The length of cargo-protein transmembrane segments drives secretory transport by facilitating cargo concentration in export domains

The cellular destination of secretory proteins is determined by interactions of their targeting motifs with coat-protein complexes. The transmembrane domain (TMD) of secretory proteins also plays a central role in their transport and targeting. However, a comprehensive model that considers both TMD- and targeting-sequence-mediated transport has never been advanced. We focused on the secretory transport of two fluorescently tagged membrane proteins: vesicular stomatitis virus G tsO45 (VSVG), which is a cargo protein that is a thermoreversible mutant, and the Golgi-resident protein GalT-CFP. A quantitative approach was applied to analyze, in living cells, secretory transport dynamics, as well as cargo concentration of YFP-tagged VSVG mutants with one, three, five, seven, eight or nine amino acids deleted from their TMD, as well as two or four amino acids added to their TMD. Changes in TMD length affected secretory transport dynamics and the extent of cargo concentration in the ER exit sites, demonstrating that the capacity of the transport machinery to concentrate cargo depends on the length of the TMD of the cargo protein.