Antibodies to the Golgi complex and the rough endoplasmic reticulum

Developmental regulation of villin gene expression in the epithelial cell lineages of mouse digestive and urogenital tracts

The expression of villin, an actin-binding protein and major structural component of the brush border of specialized absorptive cells, was studied during mouse embryogenesis. We show that the ontogeny of villin expression is limited to the epithelial cell lineages of the digestive and uro-genital tracts and accounts for the tissue-specific expression observed in adult mice. This spatiotemporal pattern of villin expression is distinctive in sequence, intensity, regional distribution and polarization. During the development of the primitive gut, villin is faintly and discontinuously expressed in the invaginating foregut but it is expressed in every cell bordering the hindgut pocket. Later, villin expression increases along the developing intestine and concentrates in the brush border of the epithelium bordering the villi. In gut derivatives, villin is present in liver and pancreas primordia but only biliary and pancreatic cells maintain a faint villin expression as observed in adults. In the urogenital tract, mesonephric tubules are the first mesodermal derived structures to express villin. This expression is maintained in the ductuli efferents, paradidymis and epoophoron. Villin then appears in the proximal metanephric tubules and later increases and concentrates in the brush border of the renal proximal tubular epithelial cells. Thus villin expression can be considered as an early marker of the endodermal cell lineage during the development of the digestive system. Conversely, during the development of the excretory and genital system, villin is only expressed after the mesenchyme/epithelium conversion following the appearance of tubular structures. These observations emphasize the multiple levels of regulation of villin gene activity that occur during mouse embryogenesis and account for the strict pattern of tissue-specific expression observed in adults. In the future, regulatory elements of the villin gene may be used to target the early expression of oncogenes to the digestive and urogenital tracts of transgenic mice.

The endoplasmic reticulum-sarcoplasmic reticulum connection: distribution of endoplasmic reticulum markers in the sarcoplasmic reticulum of skeletal muscle fibers

The skeletal muscle sarcoplasmic reticulum (SR) was investigated for the presence of well-known endoplasmic reticulum (ER) markers: the lumenal protein BiP and a group of membrane proteins recognized by an antibody raised against ER membrane vesicles. Western blots of SR fractions revealed the presence of BiP in fast- and slow-twitch muscles of the rabbit as well as in rat and chicken muscles. Analyses of purified SR subfractions, together with cryosection immunofluorescence and immunogold labeling, revealed BiP evenly distributed within the longitudinal SR and the terminal cisternae. Within the terminal cisternae BiP appeared not to be mixed with calsequestrin but to be distributed around the aggregates of the latter Ca2+ binding protein. Of the various membrane markers only calnexin (91 kDa) was found to be distributed within both SR subfractions, whereas the other markers (apparent molecular masses of 64 kDa and 58 kDa and a doublet around 28 kDa) were concentrated in the terminal cisternae. These results suggest that the SR is a specialized ER subcompartment in which general markers, such as the ones we have investigated, coexist with the major SR proteins specifically responsible for Ca2+ uptake, storage, and release. The differential distribution of the ER markers reveals new aspects of the SR molecular structure that might be of importance for the functioning of the endomembrane system.

The endoplasmic reticulum of Purkinje neuron body and dendrites: molecular identity and specializations for Ca2+ transport

Immunofluorescence and immunogold labeling, together with sucrose gradient separation and Western blot analysis of microsomal subfractions, were employed in parallel to probe the endoplasmic reticulum in the cell body and dendrites of rat cerebellar Purkinje neurons. Two markers, previously investigated in non-nerve cells, the membrane protein p91 (calnexin) and the lumenal protein BiP, were found to be highly expressed and widely distributed to the various endoplasmic reticulum sections of Purkinje neurons, from the cell body to dendrites and dendritic spines. An antibody (denominated anti-rough-surfaced endoplasmic reticulum), which recognized two membrane proteins, p14 and p40, revealed a similar immunogold labeling pattern. However, centrifugation results consistent with a widespread distribution were obtained for p14 only, while p40 was concentrated in the rough microsome-enriched subfractions. The areas enriched in the inositol 1,4,5-triphosphate receptor and thus presumably specialized in Ca2+ transport (stacks of multiple smooth-surfaced cisternae; the dendritic spine apparatus) also exhibited labeling for BiP and p91, and were positive for the anti-rough-surfaced endoplasmic reticulum antibody (presumably via the p14 antigen). Additional antibodies, that yielded inadequate immunocytochemical signals, were employed only by Western blotting of the microsomal subfractions, while the ryanodine receptor was studied by specific binding. The latter receptor and the Ca2+ ATPase, known in other species to be concentrated in Purkinje neurons, exhibited bimodal distributions with a peak in the light and another in the heavy subfractions. A similar distribution was also observed with another lumenal protein, protein disulfide isomerase. Taken as a whole, the results that we have obtained suggest the existence in the endoplasmic reticulum of Purkinje neurons of two levels of organization; the first identified by widespread, probably general markers (BiP, p91, possibly p14 and others), the second by specialization markers, such as the inositol 1,4,5-triphosphate receptor and, possibly, p40, which appear restricted to areas where specific functions appear to be localized.

Local expression and exocytosis of viral glycoproteins in multinucleated muscle cells

We have analyzed the distribution of enveloped viral infections in multinucleated L6 muscle cells. A temperature-sensitive vesicular stomatitis virus (mutant VSV ts045) was utilized at the nonpermissive temperature (39 degrees C). As expected, the glycoprotein (G protein) of this mutant was restricted to the ER when the multinucleated cells were maintained at 39 degrees C. We demonstrate that this G protein remained localized when the infection was performed at low dose. By 4 h after infection the G protein patches spanned an average of 220 microns. The localization was independent of nuclear positions, showing that the ER was a peripheric structure. Thus, the infection did not recognize nuclear domains characteristic of nuclearly encoded proteins. After release of the 39 degrees C block, transport through a perinuclear compartment into a restricted surface domain lying above the internal G protein patch occurred. Accordingly, the transport pathway was locally restricted. After a 16-h infection the G protein spanned 420 microns, while the matrix protein occupied 700-800 microns of the myotube length. Double infection of multinucleated L6 muscle cells with Semliki Forest virus and VSV at high multiplicities showed that the glycoprotein of each virus occupied intracellular domains which were devoid of the other respective glycoprotein. Taken together, these findings indicate that the viral glycoproteins did not range far from their site of synthesis within the ER or other intracellular membrane compartments in these large cells. This result also suggests that relocation of viral RNA synthesis occurred slowly.

Partitioning of the Golgi apparatus in rat primary and secondary spermatocytes during meiosis

We examined the disassembly and reformation of the Golgi apparatus as a function of the cycle of the seminiferous epithelium in adult rats during stages XIII and XIV, i.e., just prior to and during meiosis I and II. Serial section analysis of primary spermatocytes at metaphase I demonstrated the presence of two Golgi complexes. At the ultrastructural level, these Golgi complexes were shown to be composed of stacks of cisternae and vesicles, with each stack having a varying number of saccules. Although Golgi complex intermediates resulting from the process of organelle disassembly were not clearly identified in diplotene spermatocytes immediately prior to nuclear envelope vesiculation, we did observe clusters of vesicles resembling the "nuage," with each cluster varying in size and number of vesicles. Meiosis I results in the formation of secondary spermatocytes that exhibit a well-formed spherical Golgi complex approximately half the size of the diplotene spermatocyte Golgi. Next, secondary spermatocytes enter meiosis II. In contrast to metaphase I, during metaphase II reformation of the Golgi apparatus into stacks was not observed and only small clusters of vesicles at two poles of dividing cells were detected. In addition, "nuage"-like structures were not identified during meiosis II. Our results begin to characterize the process by which Golgi apparatus partitioning is accomplished during meiosis, presumably resulting in the delivery of equal complements of this organelle to four round spermatids. We suggest that partitioning of the Golgi apparatus takes place prior to metaphase I and that the two steps of meiosis may exhibit subtle differences with respect to Golgi partitioning.

The 17-residue transmembrane domain of beta-galactoside alpha 2,6-sialyltransferase is sufficient for Golgi retention

beta-Galactoside alpha 2,6-sialyltransferase (ST) is a type II integral membrane protein of the Golgi apparatus involved in the sialylation of N-linked glycans. A series of experiments has shown that the 17-residue transmembrane domain of ST is sufficient to confer localization to the Golgi apparatus when transferred to the corresponding region of a cell surface type II integral membrane protein. Lectin affinity chromatography of chimeric proteins bearing this 17-residue sequence suggests that these chimeric proteins are localized in the trans-Golgi cisternae and/or trans-Golgi network. Further experiments suggest that this 17-residue sequence functions as a retention signal for the Golgi apparatus.

Mitosis and inhibition of intracellular transport stimulate palmitoylation of a 62-kD protein

Recent studies suggest that a cycle of acylation/deacylation is involved in the vesicular transport of proteins between intracellular compartments at both the budding and the fusion stage (Glick, B. S., and J. E. Rothman. 1987. Nature (Lond.). 326:309-312). Since a number of cellular processes requiring vesicular transport are inhibited during mitosis, we examined the fatty acylation of proteins in interphase and mitotic cells. We have identified a major palmitoylated protein with an apparent molecular weight of 62,000 (p62), whose level of acylation increases 5-10-fold during mitosis. Acylation was reversible and p62 was no longer palmitoylated in cells that have exited mitosis and entered G1. p62 is tightly bound to the cytoplasmic side of membranes, since it was sensitive to digestion with proteases in the absence of detergent and was not removed by treatment with 1 M KCl. p62 is removed from membranes by nonionic detergents or concentrations of urea greater than 4 M. The localization of p62 by subcellular fractionation is consistent with it being in the cis-Golgi or the cis-Golgi network. A palmitoylated protein of the same molecular weight was also observed in interphase cells treated with inhibitors of intracellular transport, such as brefeldin A, monensin, carbonylcyanide m-chlorophenylhydrazone, or aluminum fluoride. The protein palmitoylated in the presence of brefeldin A was shown to be the same as that palmitoylated during mitosis using partial proteolysis. Digestion with two enzymes, alkaline protease and endoprotease lys-C, generated the same 3H-palmitate-labeled peptide fragments from p62 from mitotic or brefeldin A-treated cells. We suggest that the acylation and deacylation of p62 may be important in vesicular transport and that this process may be regulated during mitosis.

s-cyclophilin is retained intracellularly via a unique COOH-terminal sequence and colocalizes with the calcium storage protein calreticulin

Cyclophilins (cyclosporin A-binding proteins) are conserved, ubiquitous, and abundant proteins that accelerate the isomerization of XaaPro peptide bonds and the refolding of proteins in vitro. s-Cyclophilin is a member of the cyclophilin family with unique NH2- and COOH-terminal extensions, and with a signal sequence. We now report that s-cyclophilin is retained in the cell, and that the conserved s-cyclophilin-specific COOH-terminal extension VEKPFAIAKE is sufficient to direct a secretory protein to s-cyclophilin containing structures. Antibodies to s-cyclophilin-specific peptides were produced and the location of the protein was determined by an immunocytochemical study at the light microscopic level. s-Cyclophilin colocalized with the Ca(2+)-binding protein calreticulin and, to a lesser extent, with the microsomal Ca(2+)-ATPase in the myogenic cell line L6, and with the Ca(2+)-binding protein calsequestrin in skeletal muscle. In activated platelets, s-cyclophilin immunoreactivity was detected in a ring-like structure that might correspond to the Ca(2+)-storing and -releasing dense tubular network. In spreading cells, s-cyclophilin containing vesicular structures accumulated at actin-rich protrusion sites. While s-cyclophilin consistently codistributed with Ca2+ storage site markers, the distribution of s-cyclophilin immunoreactivity was not identical to that of ER markers. To determine whether the COOH-terminal extension of s-cyclophilin was involved in its intracellular transport we added this sequence to the COOH-terminus of the secretory protein glia-derived nexin. Appropriate constructs were expressed transiently in cultured cells and proteins were detected with specific antibodies. We found that glia-derived nexin with the COOH-terminal sequence VEKPFAIAKE (but not with the control sequence GLVVMNIT) colocalized with endogenous s-cyclophilin, indicating that the sequence contained retention information. These results indicate that s-cyclophilin is a retained component of an intracellular organelle and that it may accumulate in specialized portions of the ER, and possibly in calciosomes. Because of its conserved structure, widespread distribution, and abundance s-cyclophilin may be a useful marker to study the biogenesis and distribution of ER subcompartments.

Monoclonal antibody against ependymoma-derived cell line

Mouse myeloma cells were fused with spleen cells from mice that had been immunized with a human ependymoma derived cell line, KMS II. Hybridomas producing monoclonal antibodies (MAbs) were screened and cloned. Specificity of the antibody was determined by enzyme-linked immunosorbent assay (ELISA) and/or indirect immunofluorescence assay. One of the MAbs, designated Ep-C4 (subclass = IgG1), reacted with two cell lines derived from ependymoma but did not react with 17 cell lines derived from other types of brain tumor nor with 4 neuroblastoma cell lines or 19 cell lines derived from carcinoma, hematopoietic tumors and amnion. Indirect immunofluorescence and immuno-electron microscopy studies revealed that the antigen recognized by MAb Ep-C4 was located on cell surface membrane. The membrane antigen of KMS II cells, immunoprecipitated by MAb Ep-C4, was a protein of 81,000 dalton. The reactivity of MAb Ep-C4 was further examined using immunofluorescence and/or immunoperoxidase methods and frozen sections and short-term cultures of various types of brain tumors. No cross-reactivity with normal adult or fetal brain tissues was detected by absorption assay and immunoperoxidase staining. Our results suggest that the antigen defined by MAb Ep-C4 is specific for ependymoma cells, and different from the antigens of glioma cells or other neuroectodermal-derived cells previously described.

A monoclonal antibody recognizing Golgi apparatus produced using affinity purified material from a patient with connective tissue disease

Serum antibodies recognizing the Golgi apparatus have been reported in patients with connective tissue diseases, but little is known of their significance. Serum from a systemic lupus erythematosus patient with polymyositis was found to have high titers of anti-Golgi apparatus antibody. This serum recognized a 64 kD polypeptide in immunoblotting with HEp-2 cells. To verify that the 64 kD polypeptide was associated with the Golgi apparatus and to characterize which Golgi component was recognized, a monoclonal antibody was produced. IgG, isolated from this serum, was used in affinity chromatography to produce purified material which was used to generate a mouse monoclonal antibody. The monoclonal antibody had an indirect immunofluorescent pattern identical to that produced by the patient's serum, and similarly recognized a 64kD polypeptide in immunoblotting. A 59 kD polypeptide was also recognized by the monoclonal antibody, suggesting that the antigens recognized by the monoclonal and serum antibodies may be only partially identical. The antigen appears to be a glycoprotein and an integral component of the Golgi cisternae membranes.

The membrane spanning domain of beta-1,4-galactosyltransferase specifies trans Golgi localization

Chimeric cDNAs were constructed so as to generate hybrid proteins in which different parts of the N-terminal domain of the human invariant chain were replaced by equivalent sequences from the trans Golgi resident enzyme, beta-1,4-galactosyltransferase. The cytoplasmic and membrane spanning domains of galactosyltransferase were found to be sufficient to retain all of the hybrid invariant chain in trans Golgi cisternae as judged by indirect immunofluorescence, treatment with brefeldin A and immuno-electron microscopy. As few as ten amino acids corresponding to the lumenal half of the membrane spanning domain of the Golgi enzyme sufficed to localize most of the hybrid invariant chain to the trans cisternae. A cytoplasmic domain was necessary for complete retention as assessed by flow cytofluorometry but could be provided either by galactosyltransferase or by invariant chain. This suggests that the cytoplasmic domain plays a role accessory to the membrane spanning domain, the latter mediating compartmental specificity.

Characterization and purification of proteins which bind high-density lipoprotein. A putative cell-surface receptor

High-density lipoprotein (HDL) is shown by ligand blotting to bind membrane-associated polypeptides with sizes of 60, 100 and 210 kDa. Binding was concentration-dependent and competed by excess unlabelled HDL. All the major apolipoproteins of HDL, apoA-I, apoA-II and apoA-IV, bound independently. The 100 kDa and 210 kDa HDL-binding activities were purified from membranes of Hep3B tumour cells by ion-exchange chromatography and gel filtration. The binding activities at 100 kDa and 210 kDa co-purified. After treatment with disulphide-reducing reagent, the 210 kDa band was no longer present and an increase was observed in the amount and binding ability of the 100 kDa polypeptide. The 100 kDa binding protein labelled at the cell surface with 125I could be immunoprecipitated after cross-linking to cell-surface-bound HDL. It is proposed that this HDL-binding activity, a putative cell-surface receptor for HDL, exists totally or in part as a high-molecular-mass complex composed of 100 kDa subunits.

Electron microscopic studies of the endoplasmic reticulum in whole-mount cultured cells fixed with potassium permanganate

A method for visualizing the endoplasmic reticulum and other membrane organelles in whole-mount cells with a standard, 60-kV transmission electron microscope has been developed. By use of a new formulation of potassium permanganate as a fixative, intracellular membranes were preserved and stained, while cytosolic proteins were digested, giving a pattern of membranous organelles against a clear background, suitable for transmission EM of whole-mount cells at 60 kV. Mitochondria, lysosomes, and ER were clearly visible in whole-mount cells fixed by this method. We have employed this technique to examine the organization of the ER in a variety of different cell lines. This method also allowed visualization of the three-dimensional organization, relationships, and fine structure of mitochondria. With prolonged permanganate fixation, mitochondrial cristae were clearly visible in whole-mount cells. This method was also useful for fixation and staining of thin sections, and allowed examination of thicker sections than previously possible, thus giving improved imaging of organelle relationships and fine structure. Using this method, we have examined the ER, mitochondria, and Golgi in thin section.

Redox regulation of a protein tyrosine kinase in the endoplasmic reticulum

The subcellular localization of the mouse Ltk transmembrane protein tyrosine kinase was studied in transfected COS cells, a mature B lymphocyte line, and a low expressing transfected lymphocyte clone. Indirect immunofluorescence and immunogold staining of COS transfectants and endoglycosidase analysis of both COS transfectants and lymphocytes indicate the unusual localization of Ltk to the endoplasmic reticulum (ER). Ltk resembles a receptor tyrosine kinase; it has a short, glycosylated, and cysteine-rich N-terminal domain. Yet, it appears to function in a ligand-independent mechanism: its in vivo catalytic activity is markedly enhanced by alkylating and thiol-oxidizing agents, and the active fraction of the protein occurs as disulfide-linked multimers. The catalytic activity of Ltk in the ER may be regulated via changes in the cellular redox potential, a novel mechanism for regulating protein tyrosine kinases. The ability to respond to redox changes in the cell may, however, be shared with certain receptor kinases during their passage through the ER.

A recycling pathway between the endoplasmic reticulum and the Golgi apparatus for retention of unassembled MHC class I molecules

Assembly of class I major histocompatibility complex (MHC) molecules involves the interaction of two distinct polypeptides (the heavy and light chains) with peptide antigen. Cell lines synthesizing both chains but expressing low levels of MHC class I molecules on their surface as a result of a failure in assembly and transport have been identified. We now report that although the apparent steady-state distribution in these cells of class I molecules is in the endoplasmic reticulum (ER), the molecules in fact are recycled between the ER and Golgi, rather than retained in the ER. This explains the failure of class I molecules to negotiate the secretory pathway. Class I molecules do not seem to be modified by Golgi enzymes, suggesting that the proteins do not reach the Golgi apparatus during recycling. But morphological and subcellular fractionation evidence indicates that they pass through the cis Golgi or a Golgi-associated organelle, which we postulate to be the recycling organelle. This compartment, which we call the 'cis-Golgi network', would thereby be a sorting organelle that selects proteins for return to the ER.

Characterization of A 54-kD protein of the inner nuclear membrane: evidence for cell cycle-dependent interaction with the nuclear lamina

Using a mAb (R-7), we have characterized a 54-kD protein of the chicken nuclear envelope. Based on its biochemical properties and subnuclear distribution p54 is likely to be an integral membrane component specific to the inner nuclear membrane. Fractionation experiments indicate that p54 interacts, directly or indirectly, with the nuclear lamina, and analysis of p54 in cultured cells suggests that this interaction is controlled by cell cycle-dependent posttranslational modification, most likely phosphorylation. Modification of p54 results in a slightly reduced electrophoretic mobility, and it converts the protein from a detergent-resistant to a detergent-extractable form. Detergent solubilization of p54 can be induced in vivo by treating isolated nuclei or nuclear envelopes with highly purified cdc2 kinase, one of the most prominent kinases active in mitotic cells. These results suggest that mitotic phosphorylation of p54 might contribute to control nuclear envelope dynamics during mitosis in vivo.

Regulating the retention of T-cell receptor alpha chain variants within the endoplasmic reticulum: Ca(2+)-dependent association with BiP

Immunoglobulin heavy chain binding protein (BiP, GRP 78) coprecipitates with soluble and membrane-associated variants of the T-cell antigen receptor alpha chain (TCR-alpha) which are stably retained within the ER. Chelation of Ca2+ during solubilization of cells leads to the dissociation of BiP from the TCR-alpha variants, which is dependent upon the availability of Mg2+ and hydrolyzable ATP; this suggests that Ca2+ levels can serve to modulate the association/dissociation of these proteins with BiP. In vivo treatment of cells expressing either the soluble or membrane-anchored TCR-alpha variants with the Ca2+ ionophore, A23187, or an inhibitor of an ER Ca(2+)-ATPase, thapsigargin, or the membrane-permeant Ca2+ chelator BAPTA-AM, results in the redistribution of these proteins out of the ER and their subsequent secretion or cell surface expression. Under the same assay conditions, no movement of BiP out of the ER is observed. Taken together, these observations indicate that decreased Ca2+ levels result in the dissociation of a protein bound to BiP, leading to its release from ER retention. These data suggest that the intracellular fate of newly synthesized proteins stably associated with BiP can be regulated by Ca2+ levels in the ER.

Parallel pathways of cell cycle control during Xenopus egg activation

Transit from M phase into interphase in many eukaryotic cells is preceded by an increase in intracellular free calcium ([Ca2+]i), which may act via calcium-dependent enzymes to trigger the M-phase/interphase transition. To test the role of the calcium- and phospholipid-dependent enzyme protein kinase C (PKC) in the M-phase/interphase transition, PKC was activated in M-phase-arrested Xenopus eggs by treatment with the phorbol ester phorbol 12-myristate 13-acetate under conditions that prevent a rise in [Ca2+]i and activation of other calcium-dependent enzymes. Under these conditions, several cellular events characteristic of transit into interphase occur: sperm chromatin decondenses, the Golgi and the nuclear envelope reassemble, and endocytosis resumes. These events are also triggered by treatment of eggs with the diacylglycerol 1,2-dioctanoyl-sn-glycerol. Surprisingly, the activity of M-phase-promoting factor (MPF), a universal regulator of M phase, remains high under these conditions. If [Ca2+]i is subsequently raised, MPF activity is rapidly destroyed. Similarly, lysates made from eggs treated with phorbol 12-myristate 13-acetate support sperm chromatin decondensation in vitro and yet retain high MPF activity, measured either as the ability to induce meiotic resumption in oocytes or as histone H1 kinase activity. These effects are not triggered by the 4 alpha-phorbol ester isomer, which does not activate PKC, and are sensitive to the PKC "pseudosubstrate" peptide. The results suggest that two, parallel signals are generated by the rise in [Ca2+]i both of which contribute to cell cycle regulation. One pathway inactivates MPF; the other pathway activates PKC.

Characterization of a novel heparan sulfate proteoglycan found in the extracellular matrix of liver sinusoids and basement membranes

A novel heparan sulfate proteoglycan (HSPG) present in the extracellular matrix of rat liver has been partially characterized. Proteoglycans were purified from a high salt extract of total microsomes from rat liver and found to consist predominantly (approximately 90%) of HSPG. A polyclonal antiserum raised against this fraction specifically recognized HSPG by immunoprecipitation and immunoblotting. The intact, fully glycosylated HSPG migrated as a broad smear (150-300 kD) by SDS-PAGE, but after deglycosylation with trifluoromethanesulfonic acid only a single approximately 40-kD band was seen. By immunocytochemistry this HSPG was localized in the perisinusoidal space of Disse associated with irregular clumps of basement membrane-like extracellular matrix material, some of which was closely associated with the hepatocyte sinusoidal cell surface. It was also localized in biosynthetic compartments (rough ER and Golgi cisternae) of hepatocytes, suggesting that this HSPG is synthesized and deposited in the space of Disse by the hepatocyte. The anti-liver HSPG IgG also stained basement membranes of hepatic blood vessels and bile ducts as well as those of kidney and several other organs (heart, pancreas, and intestine). An antibody that recognizes the basement membrane HSPG found in the rat glomerular basement membrane did not precipitate the 150-300-kD rat liver HSPG. We conclude that the liver sinusoidal space of Disse contains a novel population of HSPG that differs in its overall size, its distribution and in the size of its core protein from other HSPG (i.e., membrane-intercalated HSPG) previously described in rat liver. It also differs in its core protein size from HSPG purified from other extracellular matrix sources. This population of HSPG appears to be a member of the basement membrane HSPG family.

Nonlysosomal, pre-Golgi degradation of unassembled asialoglycoprotein receptor subunits: a TLCK- and TPCK-sensitive cleavage within the ER

The human asialoglycoprotein receptor subunit H2a is cotranslationally inserted into the ER membrane. When expressed together with subunit H1 in mouse fibroblasts part forms a hetero-oligomer that is transported to the cell surface, but when expressed alone it is all rapidly degraded. Degradation is insensitive to lysosomotropic agents and the undegraded precursor is last detected in the ER region of the cell. Small amounts of an intermediate 35-kD degradation product can be detected (Amara, J. F., G. Lederkremer, and H. F. Lodish. 1989. J. Cell Biol. 109:3315). We show here that the oligosaccharides on both precursor H2a and the 35-kD fragment are Man6-9GlcNAc2, structures typically found in pre-Golgi compartments. Subcellular fractionation shows that the intermediate degradation product does not cofractionate with the lysosomal enzyme beta-galactosidase, but is found in a part of the ER that contains ribosomes. Thus the intermediate degradation product is localized in the ER, indicating that the initial degradation event does take place in the ER. All degradation of H2a, including the initial endoproteolytic cleavage generating the 35-kD intermediate, is blocked by the protease inhibitors N-tosyl-L-lysine chloromethyl ketone and N-tosyl-L-phenylalanine chloromethyl ketone. These drugs do not inhibit ER-to-Golgi transport of H1. Depleting the cells of ATP or inhibiting protein synthesis allows the initial endoproteolytic cleavage to occur, but blocks further degradation of the 35-kD intermediate; thus we can convert all cellular H2 into the 35-kD intermediate. Approximately 50% of H2b, a splicing variant differing from H2a by a five amino acid deletion, can be transported to the cell surface, and the rest appears to be degraded by the same pathway as H2a, both when expressed alone in fibroblasts and together with H1 in HepG2 cells. Addition of N-tosyl-L-lysine chloromethyl ketone or N-tosyl-L-phenylalanine chloromethyl ketone blocks degradation of the approximately 50% that is not transported, but does not affect the fraction of H2b that moves to the Golgi region. Thus, a protein destined for degradation will not be transported to the Golgi region if degradation is inhibited.

GTP-dependent membrane fusion during hepatocarcinogenesis and liver regeneration

Rough microsomes were isolated from homogenates of livers of rats bearing hepatomas as well as from homogenates of livers of rats 24 and 48 h after partial hepatectomy. When incubated in the presence of GTP in a cell-free system to assay membrane fusion these membranes were observed to have a greater capacity (1.4 to 5 fold) for GTP-dependent fusion than homologous membranes from control non-proliferating liver tissue. The enhanced GTP-dependent membrane fusion may reflect changes in membrane properties related to cell proliferation.

Intracellular Ca2+ stores in chicken Purkinje neurons: differential distribution of the low affinity-high capacity Ca2+ binding protein, calsequestrin, of Ca2+ ATPase and of the ER lumenal protein, Bip

To identify intracellular Ca2+ stores, we have mapped (by cryosection immunofluorescence and immunogold labeling) the distribution in the chicken cerebellar cortex of an essential component, the main low affinity-high capacity Ca2+ binding protein which in this tissue has been recently shown undistinguishable from muscle calsequestrin (Volpe, P., B. H. Alderson-Lang, L. Madeddu, E. Damiani, J. H. Collins, and A. Margreth. 1990. Neuron. 5:713-721). Appreciable levels of the protein were found exclusively within Purkinje neurons, distributed to the cell body, the axon, and the elaborate dendritic tree, with little labeling, however, of dendritic spines. At the EM level the protein displayed a dual localization: within the ER (rough- and smooth-surfaced cisternae, including the cisternal stacks recently shown [in the rat] to be highly enriched in receptors for inositol 1,4,5-triphosphate) and, over 10-fold more concentrated, within a population of moderately dense, membrane-bound small vacuoles and tubules, identified as calciosomes. These latter structures were widely distributed both in the cell body (approximately 1% of the cross-sectional area, particularly concentrated near the Golgi complex) and in the dendrites, up to the entrance of the spines. The distribution of calsequestrin was compared to those of another putative component of the Ca2+ stores, the membrane pump Ca2+ ATPase, and of the ER resident lumenal protein, Bip. Ca2+ ATPase was expressed by both calciosomes and regular ER cisternae, but excluded from cisternal stacks; Bip was abundant within the ER lumena (cisternae and stacks) and very low within calciosomes (average calsequestrin/Bip immunolabeling ratios were approximately 0.5 and 36.5 in the two types of structure, respectively). These results suggest that ER cisternal stacks do not represent independent Ca2+ stores, but operate coordinately with the adjacent, lumenally continuous ER cisternae. The ER and calciosomes could serve as rapidly exchanging Ca2+ stores, characterized however by different properties, in particular, by the greater Ca2+ accumulation potential of calciosomes. Hypotheses of calciosome biogenesis (directly from the ER or via the Golgi complex) are discussed.

Purification of marginal plates from bovine renal peroxisomes: identification with L-alpha-hydroxyacid oxidase B

The matrix of mammalian peroxisomes frequently contains crystalline inclusions. The most common inclusions are membrane associated plate-like "marginal plates" of hitherto unknown nature in renal peroxisomes and central polytubular "cores" composed of urate oxidase in hepatic peroxisomes. In bovine kidney, peroxisomes of proximal tubules exhibit peculiar angular shapes that are caused by multiple marginal plates (Zaar, K., and H.D. Fahimi. 1990. Cell Tissue Res. 260:409-414). Enriched or highly purified peroxisome preparations from this source were used to purify and characterize marginal plates. By SDS-PAGE, one major polypeptide of Mr 33,500 was observed that corresponded to the marginal plate protein. This polypeptide was identified by its enzymatic activity as well as by immunoblotting and preembedding immunocytochemistry as the isozyme B of L-alpha-hydroxyacid oxidase (EC 1.4.3.2). Morphologically, marginal plates were revealed to consist of rectangular straight-edged sheets, exhibiting a defined crystalline lattice structure. The sheets apparently are composed of a single layer of protomers which associate laterally to form a plate-like structure. As deduced from the negative staining results and the additional information of the thickness of marginal plates, each protomer seems to consist of eight subunits forming a cube-like array. The tendency of L-alpha-hydroxyacid oxidase B to self-associate in vitro (Philips, D.R., J.A. Duley, D.J. Fennell, and R.S. Holmes. 1976. Biochim. Biophys. Acta. 427:679-687) corresponds to the mode of association of cubical protomers to form the so-called marginal plates in renal peroxisomes.

The BOS1 gene encodes an essential 27-kD putative membrane protein that is required for vesicular transport from the ER to the Golgi complex in yeast

We recently described the identification of BOS1 (Newman, A., J. Shim, and S. Ferro-Novick. 1990. Mol. Cell. Biol. 10:3405-3414.). BOS1 is a gene that in multiple copy suppresses the growth and secretion defect of bet1 and sec22, two mutants that disrupt transport from the ER to the Golgi complex in yeast. The ability of BOS1 to specifically suppress mutants blocked at a particular stage of the secretory pathway suggested that this gene encodes a protein that functions in this process. The experiments presented in this study support this hypothesis. Specifically, the BOS1 gene was found to be essential for cellular growth. Furthermore, cells depleted of the Bos1 protein fail to transport pro-alpha-factor and carboxypeptidase Y (CPY) to the Golgi apparatus. This defect in export leads to the accumulation of an extensive network of ER and small vesicles. DNA sequence analysis predicts that Bos1 is a 27-kD protein containing a putative membrane-spanning domain. This prediction is supported by differential centrifugation experiments. Thus, Bos1 appears to be a membrane protein that functions in conjunction with Bet1 and Sec22 to facilitate the transport of proteins at a step subsequent to translocation into the ER but before entry into the Golgi apparatus.

Increase in the expression of a family of small guanosine triphosphate-binding proteins, rab proteins, during induced phagocyte differentiation

Rab is a newly identified family of small G-proteins that share 35-70% homology with the yeast Sec4p and Ypt1p involved in the regulation of the secretory pathway. Mature phagocytes display functions requiring organized intracellular traffic and, for this reason, we questioned whether phagocyte differentiation could correlate with the increased expression of rab proteins. Rabbit antisera raised against the recombinant proteins rab1Ap, 2p, 4p, and 6p were able to detect the corresponding proteins in the human monoblast leukemic cell line U937. When these cells were induced to differentiate into monocyte/macrophage-like cells displaying functional characteristics of a normal phagocyte, rab1Ap, 2p, 4p, and 6p were increased and this correlated with an increase in the rab transcripts. Using a rab5 probe, we also observed an increased expression of the rab5 gene in differentiated cells. Similarly, differentiation of the human leukemic myeloblast HL60 cell line along either monocyte or granulocyte pathways induced an increased expression of the rab proteins. Rab proteins were also detected in human neutrophils and in guinea pig alveolar macrophages. As degranulation is one of the phagocyte functions acquired in the late stage of differentiation, we investigated whether rab proteins would be involved in this process. Although rab proteins were tightly membrane bound, none of them was detected in the specific or azurophil granules purified from human neutrophils. The increased expression of rab proteins in mature phagocytes suggests that they may promote functions highly developed in these cells.

Immunolocalization of the cellular src protein in interphase and mitotic NIH c-src overexpresser cells

The mouse mAb, mAb 327, that recognizes specifically both pp60v-src and pp60c-src in a wide variety of cells, has been used to determine precisely the various locations of pp60c-src in NIH c-src overexpresser cells, using the technique of immunofluorescence microscopy. In interphase cells, the protein exhibits two main distributions: one that appears uniform and in association with the cell surface and the other that is patchy and juxtanuclear and coincides with the centrosomes. The juxtanuclear aggregation of pp60c-src-containing patches depends on microtubules and does not seem to occur within the Golgi apparatus and the rough ER. At the G2-to-M-phase transition, a drastic change in the localization patterns of pp60c-src takes place. We also report experiments in which the NIH c-src overexpresser cells were exposed to Con A for various times to induce a redistribution of the cell surface Con A receptors. We show that, at each stage of the Con A-mediated endocytotic process, the Con A-receptor complexes redistribute into structures to which pp60c-src appears also to be associated: at first, into patches that form at the cell surface level and then, into a cap that stands at the cell center in a juxtanuclear position and that coincides with the Golgi apparatus. During this capping process, pp60c-src-containing vesicles continue to accumulate in a centriolar spot, as in interphase, Con A-untreated cells, from which Con A is excluded. The significance of the intracellular locations of pp60c-src to the possible functions of the protein is discussed. Also, the distribution patterns of the cellular protein in the NIH c-src overexpresser cells are compared with those of pp60v-src in RSV-transformed cells. The differences observed are discussed in relation with the differences in transforming capacities of the two proteins. Finally, the possible physiological significance of the association between pp60c-src and the structures generated after the binding of Con A to its surface receptors is addressed.

Dissociation of a 110-kD peripheral membrane protein from the Golgi apparatus is an early event in brefeldin A action

Brefeldin A (BFA) has a profound effect on the structure of the Golgi apparatus, causing Golgi proteins to redistribute into the ER minutes after drug treatment. Here we describe the dissociation of a 110-kD cytoplasmically oriented peripheral membrane protein (Allan, V. J., and T. E. Kreis. 1986. J. Cell Biol. 103:2229-2239) from the Golgi apparatus as an early event in BFA action, preceding other morphologic changes. In contrast, other peripheral membrane proteins of the Golgi apparatus were not released but followed Golgi membrane into the ER during BFA treatment. The 110-kD protein remained widely dispersed throughout the cytoplasm during drug treatment, but upon removal of BFA it reassociated with membranes during reformation of the Golgi apparatus. Although a 30-s exposure to the drug was sufficient to cause the redistribution of the 110-kD protein, removal of the drug after this short exposure resulted in the reassociation of the 110-kD protein and no change in Golgi structure. If cells were exposed to BFA for 1 min or more, however, a portion of the Golgi membrane was committed to move into and out of the ER after removal of the drug. ATP depletion also caused the reversible release of the 110-kD protein, but without Golgi membrane redistribution into the ER. These findings suggest that the interaction between the 110-kD protein and the Golgi apparatus is dynamic and can be perturbed by metabolic changes or the drug BFA.

Differentiation of a calsequestrin-containing endoplasmic reticulum during sea urchin oogenesis

We have used light and electron microscopic immunolocalization to study the distribution of a sea urchin calsequestrin-like protein (SCS) during sea urchin oogenesis. SCS was localized exclusively in the lumen of the endoplasmic reticulum (ER) and in the nuclear envelope of oocytes of all maturation stages. Immunoelectron microscopy also revealed that SCS is not present in golgi complexes of oocytes. Double label immunofluorescent staining of frozen sections of ovary with the SCS antiserum and an antibody to the cortical granule protein hyalin indicated a dramatic morphogenesis of the SCS-containing ER (SCS-ER) coincident with oocyte maturation. This differentiation included an apparent increase in the amount and complexity of the cytoplasmic SCS-ER network, the transient appearance of stacks of SCS-ER cisternae in synthetically active vitellogenic oocytes, and the restructuring of the SCS-ER in the cortex. Immunofluorescence of isolated oocyte cortices showed a plasma membrane-associated SCS-ER which was much less dense and regular than that found surrounding the cortical granules in the mature unfertilized egg cortex. Cytoplasmic and cortical microtubule arrays are present in oocytes and may provide the basis for the SCS-ER distributional dynamics. The results of this study underscore the dynamic nature of ER and how it's organization reflects cellular functions. We suggest that the establishment during oogenesis of the dense SCS-ER tubuloreticulum provides the egg with the calcium sequestration and release apparatus that regulates calcium fluxes during egg activation and early development.

Identification, sequencing and expression of an integral membrane protein of the trans-Golgi network (TGN38)

Organelle-specific integral membrane proteins were identified by a novel strategy which gives rise to monospecific antibodies to these proteins as well as to the cDNA clones encoding them. A cDNA expression library was screened with a polyclonal antiserum raised against Triton X-114-extracted organelle proteins and clones were then grouped using antibodies affinity-purified on individual fusion proteins. The identification, molecular cloning and sequencing are described of a type 1 membrane protein (TGN38) which is located specifically in the trans-Golgi network.

Helix pomatia agglutinin binds specifically to the Golgi apparatus in cultured human fibroblasts and reveals two Golgi apparatus-specific glycoproteins

Fluorochrome-coupled Helix pomatia agglutinin (HPA), but not other lectin-conjugates with the same nominal specificity, bound specifically to the Golgi apparatus in cultured human fibroblasts, revealing a cytoplasmic juxtanuclear reticular structure. Unlike other Golgi-binding lectins the HPA-conjugates did not bind to the cell surface membrane or pericellular matrix. Experiments with 35S-methionine-labeled cells showed that HPA recognized two glycoproteins of Mr 170,000 and 400,000 among the secreted products of fibroblasts and two major cellular glycoproteins of Mr 40,000 and Mr 180,000 in Triton X-100 extracts of the cells. The two cellular HPA-binding polypeptides were also found in cells depleted of secretory products and in cells pulse-labeled shortly with 35S-methionine and then chased with methionine containing medium up to 12 h. These findings suggest that the two cellular glycoproteins recognized by HPA are retained in the Golgi apparatus and are therefore not precursors of secretory proteins. The results suggest that there are two endogenous, Golgi apparatus-specific glycoproteins in cultured human fibroblasts with terminal non-reducing O-glycosidic N-acetyl galactosaminyl residues.

Role of the intradiscal domain in rhodopsin assembly and function

The role of the intradiscal polypeptide loops in bovine rhodopsin has been investigated by deletions in the N-terminal tail and in loops B-C, D-E, and E-F as well as by single amino acid substitutions in the D-E loop. Mutants with three types of phenotypes were observed. Type I mutants showed a rhodopsin-like chromophore and glycosylation. Type II mutants did not regenerate the chromophore and showed abnormal glycosylation. Type III mutants showed poor chromophore regeneration and abnormal glycosylation. Reduced transducin activation was shown by some type I and III mutants. Single amino acid substitutions in the D-E loop gave mostly type I mutants. Deletions in loops B-C, D-E, and F-G gave type II mutants, whereas deletions in the N-terminal tail produced type III mutants. Systematic deletions of two adjacent amino acids in loop D-E indicated that the amino acid sequences 171-182 and 189-192 were essential to rhodopsin structure. Immunofluorescence double-staining and transmission electron microscopy of one type II mutant (with residues 189 and 190 deleted) showed that it was mostly in the endoplasmic reticulum, whereas the wild-type protein was in the plasma membrane. We conclude that the first step in the assembly of the rhodopsin molecule is the formation of a three-dimensional structure in the intradiscal domain involving the bulk of the out-of-the-membrane polypeptide segments followed by the linkage of Cys-110 and Cys-187 through a disulfide bond.

Intracellular processing and transport of NH2-terminally truncated forms of a hemagglutinin-neuraminidase type II glycoprotein

Six amino-terminal deletion mutants of the NH2-terminally anchored (type II orientation) hemagglutinin-neuraminidase (HN) protein of parainfluenza virus type 3 were expressed in tissue culture by recombinant SV-40 viruses. The mutations consisted of progressive deletions of the cytoplasmic domain and, in some cases, of the hydrophobic signal/anchor. Three activities were dissociated for the signal/anchor: membrane insertion, translocation, and anchoring/transport. HN protein lacking the entire cytoplasmic tail was inserted efficiently into the membrane of the endoplasmic reticulum but was translocated inefficiently into the lumen. However, the small amounts that were successfully translocated appeared to be processed subsequently in a manner indistinguishable from that of parental HN. Thus, the cytoplasmic domain was not required for maturation of this type II glycoprotein. Progressive deletions into the membrane anchor restored efficient translocation, indicating that the NH2-terminal 44 amino acids were fully dispensable for membrane insertion and translocation and that a 10-amino acid hydrophobic signal sequence was sufficient for both activities. These latter HN molecules appeared to be folded authentically as assayed by hemagglutination activity, reactivity with a conformation-specific antiserum, correct formation of intramolecular disulfide bonds, and homooligomerization. However, most (85-90%) of these molecules accumulated in the ER. This showed that folding and oligomerization into a biologically active form, which presumably represents a virion spike, occurs essentially to completion within that compartment but is not sufficient for efficient transport through the exocytotic pathway. Protein transport also appeared to depend on the structure of the membrane anchor. These latter mutants were not stably integrated in the membrane, and the small proportion (10-15%) that was processed through the exocytotic pathway was secreted. The maturation steps and some of the effects of mutations described here for a type II glycoprotein resemble previous observations for prototypic type I glycoproteins and are indicative of close similarities in these processes for proteins of both membrane orientations.

Small GTP-binding protein associated with Golgi cisternae

Eukaryotic cells seem to use GTP hydrolysis to regulate vesicular traffic in exocytosis and endocytosis. The best evidence for this comes from studies on the yeast Saccharomyces cerevisiae that have identified two small Ras-related GTP-binding proteins, Sec4p and Ypt1p, which control distinct stages of the secretory pathway. In mammalian cells the effects of a non-hydrolysable GTP analogue, GTP-gamma S, on different transport events have suggested that they also have proteins functionally related to yeast Sec4p and Ypt1p. The rab genes have recently been cloned and sequenced for rat and human and their proteins have highly conserved domains in common with Sec4p and Ypt1p (including a putative effector binding site). They are therefore good candidates for GTP-binding proteins involved in intracellular transport in mammalian cells. One of the Rab proteins (Rab1p) is the mammalian counterpart of Ypt1p (ref. 13). Here we report the localization of the protein Rab6p to the Golgi apparatus in several cell types. By immunolabelling and electron microscopy, Rab6p appears to be concentrated predominantly on the medial and trans cisternae and distributed over their entire surface.

Studies on the mechanisms of autophagy: formation of the autophagic vacuole

Autophagic vacuoles form within 15 min of perfusing a liver with amino acid-depleted medium. These vacuoles are bound by a "smooth" double membrane and do not contain acid phosphatase activity. In an attempt to identify the membrane source of these vacuoles, I have used morphological techniques combined with immunological probes to localize specific membrane antigens to the limiting membranes of newly formed or nascent autophagic vacuoles. Antibodies to three integral membrane proteins of the plasma membrane (CE9, HA4, and epidermal growth factor receptor) and one of the Golgi apparatus (sialyltransferase) did not label these vacuoles. Internalized epidermal growth factor and its membrane receptor were not found in nascent autophagic vacuoles but were present in lysosome-like degradative autophagic vacuoles. All these results suggested that autophagic vacuoles were not formed from plasma membrane, Golgi apparatus, or endosome constituents. Antisera prepared against integral membrane proteins (14, 25, and 40 kD) of the RER was found to label the inner and outer limiting membranes of almost all nascent autophagic vacuoles. In addition, ribophorin II was identified at the limiting membranes of many nascent autophagic vacuoles. Finally, secretory proteins, rat serum albumin and alpha 2u-globulin, were localized to the lumen of the RER and to the intramembrane space between the inner and outer membranes of some of these vacuoles. The results were consistent with the formation of autophagic vacuoles from ribosome-free regions of the RER.

Studies on the mechanisms of autophagy: maturation of the autophagic vacuole

Data presented in the accompanying paper suggests nascent autophagic vacuoles are formed from RER (Dunn, W. A. 1990. J. Cell Biol. 110:1923-1933). In the present report, the maturation of newly formed or nascent autophagic vacuoles into degradative vacuoles was examined using morphological and biochemical methods combined with immunological probes. Within 15 min of formation, autophagic vacuoles acquired acid hydrolases and lysosomal membrane proteins, thus becoming degradative vacuoles. A previously undescribed type of autophagic vacuole was also identified having characteristics of both nascent and degradative vacuoles, but was different from lysosomes. This intermediate compartment contained only small amounts of cathepsin L in comparison to lysosomes and was bound by a double membrane, typical of nascent vacuoles. However, unlike nascent vacuoles vet comparable to degradative vacuoles, these vacuoles were acidic and contained the lysosomal membrane protein, lgp120, at the outer limiting membrane. The results were consistent with the stepwise acquisition of lysosomal membrane proteins and hydrolases. The presence of mannose-6-phosphate receptor in autophagic vacuoles suggested a possible role of this receptor in the delivery of newly synthesized hydrolases from the Golgi apparatus. However, tunicamycin had no significant effect on the amount of mature acid hydrolases present in a preparation of autophagic vacuoles isolated from a metrizamide gradient. Combined, the results suggested nascent autophagic vacuoles mature into degradative vacuoles in a stepwise fashion: (a) acquisition of lysosomal membrane proteins by fusing with a vesicle deficient in hydrolytic enzymes (e.g., prelysosome); (b) vacuole acidification; and (c) acquisition of hydrolases by fusing with preexisting lysosomes or Golgi apparatus-derived vesicles.

Identification of a novel nucleotide-sensitive microtubule-binding protein in HeLa cells

A protein of Mr 170,000 (170K protein) has been identified in HeLa cells, using an antiserum raised against HeLa nucleotide-sensitive microtubule-binding proteins. Affinity-purified antibodies specific for this 170K polypeptide were used for its characterization. In vitro sedimentation of the 170K protein with taxol microtubules polymerized from HeLa high-speed supernatant is enhanced in the presence of an ATP depleting system, but unaffected by the non-hydrolyzable ATP analogue AMP-PNP. In addition, it can be eluted from taxol microtubules by ATP or GTP, as well as NaCl. Thus it shows microtubule-binding characteristics distinct from those of the previously described classes of nucleotide-sensitive microtubule-binding proteins, the motor proteins kinesin and cytoplasmic dynein, homologues of which are also present in HeLa cells. The 170K protein sediments on sucrose gradients at approximately 6S, separate from kinesin (9.5S) and cytoplasmic dynein (20S), further indicating that it is not associated with these motor proteins. Immunofluorescence localization of the 170K protein shows a patchy distribution in interphase HeLa cells, often organized into linear arrays that correlate with microtubules. However, not all microtubules are labeled, and there is a significant accumulation of antigen at the peripheral ends of microtubules. In mitotic cells, 170K labeling is found in the spindle, but there is also dotty labeling in the cytoplasm. After depolymerization of microtubules by nocodazole, the staining pattern is also patchy but not organized in linear arrays, suggesting that the protein may be able to associate with other intracellular structures as well as microtubules. In vinblastine-treated cells, there is strong labeling of tubulin paracrystals, and random microtubules induced in vivo by taxol are also labeled by the antibodies. These immunofluorescence labeling patterns are stable to extraction of cells with Triton X-100 before fixation, further suggesting an association of the protein with cytoplasmic structures. In vivo, therefore, the 170K protein appears to be associated with a subset of microtubules at discrete sites. Its in vitro behavior suggests that it belongs to a novel class of nucleotide-sensitive microtubule-binding proteins.

A new recombinant DNA strategy for the molecular cloning of rare membrane proteins

We have constructed a cDNA library in the plasmid expression vector pUEX enriched in sequences encoding membrane proteins. The procedure involved positive selection of sequences common to two different rat tissues (thus excluding tissue-specific mRNA) followed by positive selection between this material and RNA extracted from membrane bound polysomes (thus excluding cytoplasmic proteins). The resultant library prepared from rat kidney cDNA hybridized with rat liver poly(A)+ RNA, contained 30,000 clones and was shown to be enriched in cDNAs encoding membrane proteins. Seventeen clones selected because they encode large fusion proteins were shown to be single copy in the library, and not present in nucleotide data banks. Thus the strategy is particularly suitable for cloning low abundance cDNAs encoding membrane proteins.

Identification of a Golgi-associated protein that undergoes mitosis dependent phosphorylation and relocation

By means of a monoclonal antibody (BH3), we have identified a 57-kD protein (p57) that in interphase is restricted largely to the perinuclear region of the cell. Double label immunofluorescence microscopy suggests localization of p57 to the Golgi complex and associated membranous structures. Protease protection experiments and chemical extractability indicate that p57 is a peripheral membrane protein exposed to the cytoplasm. p57 displays unique behavior during mitosis. At the end of G2 or in early prophase, p57 leaves the perinuclear region and accumulates very rapidly within the nucleus, at a time when the nuclear envelope is still intact and before nuclear lamina disassembly. This relocation of p57 coincides with its hyperphosphorylation on serine and threonine residues. After nuclear envelope breakdown p57 becomes uniformly distributed throughout the mitotic cytoplasm until in late telophase when it returns to its perinuclear location and is once again excluded from the nucleus. The behavior of p57 during mitosis suggests that it may play a role in the cellular reorganization evident during mitotic prophase.