is unknown whether the TRIM72 oligomer can sense or induce specific features of membrane formation, such as convex or concave surfaces. Therefore, this study aimed to comprehensively analyze the coiled-coil structure of TRIM72 and to visualize TRIM72-bound liposomes using cryo-transmission electron microscopy (cryo-TEM). The results indicated that TRIM72 can sense any type of membrane curvature to induce membrane patch formation via self-oligomerization.