A model for collagen secretion by intercompartmental continuities

Newly synthesized secretory proteins are exported from endoplasmic reticulum (ER) at specialized subcompartments called exit sites (ERES). Cargoes like procollagen are too large for export by the standard COPII-coated vesicle of 60 nm average diameter. We have previously suggested that procollagen is transported from the ER to the next secretory organelle, the ERGIC, in TANGO1-dependent inter-organelle tunnels. Here, we show that intrinsically disordered domains of TANGO1 in the ER lumen generate an entropic contraction that pulls procollagen towards the ERES. Molecular gradients of pH and HSP47 between the ER and ERGIC generate a force in the range of tens of femtoNewtons (fN), which is sufficient to propel procollagen from the ER at a speed of [~]1 nm.s-1. This calculated speed and the quantities of collagen secreted are similar to its observed physiological secretion rate in fibroblasts, consistent with the proposal that ER export is the rate limiting step for procollagen secretion. Our theoretical model explains how cells can utilize molecular gradients to export procollagens at a rate commensurate with physiological needs. Significance StatementProcollagen cannot be exported from the endoplasmic reticulum (ER) by standard COPII-coated vesicle of 60 nm average diameter. We have previously suggested that collagen is transported from the ER to the next secretory organelle, the ERGIC, in TANGO1-dependent inter-organelle tunnels. ER and ERGIC differ in molecular composition including their pH and protein composition. We propose a mechanical/entropic ratchet model whereby molecular gradients of pH and the collagen chaperone HSP47, provide the energy to propel procollagen from the ER at a speed that matches the physiological rate of collagen secretion.