α1-Antitrypsin is a serine protease inhibitor produced in the liver that is responsible for the regulation of pulmonary inflammation. The commonest pathogenic gene mutation yields Z-α1-antitrypsin, which has a propensity to self-associate forming polymers that become trapped in inclusions of endoplasmic reticulum (ER). It is unclear whether these inclusions are connected to the main ER network in Z-α1-antitrypsin-expressing cells. Using live cell imaging, we found that despite inclusions containing an immobile matrix of polymeric α1-antitrypsin, small ER resident proteins can diffuse freely within them. Inclusions have many features to suggest they represent fragmented ER, and some are physically separated from the tubular ER network, yet we observed cargo to be transported between them in a cytosol-dependent fashion that is sensitive to N-ethylmaleimide and dependent on Sar1 and sec22B. We conclude that protein recycling occurs between ER inclusions despite their physical separation.-Dickens, J. A., Ordóñez, A., Chambers, J. E., Beckett, A. J., Patel, V., Malzer, E., Dominicus, C. S., Bradley, J., Peden, A. A., Prior, I. A., Lomas, D. A., Marciniak, S. J. The endoplasmic reticulum remains functionally connected by vesicular transport after its fragmentation in cells expressing Z-α1-antitrypsin.J.A.D. was funded by a Medical Research Council (MRC) Clinical Research Training Fellowship and a starter grant from the Academy of Medical Sciences; S.J.M. is a MRC Senior Clinical Research Fellow. D.A.L. is funded by the MRC and by the University College London Hospitals National Institute for Health Research (NIHR) Biomedical Research Centre (London, United Kingdom), and is an NIHR Senior Investigator. The work was also supported by the Alpha1 Foundation. The Cambridge Institute for Medical Research microscopy core facility is supported by a Wellcome Trust Strategic Award (100140) and a Wellcome Trust equipment grant (093026).This is the final version of the article. It first appeared from the Federation of American Societies for Experimental Biology via https://doi.org/10.1096/fj.201600430
