Coat proteins in intracellular membrane transport.

Transport of newly synthesized material from the endoplasmic reticulum (ER) towards the Golgi complex, through the Golgi cisternae, and out of the trans-Golgi network (TGN) is thought to be mediated by vesicular carriers. Different types of vesicle are involved in this biosynthetic membrane traffic. All are coated with protein complexes on their cytoplasmic surface. COP-coated vesicles have recently been implicated in transport of cargo from the ER to the TGN, and clathrin-coated vesicles from the TGN to endosomes, but the carriers moving material to the cell surface are still unknown. Sequence homologies between subunits of the COP- and the clathrin-adaptor complexes suggest that coat proteins may belong to a family of proteins with related functions. The precise role of the coat proteins is not fully understood, although they have been implicated in clustering of cargo into buds and in budding of vesicles. In addition, coat proteins may play an essential role in targeting of transport intermediates and may serve to regulate membrane fusion.

Evidence for a COP-I-independent transport route from the Golgi complex to the endoplasmic reticulum.

The cytosolic coat-protein complex COP-I interacts with cytoplasmic 'retrieval' signals present in membrane proteins that cycle between the endoplasmic reticulum (ER) and the Golgi complex, and is required for both anterograde and retrograde transport in the secretory pathway. Here we study the role of COP-I in Golgi-to-ER transport of several distinct marker molecules. Microinjection of anti-COP-I antibodies inhibits retrieval of the lectin-like molecule ERGIC-53 and of the KDEL receptor from the Golgi to the ER. Transport to the ER of protein toxins, which contain a sequence that is recognized by the KDEL receptor, is also inhibited. In contrast, microinjection of anti-COP-I antibodies or expression of a GTP-restricted Arf-1 mutant does not interfere with Golgi-to-ER transport of Shiga toxin/Shiga-like toxin-1 or with the apparent recycling to the ER of Golgi-resident glycosylation enzymes. Overexpression of a GDP-restricted mutant of Rab6 blocks transport to the ER of Shiga toxin/Shiga-like toxin-1 and glycosylation enzymes, but not of ERGIC-53, the KDEL receptor or KDEL-containing toxins. These data indicate the existence of at least two distinct pathways for Golgi-to-ER transport, one COP-I dependent and the other COP-I independent. The COP-I-independent pathway is specifically regulated by Rab6 and is used by Golgi glycosylation enzymes and Shiga toxin/Shiga-like toxin-1.

Breaking the COPI monopoly on Golgi recycling.

The unexpected discovery of a transport pathway from the Golgi to the endoplasmic reticulum (ER) independent of COPI coat proteins sheds light on how Golgi resident enzymes and protein toxins gain access to the ER from as far as the trans Golgi network. This new pathway provides an explanation for how membrane is recycled to allow for an apparent concentration of anterograde cargo at distinct stages of the secretory pathway. As signal-mediated COPI-dependent recycling also involves the concentration of resident proteins into retrograde COPI vesicles, the main bulk of lipids must be recycled, possibly through a COPI-independent pathway.

Intracellular movement of green fluorescent protein-tagged phosphatidylinositol 3-kinase in response to growth factor receptor signaling.

Phosphatidylinositol 3-kinase (PI 3-kinase) is a lipid kinase which has been implicated in mitogenesis, protein trafficking, inhibition of apoptosis, and integrin and actin functions. Here we show using a green fluorescent protein-tagged p85 subunit that phosphatidylinositol 3-kinase is distributed throughout the cytoplasm and is localized to focal adhesion complexes in resting NIH-3T3, A431, and MCF-7 cells. Ligand stimulation of an epidermal growth factor receptor/c-erbB-3 chimera expressed in these cells results in a redistribution of p85 to the cell membrane which is independent of the catalytic activity of the enzyme and the integrity of the actin cytoskeleton. The movement is, however, dependent on the phosphorylation status of the erbB-3 chimera. Using rhodamine-labeled epidermal growth factor we show that the phosphatidylinositol 3-kinase and the receptors colocalize in discrete patches on the cell surface. Low concentrations of ligand cause patching only at the periphery of the cells, whereas at high concentrations patches were seen over the whole cell surface. Using green fluorescent protein-tagged fragments of p85 we show that binding to the receptor requires the NH(2)-terminal part of the protein as well as its SH2 domains.

An ordered inheritance strategy for the Golgi apparatus: visualization of mitotic disassembly reveals a role for the mitotic spindle.

During mitosis, the ribbon of the Golgi apparatus is transformed into dispersed tubulo-vesicular membranes, proposed to facilitate stochastic inheritance of this low copy number organelle at cytokinesis. Here, we have analyzed the mitotic disassembly of the Golgi apparatus in living cells and provide evidence that inheritance is accomplished through an ordered partitioning mechanism. Using a Sar1p dominant inhibitor of cargo exit from the endoplasmic reticulum (ER), we found that the disassembly of the Golgi observed during mitosis or microtubule disruption did not appear to involve retrograde transport of Golgi residents to the ER and subsequent reorganization of Golgi membrane fragments at ER exit sites, as has been suggested. Instead, direct visualization of a green fluorescent protein (GFP)-tagged Golgi resident through mitosis showed that the Golgi ribbon slowly reorganized into 1-3-micron fragments during G2/early prophase. A second stage of fragmentation occurred coincident with nuclear envelope breakdown and was accompanied by the bulk of mitotic Golgi redistribution. By metaphase, mitotic Golgi dynamics appeared to cease. Surprisingly, the disassembly of mitotic Golgi fragments was not a random event, but involved the reorganization of mitotic Golgi by microtubules, suggesting that analogous to chromosomes, the Golgi apparatus uses the mitotic spindle to ensure more accurate partitioning during cytokinesis.

Microtubules are stabilized in confluent epithelial cells but not in fibroblasts.

Rhodamine-tagged tubulin was microinjected into epithelial cells (MDCK) and fibroblasts (Vero) to characterize the dynamic properties of labeled microtubules in sparse and confluent cells. Fringe pattern fluorescence photobleaching revealed two components with distinct dynamic properties. About one-third of the injected tubulin diffused rapidly in the cytoplasm with a diffusion coefficient of 1.3-1.6 x 10(-8) cm2/s. This pool of soluble cytoplasmic tubulin was increased to greater than 80% when cells were treated with nocodazole, or reduced to approximately 20% upon treatment of cells with taxol. Fluorescence recovery of the remaining two-thirds of labeled tubulin occurred with an average half-time (t1/2) of 9-11 min. This pool corresponds to labeled tubulin associated with microtubules, since it was sensitive to treatment of cells with nocodazole and since taxol increased its average t1/2 to greater than 22 min. Movement of photobleached microtubules in the cytoplasm with rates of several micrometers per minute was shown using very small interfringe distances. A significant change in the dynamic properties of microtubules occurred when MDCK cells reached confluency. On a cell average, microtubule half-life was increased about twofold to approximately 16 min. In fact, two populations of cells were detected with respect to their microtubule turnover rates, one with a t1/2 of approximately 9 min and one with a t1/2 of greater than 25 min. Correspondingly, the rate of incorporation of microinjected tubulin into interphase microtubules was reduced about twofold in confluent MDCK cells. In contrast to the MDCK cells, no difference in microtubule dynamics was observed in sparse and confluent populations of Vero fibroblasts, where the average microtubule half-life was approximately 10 min. Thus, microtubules are significantly stabilized in epithelial but not fibroblastic cells grown to confluency.

Regulation of the cell cycle by the cdk2 protein kinase in cultured human fibroblasts.

In mammalian cells inhibition of the cdc2 function results in arrest in the G2-phase of the cell cycle. Several cdc2-related gene products have been identified recently and it has been hypothesized that they control earlier cell cycle events. Here we have studied the relationship between activation of one of these cdc2 homologs, the cdk2 protein kinase, and the progression through the cell cycle in cultured human fibroblasts. We found that cdk2 was activated and specifically localized to the nucleus during S phase and G2. Microinjection of affinity-purified anti-cdk2 antibodies but not of affinity-purified anti-cdc2 antibodies, during G1, inhibited entry into S phase. The specificity of these effects was demonstrated by the fact that a plasmid-driven cdk2 overexpression counteracted the inhibition. These results demonstrate that the cdk2 protein kinase is involved in the activation of DNA synthesis.

Transcription-dependent colocalization of the U1, U2, U4/U6, and U5 snRNPs in coiled bodies.

We have recently shown that discrete foci are present in the nuclei of mammalian cells in which each of the U1, U2, U4/U6, and U5 snRNPs involved in pre-mRNA splicing, and the non-snRNP-splicing factor U2AF, are concentrated (Carmo-Fonseca, M., D. Tollervey, R. Pepperkok, S. Barabino, A. Merdes, C. Brunner, P. D. Zamore, M. R. Green, E. Hurt, and A. I. Lamond. 1991. EMBO (Eur. Mol. Biol. Organ.) J. 10:195-206; Carmo-Fonseca, M., R. Pepperkok, B. S. Sproat, W. Ansorge, M. S. Swanson, and A. I. Lamond. 1991 EMBO (Eur. Mol. Biol. Organ.) J. 10:1863-1873). Here, we identify these snRNP-rich organelles as coiled bodies. snRNPs no longer concentrate in coiled bodies after cells are treated with the transcription inhibitors alpha-amanitin or actinomycin D. snRNP association with coiled bodies is also disrupted by heat shock. This indicates that the association of snRNPs with coiled bodies may be connected with the metabolism of nascent transcripts. A novel labeling method is described which shows both the RNA and protein components of individual snRNPs colocalizing in situ. Using this procedure all spliceosomal snRNPs are seen distributed in a nonhomogeneous pattern throughout the nucleoplasm, excluding nucleoli. They are most concentrated in coiled bodies, but in addition are present in "speckled" structures which are distinct from coiled bodies and which contain the non-snRNP splicing factor SC-35. U1 snRNP shows a more widespread nucleoplasmic staining, outside of coiled bodies and "speckled" structures, relative to the other snRNPs. The association of snRNPs with "speckles" is disrupted by heat shock but enhanced when cells are treated with alpha-amanitin.

Partitioning of the Golgi apparatus during mitosis in living HeLa cells.

The Golgi apparatus of HeLa cells was fluorescently tagged with a green fluorescent protein (GFP), localized by attachment to the NH2-terminal retention signal of N-acetylglucosaminyltransferase I (NAGT I). The location was confirmed by immunogold and immunofluorescence microscopy using a variety of Golgi markers. The behavior of the fluorescent Golgi marker was observed in fixed and living mitotic cells using confocal microscopy. By metaphase, cells contained a constant number of Golgi fragments dispersed throughout the cytoplasm. Conventional and cryoimmunoelectron microscopy showed that the NAGT I-GFP chimera (NAGFP)-positive fragments were tubulo-vesicular mitotic Golgi clusters. Mitotic conversion of Golgi stacks into mitotic clusters had surprisingly little effect on the polarity of Golgi membrane markers at the level of fluorescence microscopy. In living cells, there was little self-directed movement of the clusters in the period from metaphase to early telophase. In late telophase, the Golgi ribbon began to be reformed by a dynamic process of congregation and tubulation of the newly inherited Golgi fragments. The accuracy of partitioning the NAGFP-tagged Golgi was found to exceed that expected for a stochastic partitioning process. The results provide direct evidence for mitotic clusters as the unit of partitioning and suggest that precise regulation of the number, position, and compartmentation of mitotic membranes is a critical feature for the ordered inheritance of the Golgi apparatus.

Intracellular distribution of mammalian protein kinase A catalytic subunit altered by conserved Asn2 deamidation.

The catalytic (C) subunit of protein kinase A functions both in the cytoplasm and the nucleus. A major charge variant representing about one third of the enzyme in striated muscle results from deamidation in vivo of the Asn2 residue at the conserved NH(2)-terminal sequence myrGly-Asn-Ala (Jedrzejewski, P.T., A. Girod, A. Tholey, N. König, S. Thullner, V. Kinzel, and D. Bossemeyer. 1998. Protein Sci. 7:457-469). Because of the increase of electronegativity by generation of Asp2, it is reminiscent of a myristoyl-electrostatic switch. To compare the intracellular distribution of the enzymes, both forms of porcine or bovine heart enzyme were microinjected into the cytoplasm of mouse NIH 3T3 cells after conjugation with fluorescein, rhodamine, or in unlabeled form. The nuclear/cytoplasmic fluorescence ratio (N/C) was analyzed in the presence of cAMP (in the case of unlabeled enzyme by antibodies). Under all circumstances, the N/C ratio obtained with the encoded Asn2 form was significantly higher than that with the deamidated, Asp2 form; i.e., the Asn2 form reached a larger nuclear concentration than the Asp2 form. Comparable data were obtained with a human cell line. The differential intracellular distribution of both enzyme forms is also reflected by functional data. It correlates with the degree of phosphorylation of the key serine in CREB family transcription factors in the nucleus. Microinjection of myristoylated recombinant bovine Calpha and the Asn2 deletion mutant of it yielded N/C ratios in the same range as encoded native enzymes. Thus, Asn2 seems to serve as a potential site for modulating electronegativity. The data indicate that the NH(2)-terminal domain of the PKA C-subunit contributes to the intracellular distribution of free enzyme, which can be altered by site-specific in vivo deamidation. The model character for other signaling proteins starting with myrGly-Asn is discussed.

Dissociation of coatomer from membranes is required for brefeldin A-induced transfer of Golgi enzymes to the endoplasmic reticulum.

Addition of brefeldin A (BFA) to mammalian cells rapidly results in the removal of coatomer from membranes and subsequent delivery of Golgi enzymes to the endoplasmic reticulum (ER). Microinjected anti-EAGE (intact IgG or Fab-fragments), antibodies against the "EAGE"-peptide of beta-COP, inhibit BFA-induced redistribution of beta-COP in vivo and block transfer of resident proteins of the Golgi complex to the ER; tubulo-vesicular clusters accumulate and Golgi membrane proteins concentrate in cytoplasmic patches containing beta-COP. These patches are devoid of marker proteins of the ER, the intermediate compartment (IC), and do not contain KDEL receptor. Interestingly, relocation of KDEL receptor to the IC, where it colocalizes with ERGIC53 and ts-O45-G, is not inhibited under these conditions. While no stacked Golgi cisternae remain in these injected cells, reassembly of stacks of Golgi cisternae following BFA wash-out is inhibited to only approximately 50%. Mono- or divalent anti-EAGE stabilize binding of coatomer to membranes in vitro, at least as efficiently as GTP(gamma)S. Taken together these results suggest that enhanced binding of coatomer to membranes completely inhibits the BFA-induced retrograde transport of Golgi resident proteins to the ER, probably by inhibiting fusion of Golgi with ER membranes, but does not interfere with the disassembly of the stacked Golgi cisternae and recycling of KDEL receptor to the IC. These results confirm our previous results suggesting that COPI is involved in anterograde membrane transport from the ER/IC to the Golgi complex (Pepperkok et al., 1993), and corroborate that COPI regulates retrograde membrane transport between the Golgi complex and ER in mammalian cells.

An antibody against secretogranin I (chromogranin B) is packaged into secretory granules.

We have investigated the sorting and packaging of secretory proteins into secretory granules by an immunological approach. An mAb against secretogranin I (chromogranin B), a secretory protein costored with various peptide hormones and neuropeptides in secretory granules of many endocrine cells and neurons, was expressed by microinjection of its mRNA into the secretogranin I-producing cell line PC12. An mAb against the G protein of vesicular stomatitis virus--i.e., against an antigen not present in PC12 cells--was expressed as a control. The intracellular localization and the secretion of the antibodies was studied by double-labeling immunofluorescence using the conventional and the confocal microscope, as well as by pulse-chase experiments. The secretogranin I antibody, like the control antibody, was transported along the secretory pathway to the Golgi complex. However, in contrast to the control antibody, which was secreted via the constitutive pathway, the secretogranin I antibody formed an immunocomplex with secretogranin I, was packaged into secretory granules, and was released by regulated exocytosis. Our results show that a constitutive secretory protein, unaltered by genetic engineering, can be diverted to the regulated pathway of secretion by its protein-protein interaction with a regulated secretory protein. The data also provide the basis for immunologically studying the role of luminally exposed protein domains in the biogenesis and function of regulated secretory vesicles.

Involvement of the transmembrane protein p23 in biosynthetic protein transport.

Here, we report the localization and characterization of BHKp23, a member of the p24 family of transmembrane proteins, in mammalian cells. We find that p23 is a major component of tubulovesicular membranes at the cis side of the Golgi complex (estimated density: 12,500 copies/micron2 membrane surface area, or approximately 30% of the total protein). Our data indicate that BHKp23-containing membranes are part of the cis-Golgi network/intermediate compartment. Using the G protein of vesicular stomatitis virus as a transmembrane cargo molecule, we find that p23 membranes are an obligatory station in forward biosynthetic membrane transport, but that p23 itself is absent from transport vesicles that carry the G protein to and beyond the Golgi complex. Our data show that p23 is not present to any significant extent in coat protein (COP) I-coated vesicles generated in vitro and does not colocalize with COP I buds and vesicles. Moreover, we find that p23 cytoplasmic domain is not involved in COP I membrane recruitment. Our data demonstrate that microinjected antibodies against the cytoplasmic tail of p23 inhibit G protein transport from the cis-Golgi network/ intermediate compartment to the cell surface, suggesting that p23 function is required for the transport of transmembrane cargo molecules. These observations together with the fact that p23 is a highly abundant component in the intermediate compartment, lead us to propose that p23 contributes to membrane structure, and that this contribution is necessary for efficient segregation and transport.

KDEL receptor (Erd2p)-mediated retrograde transport of the cholera toxin A subunit from the Golgi involves COPI, p23, and the COOH terminus of Erd2p.

A cholera toxin mutant (CTX-K63) unable to raise cAMP levels was used to study in Vero cells the retrograde transport of the toxin A subunit (CTX-A-K63), which possesses a COOH-terminal KDEL retrieval signal. Microinjected GTP-gamma-S inhibits the internalization as well as Golgi-ER transport of CTX-A-K63. The appearance of CTX-A-K63 in the Golgi induces a marked dispersion of Erd2p and p53 but not of the Golgi marker giantin. Erd2p is translocated under these conditions most likely to the intermediate compartment as indicated by an increased colocalization of Erd2p with mSEC13, a member of the mammalian coat protein II complex. IgGs as well as Fab fragments directed against Erd2p, beta-COP, or p23, a new member of the p24 protein family, inhibit or block retrograde transport of CTX-A-K63 from the Golgi without affecting its internalization or its transport to the Golgi. Anti-Erd2p antibodies do not affect the binding of CTX-A to Erd2p, but inhibit the CTX-K63-induced translocation of Erd2p and p53.

Regulation of microtubule dynamics and nucleation during polarization in MDCK II cells.

MDCK cells form a polarized epithelium when they reach confluence in tissue culture. We have previously shown that concomitantly with the establishment of intercellular junctions, centrioles separate and microtubules lose their radial organization (Bacallao, R., C. Antony, C. Dotti, E. Karsenti, E.H.K. Stelzer, and K. Simons. 1989. J. Cell Biol. 109:2817-2832. Buendia, B., M.H. Bré, G. Griffiths, and E. Karsenti. 1990. 110:1123-1136). In this work, we have examined the pattern of microtubule nucleation before and after the establishment of intercellular contacts. We analyzed the elongation rate and stability of microtubules in single and confluent cells. This was achieved by microinjection of Paramecium axonemal tubulin and detection of the newly incorporated subunits by an antibody directed specifically against the Paramecium axonemal tubulin. The determination of newly nucleated microtubule localization has been made possible by the use of advanced double-immunofluorescence confocal microscopy. We have shown that in single cells, newly nucleated microtubules originate from several sites concentrated in a region localized close to the nucleus and not from a single spot that could correspond to a pair of centrioles. In confluent cells, newly nucleated microtubules were still more dispersed. The microtubule elongation rate of individual microtubules was not different in single and confluent cells (4 microns/min). However, in confluent cells, the population of long lived microtubules was strongly increased. In single or subconfluent cells most microtubules showed a t1/2 of less than 30 min, whereas in confluent monolayers, a large population of microtubules had a t1/2 of greater than 2 h. These results, together with previous observations cited above, indicate that during the establishment of polarity in MDCK cells, microtubule reorganization involves both a relocalization of microtubule-nucleating activity and increased microtubule stabilization.

Observing proteins in their natural habitat: the living cell.

Fluorescence microscopy has played a tremendous role in uncovering the morphological features of cells and the expression pattern of proteins by immunofluorescence. Since the discovery of green-fluorescent proteins (GFPs), this technique has undergone a revival in the life sciences as the spatial distribution of ectopically expressed fusion proteins inside living cells can now be followed more easily. By further exploiting the photophysical properties of the emitted fluorescence with microspectroscopic methods, spatial information on the biochemical parameters of intracellular processes and reactions can be obtained. This possibility will not only play an important role in the understanding of biochemical reactions in signal processing and fidelity but also help to uncover the molecular mechanisms of organelle and cell morphogenesis.

Protein kinase WNK3 increases cell survival in a caspase-3-dependent pathway.

The subfamily of WNK (with no K= lysine) protein kinases has four human members and germline mutations in the WNK1 and WNK4 genes were recently found to cause pseudohypoaldosteronism type II, a familial hypertension disease. Here, we describe cloning and functional analysis of a further WNK member, human WNK3. Endogenous WNK3 protein is an active protein kinase when immunoprecipitated from cells and its overexpression increases the survival of HeLa cells by delaying the onset of apoptosis. Suppression of endogenous WNK3 protein by RNA interference accelerates the apoptotic response and promotes the activation of caspase-3. The mechanism of WNK3 action involves interaction with procaspase-3 and heat-shock protein 70. These results demonstrate a role for WNK3 in promoting cell survival and suggest a mechanism at the level of procaspase-3 activation.

Simultaneous detection of multiple green fluorescent proteins in live cells by fluorescence lifetime imaging microscopy.

The green fluorescent protein (GFP) has proven to be an excellent fluorescent marker for protein expression and localisation in living cells [1] [2] [3] [4] [5]. Several mutant GFPs with distinct fluorescence excitation and emission spectra have been engineered for intended use in multi-labelling experiments [6] [7] [8] [9]. Discrimination of these co-expressed GFP variants by wavelength is hampered, however, by a high degree of spectral overlap, low quantum efficiencies and extinction coefficients [10], or rapid photobleaching [6]. Using fluorescence lifetime imaging microscopy (FLIM) [11] [12] [13] [14] [15] [16], four GFP variants were shown to have distinguishable fluorescence lifetimes. Among these was a new variant (YFP5) with spectral characteristics reminiscent of yellow fluorescent protein [8] and a comparatively long fluorescence lifetime. The fluorescence intensities of co-expressed spectrally similar GFP variants (either alone or as fusion proteins) were separated using lifetime images obtained with FLIM at a single excitation wavelength and using a single broad band emission filter. Fluorescence lifetime imaging opens up an additional spectroscopic dimension to wavelength through which novel GFP variants can be selected to extend the number of protein processes that can be imaged simultaneously in cells.

Segregation of COPI-rich and anterograde-cargo-rich domains in endoplasmic-reticulum-to-Golgi transport complexes.

Membrane traffic between the endoplasmic reticulum (ER) and the Golgi complex is regulated by two vesicular coat complexes, COPII and COPI. COPII has been implicated in the selective packaging of anterograde cargo into coated transport vesicles budding from the ER [1]. In mammalian cells, these vesicles coalesce to form tubulo-vesicular transport complexes (TCs), which shuttle anterograde cargo from the ER to the Golgi complex [2] [3] [4]. In contrast, COPI-coated vesicles are proposed to mediate recycling of proteins from the Golgi complex to the ER [1] [5] [6] [7]. The binding of COPI to COPII-coated TCs [3] [8] [9], however, has led to the proposal that COPI binds to TCs and specifically packages recycling proteins into retrograde vesicles for return to the ER [3] [9]. To test this hypothesis, we tracked fluorescently tagged COPI and anterograde-transport markers simultaneously in living cells. COPI predominated on TCs shuttling anterograde cargo to the Golgi complex and was rarely observed on structures moving in directions consistent with retrograde transport. Furthermore, a progressive segregation of COPI-rich domains and anterograde-cargo-rich domains was observed in the TCs. This segregation and the directed motility of COPI-containing TCs were inhibited by antibodies that blocked COPI function. These observations, which are consistent with previous biochemical data [2] [9], suggest a role for COPI within TCs en route to the Golgi complex. By sequestering retrograde cargo in the anterograde-directed TCs, COPI couples the sorting of ER recycling proteins [10] to the transport of anterograde cargo.

The p24 family member p23 is required for early embryonic development.

The p24 family of type I integral-membrane proteins, which are localised in the endoplasmic reticulum (ER), the intermediate compartment and the Golgi apparatus, are thought to function as receptors for cargo exit from the ER and in transport vesicle formation. Members of the p24 family have been found in a molecular complex and are enriched in COPI-coated vesicles, which are involved in membrane traffic between the ER and Golgi complex. Although expressed abundantly, simultaneous deletion of several family members does not appear to affect cell viability and protein secretion in yeast. In order to gain more insights into the physiological roles of different p24 proteins, we generated mice deficient in the expression of one family member, p23 (also called 24delta1, see for alternative nomenclature). In contrast to yeast genetics, in mice disruption of both p23 alleles resulted in early embryonic lethality. Inactivation of one allele led not only to reduced levels of p23 itself but also to reduced levels of other family members. The reduction in steady-state protein levels also induced structural changes in the Golgi apparatus, such as the formation of dilated saccules. The generation of mice deficient in p23 expression has revealed an essential and non-redundant role for p23 in the earliest stages of mammalian development. It has also provided genetic evidence for the participation of p24 family members in oligomeric complexes and indicates a structural role for these proteins in maintaining the integrity of the early secretory pathway.