A transferrin receptor-based vector enables robust Type-II membrane protein display on mammalian cells

Cell surface protein display is widely used in biotechnology as a platform for interrogating and engineering protein function in mammalian systems. Current display systems, including pDisplay™, are based on type-I membrane topology and enforce an extracellular N-terminus and cytosolic C-terminus. While suitable for secreted proteins and antibody fragments, they are incompatible with type-II membrane proteins, which instead possess an inverted topology. Despite encompassing an important class of proteins, type-II proteins lack robust and generalisable surface display tools. Here, we report a transferrin receptor 1 (TfR1)-based vector for type-II membrane protein display in mammalian cells. By leveraging the native cytoplasmic and transmembrane architecture of TfR1, the system enforces a correct type-II topology and enables efficient trafficking to the plasma membrane. Using TMEM106b as a representative type-II protein, we show that TfR1 fusion drives robust cell-surface localisation, confirmed by fluorescence-activated cell sorting and confocal microscopy.