Electron microscopy reveals multibudded tubules
Because the diameter of COPII-coated tubules is below the optical resolution of confocal microscopy, we used EM for a more detailed examination. Cryo-EM revealed that the COPII-coated tubules carry symmetric constrictions at regular intervals (Fig. 3). In other words, they consist of unfissioned vesicles, like beads on a string. The size of the connected vesicles in cryo-EM was rather homogeneous (Fig. 3a–h), although variations occurred, for instance at the end of a string (Fig. 3e). Apart from the prominent connected protein-coated vesicles, single vesicles as well as larger liposomes were observed (Fig. 3a,d–h).
Most beads-on-a-string vesicles were linearly connected, with occasional branches. Clover shapes, which appeared as branches, actually comprised two crossed-over tubules (Fig. 3a,b,f). Some of the constricted tubules deposited on the carbon support of the EM grid were up to several micrometers long and completely straight (Fig. 3f), whereas others apparently became entangled during the preparation (Fig. 3a). Except for the constrictions, tubules were mostly uniform in thickness. The appearance of the tubules in cryo-EM hence agreed well with their appearance by confocal microscopy. Furthermore, the cryo-EM images clearly showed protein coat on the lipid bilayer (Fig. 3b,d,e, arrows), even at the ‘necks' between bulges, explaining the great persistence length. Whereas the typical strings of vesicles were all coated with protein, some of the larger liposomes appeared coated (Fig. 3d,e arrows) and others not (Fig. 3e, arrowhead). This observation seems to suggest that - despite the preference of the COPII coat for high-curvature structures seen by fluorescence – COPII proteins have some flexibility for binding to lower curvature membranes. The ability of the COPII coat to bind to membranes of different curvatures merits further investigation to determine how COPII cages form on ‘oversized' cargo such as procollagen and chylomicrons1213.