A model for collagen secretion by intercompartmental continuities.

Newly synthesized secretory proteins are exported from the 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 ER-Golgi intermediate compartment (ERGIC), in TANGO1-dependent interorganelle tunnels. In the theoretical model presented here, we suggest that intrinsically disordered domains of TANGO1 in the ER lumen induce an entropic contraction, which exerts a force that draws procollagen toward the ERES. Within this framework, molecular gradients of pH and/or HSP47 between the ER and ERGIC create a force in the order of tens of femto-Newtons. This force is substantial enough to propel procollagen from the ER at a speed of approximately 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. Hence, the mechanism we propose is theoretically adequate to explain how cells can utilize molecular gradients and export procollagens at a rate commensurate with physiological needs.

Membrane Tubulation with a Biomembrane Force Probe.

Tubulation is a common cellular process involving the formation of membrane tubes ranging from 50 nm to 1 µm in diameter. These tubes facilitate intercompartmental connections, material transport within cells and content exchange between cells. The high curvature of these tubes makes them specific targets for proteins that sense local geometry. In vitro, similar tubes have been created by pulling on the membranes of giant unilamellar vesicles. Optical tweezers and micromanipulation are typically used in these experiments, involving the manipulation of a GUV with a micropipette and a streptavidin-coated bead trapped in optical tweezers. The interaction forms streptavidin/biotin bonds, leading to tube formation. Here, we propose a cost-effective alternative using only micromanipulation techniques, replacing optical tweezers with a Biomembrane Force Probe (BFP). The BFP, employing a biotinylated erythrocyte as a nanospring, allows for the controlled measurement of forces ranging from 1 pN to 1 nN. The BFP has been widely used to study molecular interactions in cellular processes, extending beyond its original purpose. We outline the experimental setup, tube formation and characterization of tube dimensions and energetics, and discuss the advantages and limitations of this approach in studying membrane tubulation.