UBL3

UBL3 Full Name

ubiquitin-like 3

UBL3 Introduction

UBL3, also called membrane-anchored ubiquitin-fold protein (MUB) or HCG-1, is a ~15 kDa ubiquitin-like protein distinguished by a C-terminal CAAX prenylation motif. Unlike canonical ubiquitin or UBLs, UBL3 is lipidated at this motif, anchoring it to membranes rather than being conjugated to substrates. It was first identified as a novel ubiquitin-like gene through cloning and chromosomal mapping studies. The prenylation and resulting membrane localization actually prevent UBL3 from attaching to target proteins; instead, the protein itself functions as a post-translational modifier that directs selected cargo into small extracellular vesicles (sEVs). Like other CAAX-containing proteins, the C-terminal motif of UBL3 is geranylgeranylated, a lipid modification that inserts the protein into the cytosolic leaflet of membranes and is essential for its vesicular function, because only the lipid-anchored form is efficiently incorporated into budding vesicles. The MUB family to which UBL3 belongs is conserved from plants to animals, pointing to an ancient strategy for coupling a ubiquitin fold to membrane tethering.

Figure 1. UBL3 as a new protein-sorting factor, joining Ub and UBL. (Source: Takanashi Y, et al. 2022)

Functionally, UBL3 has emerged as a protein-sorting factor for sEVs, and UBL3 knockout markedly reduces sEV protein content, underscoring its central role in loading vesicles with specific clients such as oncogenic Ras. In the immune system, UBL3 is required for MARCH1-mediated ubiquitination of MHC class II and CD86 in dendritic cells and macrophages, directly influencing antigen presentation to T cells. The requirement for UBL3 in MARCH1 ubiquitination highlights a direct link between a ubiquitin-like sorting factor and adaptive immune regulation. Structural and cell-biological work positions UBL3 within a family of MUB proteins that bridge membrane localization and vesicular trafficking, illustrating a non-canonical branch of the ubiquitin-fold world in which the fold is repurposed for lipid anchoring and cargo sorting rather than proteolytic signaling. By coupling a ubiquitin-like scaffold to prenylation, UBL3 provides a concise example of how UBL domains can be co-opted for membrane biology.

Alternate Names for UBL3

UBL3; ubiquitin-like 3; PNSC1; ubiquitin-like protein 3; DKFZP434K151; FLJ32018; HCG 1; DKFZp434K151; FLJ32018; HCG 1; HCG1; HsMUB; Membrane-anchored ubiquitin-fold protein; MUB; OTTHUMP00000018193; PNSC1; Protein HCG 1; Protein HCG-1; Ubiquitin like 3; U

• 1. Takanashi Y, et al. Ubiquitin-like 3 as a new protein-sorting factor for small extracellular vesicles. Cell Struct Funct. 2022;47(1):1-18.
  2. Chadwick BP, et al. Cloning, mapping and expression of UBL3, a novel ubiquitin-like gene. Gene. 1999 Jun 11;233(1-2):189-95.