Silencing of mammalian Sar1 isoforms reveals COPII-independent protein sorting and transport.
Cutrona MB, Beznoussenko GV, Fusella A, Martella O, Moral P, Mironov AA.
This paper describes studies of trafficking of secretory cargo from the ER in cells depleted of the small GTPase Sar1, an essential component of the COPII coat. A COPII-independent mechanism of secretory trafficking would mark a step-change in our understanding of secretory protein trafficking in cells. While we do not think that this paper requires such a re-evaluation of our thinking, it does provide some thought provoking data. By disrupting the normal mechanism of transport the authors seek to reveal COPII-independent mechanisms and challenge the primacy of COPII-dependent transport. The most important point to bear in mind when reading this paper is that siRNA knock-downs are unable to generate a complete depletion of any protein and that residual Sar1 may influence some of the results seen. Indeed in our opinion, one cannot conclusively draw the conclusion that COPII-independent ER export pathways exist in cells from this work alone.
The authors confirm that Sar1-depletion effectively inhibits COPII vesicle formation (demonstrated by EM and reduced immunofluorescence staining of COPII components). They see changes to both ER-Golgi intermediate compartment (ERGIC) formation (the compartment between the Golgi and the ER) and Golgi organisation (resulting in formation of mini-stacks), also in agreement with a reduction in secretory trafficking. Despite this, pulse-chase experiments show no reduction in the total amount of protein secretion in Sar1-depleted cells. From this, the authors conclude that alternative transport mechanisms are being used. The virus glycoprotein VSV-G also passes from the ER to the plasma membrane in Sar1-depleted cells, even when additionalCOPII subunits (Sec23A and B) are depleted in addition to Sar1A and B. Based on these and other experiments, the authors propose a COPI-dependent replacement pathway that takes VSV-G to the Golgi. Export of procollagen transport is inhibited by Sar1-depletion, but as collagens require specialized COPII-coated vesicles to accommodate their large size, they therefore are not representative of typical COPII trafficking (Jin et al., 2012). Furthermore, many other studies support the notion that procollagen secretion is exquisitely sensitive to perturbation of the early secretory pathway (Smits et al., 2010; Townley et al., 2008; Venditti et al., 2012).
These data do support the concept of a “short-loop” ER-to-Golgi trafficking pathway that involves juxtanuclear ER and possible even ER-Golgi contact sites. It is as yet unclear how possible mechanisms such as kiss and run could retainsufficient selectivity to prevent non-selective transport. It remains possible however that the small remaining amount of COPII proteins in the RNAi experiments described here is sufficient to direct COPII-dependent selectivity.
In summary the paper presents some interesting findings. Complete removal of Sar1 from cells by gene deletion is required to examine fully the existence ofCOPII-independent trafficking pathways and the relative importance of COPII-trafficking in cells.
Jin, L., K.B. Pahuja, K.E. Wickliffe, A. Gorur, C. Baumgärtel, R. Schekman, and M. Rape. 2012. Ubiquitin-dependent regulation of COPII coat size and function. Nature 482(7386):495-500. doi: 10.1038/nature10822
Smits, P., A.D. Bolton, V. Funari, M. Hong, E.D. Boyden, L. Lu, D.K. Manning, N.D. Dwyer, J.L. Moran, M. Prysak, B. Merriman, S.F. Nelson, L. Bonafe, A. Superti-Furga, S. Ikegawa, D. Krakow, D.H. Cohn, T. Kirchhausen, M.L. Warman, and D.R. Beier. 2010. Lethal skeletal dysplasia in mice and humans lacking the golginGMAP-210. N. Engl. J. Med. 362:206-216.
Townley, A.K., Y. Feng, K. Schmidt, D.A. Carter, R. Porter, P. Verkade, and D.J. Stephens. 2008. Efficient coupling of Sec23-Sec24 to Sec13-Sec31 drives COPII-dependent collagen secretion and is essential for normal craniofacial development. J. Cell Sci. 121:3025-3034.
Full disclosure: David is a member of the “Traffic” editorial board but had not role in the editing or reviewing of this paper.
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