The selective export of proteins and lipids from the endoplasmic reticulum (ER) is mediated by the coat protein complex II (COPII) that assembles onto the ER membrane. In higher eukaryotes, COPII proteins assemble at discrete sites on the membrane known as ER exit sites (ERES). Here, we identify Sec16 as the protein that defines ERES in mammalian cells. Sec16 localizes to ERES independent of Sec23/24 and Sec13/31. Overexpression, and to a lesser extent, small interfering RNA depletion of Sec16, both inhibit ER‐to‐Golgi transport suggesting that Sec16 is required in stoichiometric amounts. Sar1 activity is required to maintain the localization of Sec16 at discrete locations on the ER membrane, probably through preventing its dissociation. Our data suggest that Sar1‐GTP‐dependent assembly of Sec16 on the ER membrane forms an organized scaffold defining an ERES.

The COPII complex mediates the selective incorporation of secretory cargo and relevant machinery into budding vesicles at specialised sites on the endoplasmic reticulum membrane called transitional ER (tER). Here, we show using confocal microscopy, immunogold labelling of ultrathin cryosections and electron tomography that in human cells at steady state, Sec16 localises to cup-like structures of tER that are spatially distinct from the localisation of other COPII coat components. We show that Sec16 defines the tER, whereas Sec23-Sec24 and Sec13-Sec31 define later structures that precede but are distinct from the intermediate compartment. Steady-state localisation of Sec16 is independent of the localisation of downstream COPII components Sec23-Sec24 and Sec13-Sec31. Sec16 cycles on and off the membrane at a slower rate than other COPII components with a greater immobile fraction. We define the region of Sec16A that dictates its robust localisation of tER membranes and find that this requires both a highly charged region as well as a central domain that shows high sequence identity between species. The central conserved domain of Sec16 binds to Sec13 linking tER membrane localisation with COPII vesicle formation. These data are consistent with a model where Sec16 acts as a platform for COPII assembly at ERES.

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Transport of proteins from the endoplasmic reticulum (ER) to the Golgi is mediated by the sequential action of two coat complexes: COPII concentrates cargo for secretion at ER export sites, then COPI is subsequently recruited to nascent carriers and retrieves recycling proteins back to the ER. These carriers then move towards the Golgi along microtubules, driven by the dynein/ dynactin complexes. Here we show that the Sec23p component of the COPII complex directly interacts with the dynactin complex through the carboxy-terminal cargo-binding domain of p150 Glued . Functional assays, including measurements of the rate of recycling of COPII on the ER membrane and quantitative analyses of secretion, indicate that this interaction underlies functional coupling of ER export to microtubules. Together, our data suggest a mechanism by which membranes of the early secretory pathway can be linked to motors and microtubules for subsequent organization and movement to the Golgi apparatus.The COPII complex cycles between the cytosol and discrete sites on the ER membrane1. Directed assembly of COPII on the ER membrane is initiated by GDP/GTP exchange on the small GTPase Sar1p, catalysed by Sec12p2. This results in the sequential recruitment of two binary complexes, Sec23p-Sec24p and Sec13p-Sec31p3. Sec23p-Sec24p acts as a GTPaseactivating protein for Sar1p as well as having a direct role in cargo binding2. Sec13p-Sec31p provides the structural scaffold for membrane deformation and vesicle formation2. ER-to-Golgi transport in mammalian cells proceeds by concentration of cargo into COPIIcoated ER export sites (ERES)2, followed by formation of vesicular-tubular transport carriers (VTC)4,5, which then move in a COPI-dependent4 and dynein/dynactin-dependent6 manner along microtubule tracks towards the Golgi. The movement of membranes along cytoskeletal tracks largely determines the directionality and efficiency of membrane traffic in mammalian cells. The microtubule and actin cytoskeletons coordinate many membrane traffic processes7, notably the movement of post-Golgi membranes, endosomes and retrograde transport carriers. There is considerable evidence for microtubules functioning in cargo export from the ER and subsequent VTC motility. ERES align along microtubules in certain cell types8, Golgi-directed VTCs localize along a population of stable microtubules9,10 and translocate along microtubules towards the Golgi in a dynactin-

Young people presenting with acne are at increased risk of depression, anxiety and suicide attempts. Attention should be paid to their mental health, and the importance of asking directly regarding suicide is emphasised.

Cells expressing ricin B chain within the secretory pathway are significantly more resistant to intoxication by ricin holotoxin but not to other cytotoxins that exploit similar endocytic routes to the cytosol. Furthermore, cells expressing the related B chain of abrin are protected against both incoming abrin and ricin. These phenotypes can be correlated with the abilities of the respective B chains to form disulphide-linked A-B holotoxins, since abrin B chain forms heterodimers with either abrin or ricin A chains, whereas ricin B chain forms heterodimers with ricin A chain only. In the ricin B-expressing cells, this newly made lectin disappears with biphasic kinetics comprising a retention phase followed by slow turnover and disposal after disengagement from calnexin cycle components. Interference with ricin cytotoxicity occurs during the early retention phase when ricin B chain is associated with PDI (protein disulphide-isomerase). The data show that retrotranslocation of incoming toxin is impeded by PDI-catalysed formation of heterodimers between endogenous B and A chains derived from reduced holotoxin, thus proving that reduction of ricin occurs in the endoplasmic reticulum. In contrast with other toxins, ricin does not appear to require either proteolytic cleavage or unfolding for PDI-catalysed reduction.

A major unmet clinical need is a universal method for subcellular targeting of bioactive molecules to lysosomes. Delivery to this organelle enables either degradation of oncogenic receptors that are overexpressed in cancers, or release of prodrugs from antibody–drug conjugates. Here, we describe a general method that uses receptor crosslinking to trigger endocytosis and subsequently redirect trafficking of receptor:cargo complexes from their expected route, to lysosomes. By incubation of plasma membrane receptors with biotinylated cargo and subsequent addition of streptavidin to crosslink receptor:cargo–biotin complexes, we achieved rapid and selective lysosomal targeting of transferrin, an anti-MHC class I antibody, and the clinically approved anti-Her2 antibody trastuzumab. These three protein ligands each target a receptor with a distinct cellular function and intracellular trafficking profile. Importantly, we confirmed that crosslinking of trastuzumab increased lysosomal degradation of its cognate oncogenic receptor Her2 in breast cancer cell lines SKBR3 and BT474. These data suggest that crosslinking could be exploited for a wide range of target receptors, for navigating therapeutics through the endolysosomal pathway, for significant therapeutic benefit.

Summary Microtubule dynamics and function are regulated, at least in part, by a family of proteins that localize to microtubule plus-ends, and include EB1, CLIP-170 and the dynactin component p150Glued. Plus-end pools of these proteins, notably dynactin, have been invoked in a number of ‘search-and-capture’ mechanisms, including the attachment of microtubules to kinetochores during mitosis and to endomembranes prior to the initiation of intracellular transport. Here we show that, in mammalian cells, EB1 is required for the plus-end localization of CLIP-170, and that this is in turn required to localize p150Glued to plus-ends. Specific depletion of CLIP-170 results in defects in microtubule dynamics, cell polarization in response to scratch wounding and a loss of p150Glued from plus ends. By contrast, removal of p150Glued from plus-ends by depletion of either EB1 or CLIP-170 caused no defects in the localization of intracellular organelles, the dynamics of ER-to-Golgi transport, the efficiency of transferrin uptake or the motility of early endosomes or lysosomes. In addition to labelling microtubule plus-ends, we show that GFP-p150Glued becomes incorporated into the dynactin complex and labels small, highly dynamic, punctate structures that move along microtubules. A subset of these structures colocalizes with ER-Golgi transport intermediates. Together, these data show that the function of CLIP-170 and p150Glued in membrane trafficking is not associated with their plus-end localization.