Mechanisms for the incorporation of proteins in membranes and organelles

Expression of a novel histone 2B during mouse spermiogenesis

During mammalian spermiogenesis transitional proteins and protamines replace histones on the DNA as the chromatin condenses. While previous studies suggested that histone genes are inactive postmeiotically, we have shown both by steady-state RNA analysis and nuclear run-off transcription assays that histone 2b (H2b) transcription occurs in mouse round spermatids. In addition, a novel H2b cDNA clone has been isolated from an adult mouse testes cDNA library. The sequence of this cDNA clone predicts a protein that is extremely similar to other mouse H2b proteins, except at the carboxyl-terminus where the testes H2b contains an additional 12 amino acids, seven of which are hydrophobic. In contrast to the replication-dependent histone mRNAs, the 3' untranslated region of this cDNA contains the poly(A) addition sequence (AAUAAA) upstream of a poly(A) tract. Furthermore, the conserved hairpin structure immediately upstream of replication-dependent histone mRNA termini is not present. Northern blot analysis of RNA from embryonic, ovarian, spermatogenic, and a variety of somatic tissues reveals that this novel H2b transcript is spermatid specific. The H2b mRNA is in polyribosomes isolated from spermatogenic cells, strongly suggesting that it is translated during spermiogenesis.

Molecular cloning of syndecan, an integral membrane proteoglycan

We describe cDNA clones for a cell surface proteoglycan that bears both heparan sulfate and chondroitin sulfate and that links the cytoskeleton to the interstitial matrix. The cDNA encodes a unique core protein of 32,868 D that contains several structural features consistent with its role as a glycosamino-glycan-containing matrix anchor. The sequence shows discrete cytoplasmic, transmembrane, and NH2-terminal extracellular domains, indicating that the molecule is a type I integral membrane protein. The cytoplasmic domain is small and similar in size but not in sequence to that of the beta-chain of various integrins. The extracellular domain contains a single dibasic sequence adjacent to the extracellular face of the transmembrane domain, potentially serving as the protease-susceptible site involved in release of this domain from the cell surface. The extracellular domain contains two distinct types of putative glycosaminoglycan attachment sites; one type shows sequence characteristics of the sites previously described for chondroitin sulfate attachment (Bourdon, M. A., T. Krusius, S. Campbell, N. B. Schwartz, and E. Ruoslahti. 1987. Proc. Natl. Acad. Sci. USA. 84:3194-3198), but the other type has newly identified sequence characteristics that potentially correspond to heparan sulfate attachment sites. The single N-linked sugar recognition sequence is within the putative chondroitin sulfate attachment sequence, suggesting asparagine glycosylation as a mechanism for regulating chondroitin sulfate chain addition. Both 5' and 3' regions of this cDNA have sequences substantially identical to analogous regions of the human insulin receptor cDNA: a 99-bp region spanning the 5' untranslated and initial coding sequences is 67% identical and a 35-bp region in the 3' untranslated region is 81% identical in sequence. mRNA expression is tissue specific; various epithelial tissues show the same two sizes of mRNA (2.6 and 3.4 kb); in the same relative abundance (3:1), the cerebrum shows a single 4.5-kb mRNA. This core protein cDNA describes a new class of molecule, an integral membrane proteoglycan, that we propose to name syndecan (from the Greek syndein, to bind together).

Cytochrome P-450 from the Mesocarp of Avocado (Persea americana)

The microsomal fraction from the mesocarp of avocado (Persea americana) is one of few identified rich sources of plant cytochrome P-450. Cytochrome P-450 from this tissue has been solubilized and purified. Enzymatic assays (p-chloro-N-methylaniline demethylase) and spectroscopic observations of substrate binding suggest a low spin form of the cytochrome, resembling that in the microsomal membrane, can be recovered. However, this preparation of native protein is a mixture of nearly equal proportions of two cytochrome P-450 polypeptides that have been resolved only under denaturing conditions. Overall similarities between these polypeptides include indistinguishable amino acid compositions, similar trypsin digest patterns, and cross reactivity with the same antibody. The amino terminal sequences of both polypeptides are identical, with the exception that one of them lacks a methionine residue at the amino terminus. This sequence exhibits some similarities with the membrane targeting signal found at the amino terminus of most mammalian cytochromes P-450.

Cloning of the mRNA for the protein that crosslinks to the egg peptide speract

An apparent receptor for the egg peptide speract (Gly-Phe-Asp-Leu-Asn-Gly-Gly-Gly-Val-Gly) was identified by covalently coupling a radiolabeled speract analogue to intact spermatozoa and was then purified by DEAE-Sepharose chromatography and preparative gel electrophoresis after solubilization with Lubrol PX. The purified, crosslinked protein was digested with Staphylococcus aureus V8 protease and a resultant peptide, purified from polyacrylamide slab gel slices, was shown to have the amino acid sequence Val-Ser-Ala-Pro-Phe-Asp-Leu-Glu-Ala-Pro-Phe-Ile-Ile-Asp-Gly-Ile. Polyclonal antiserum, generated against a synthetic peptide that corresponded to the above sequence, immunoprecipitated the radiolabeled crosslinked protein and reacted with a Mr 77,000 protein on immunoblots, demonstrating that the sequenced peptide originated from the apparent receptor. A clone containing a 2.5-kilobase insert was subsequently isolated from a sea urchin testis cDNA library that contained DNA sequences encoding an open reading frame of 532 amino acids that included the above peptide sequence. The deduced amino acid sequence suggests that the protein contains a 26-residue amino-terminal signal peptide, a large extracellular domain relatively rich in cysteine (5%) that includes a four-fold repeat of about 115 amino acids, a single membrane-spanning region, and only 12 amino acid residues extending into the cytoplasm. Analysis of total RNA from Strongylocentrotus purpuratus testis by Northern blot revealed a 2.5-kilobase RNA. Preliminary data show the presence of hybridizing RNA of the same apparent size in other sea urchin species, including Arbacia punctulata, which does not respond to speract.

Biosynthesis of the Tonoplast H-ATPase from Oats

To determine whether the tonoplast-type H(+)-ATPase was differentially synthesized in various parts of the oat seedling, sections of 4-day-old oat (Avena sativa L. var Lang) seedlings were labeled in vivo with [(35)S]methionine and ATPase subunits were precipitated with polyclonal antisera. ATPase subunits were detected in all portions of the seedling with the exception of the seed. Lesser amounts of the 60 and 72 kilodalton polypeptides of the ATPase were found in apical regions (0-5 millimeter) than in maturing regions (10-15, or 20-25 millimeter from the tip) of the roots or shoots. To initiate a study of the biosynthesis of the ATPase, the intracellular site of synthesis for two peripheral ATPase subunits was investigated. Poly(A) RNA from either free or membrane-bound polysomes was isolated and translated in vitro. Message encoding the 72 kilodalton (catalytic) subunit was found predominantly in mRNA isolated from membrane-bound polysomes. In contrast, the message for the 60 kilodalton (putative regulatory) subunit was found predominantly on free polysomes. Polypeptides synthesized in vivo or obtained from RNA translated in vitro exhibited no apparent size differences (limit of resolution, approximately 1 kilodalton), suggesting the absence of cleaved precursors for the 72 or 60 kilodalton subunits. These data suggest a complex mechanism for the synthesis and assembly of the tonoplast ATPase.

Characterization of the alkane-inducible cytochrome P450 (P450alk) gene from the yeast Candida tropicalis: identification of a new P450 gene family

The P450alk gene, which is inducible by the assimilation of alkane in Candida tropicalis, was sequenced and characterized. Structural features described in promoter and terminator regions of Saccharomyces yeast genes are present in the P450alk gene and some particular structures are discussed for their possible role in the inducibility of this gene. Expression of the P450alk gene was achieved in Saccharomyces cerevisiae using the yeast alcohol dehydrogenase expression system after removal of the P450alk gene flanking regions. The resultant expressed protein had a molecular mass slightly greater than that of P450alk from C. tropicalis. This alteration did not prevent the function and the localization of P450alk expressed in S. cerevisiae, as this organism showed an acquired microsome-bound activity for the terminal hydroxylation of lauric acid. The deduced P450alk amino acid sequence was compared with members of the nine known P450 gene families. These comparisons indicated that P450alk had a low relationship with these members and was therefore the first member (A1) of a new P450 gene family (LII).

Evidence for a two-step mechanism involved in assembly of functional signal recognition particle receptor

The signal recognition particle (SRP) and SRP receptor act sequentially to target nascent secretory proteins to the membrane of the ER. The SRP receptor consists of two subunits, SR alpha and SR beta, both tightly associated with the ER membrane. To examine the biogenesis of the SRP receptor we have developed a cell-free assay system that reconstitutes SR alpha membrane assembly and permits both anchoring and functional properties to be assayed independently. Our experiments reveal a mechanism involving at least two distinct steps, targeting to the ER and anchoring of the targeted molecule on the cytoplasmic face of the membrane. Both steps can be reconstituted in vitro to restore translocation activity to ER microsomes inactivated by alkylation with N-ethyl-maleimide. The characteristics elucidated for this pathway distinguish it from SRP-dependent targeting of secretory proteins, SRP-independent ER translocation of proteins such as prepromellitin, and direct insertion mechanisms of the type exemplified by cytochrome b5.

Isolation of Chinese hamster ovary cell lines temperature conditional for the cell-surface expression of integral membrane glycoproteins

A procedure is described to select mutants of Chinese hamster ovary cells that are conditionally defective for the cell-surface expression of integral membrane glycoproteins, including the hemagglutinin (HA) of influenza virus. Using a combination of cell sorting and biochemical screening, seven cell lines were obtained that express more cell-surface HA at 32 degrees C than at 39 degrees C. The production of infectious vesicular stomatitis virus, whose growth requires insertion of an integral membrane protein into the plasma membrane, was also temperature conditional in the majority of these mutant cell lines. Five of the lines synthesized apparently normally core-glycosylated HA at the elevated temperature but the protein was neither displayed on the cell surface nor accumulated intracellularly. In these cell lines, little or no terminally glycosylated HA molecules were observed after synthesis at 39 degrees C. By contrast, the core glycosylation of HA and several other integral membrane proteins was abnormal in the remaining two cell lines at both permissive and restrictive temperatures, due to a lesion in a cellular gene(s) that affects the formation of and/or the addition of mannose-rich oligosaccharide chains to newly synthesized polypeptides. Although HA was transported to the plasma membrane at both 32 and 39 degrees C, it did not accumulate on the cell surface at the higher temperature, apparently because of an increased rate of degradation.

The MIC2 gene product: epitope mapping and structural prediction analysis define an integral membrane protein

The MIC2 locus is located in the pseudoautosomal (pairing) region of human X and Y chromosomes (Goodfellow et al., Science 234, 740-743, 1986). Despite extensive molecular analysis of MIC2 (see Darling et al., Cold Spring Harb. Symp. quant. Biol. 51, 205-211, 1986), study of the gene product has been limited (Banting et al., EMBO J. 41, 1967-1972, 1985). Here we report the combined use of monoclonal antibodies, plasmid expression vectors and structural prediction analysis to define the MIC2 gene product as an integral membrane protein. Random overlapping fragments of a cDNA, corresponding to the MIC2 locus, were cloned into the plasmid expression vector pEX1 (Stanley and Luzio, EMBO J. 3, 1429-1434, 1984) to produce "epitope libraries". Six different monoclonal antibodies, known to recognize the extracellular region of the MIC2 gene product, were used to screen these libraries. Clones recognized by these antibodies were sequenced and their sequences aligned with one another and with the complete MIC2 cDNA sequence. All antibodies tested recognized adjacent and/or overlapping epitopes in the same region of the molecule. These results complement data from a hydropathy plot of a conceptual translation of the MIC2 sequence, which demonstrated the presence of a single long hydrophobic region in the mature protein. Since the antibodies recognize the extracellular portion of the molecule, we were able to determine the orientation in the plasma membrane. This method of analysis is generally applicable where antibodies and cloned cDNAs are available.

Changes with senescence in the fine structure of the granular convoluted tubule of the submandibular gland of the mouse

Cells of the granular convoluted tubules (GCTs) of the submandibular gland of senescent male mice show structural changes indicative of functional decline. In order to define the nature of these age-related changes more clearly, the fine structure of GCT cells of 12- and 28-month-old males was compared. In old mice, there was cell-to-cell variation in the extent of these changes, with some cells of senescent males appearing no different from those of young adults. In affected cells the most striking alterations were seen in secretion granules and lysosomal elements. Secretion granules varied greatly in size, with some GCT cells having only very fine apical granules. Secondary lysosomes and large lipofuscin granules were frequent in the basal cytoplasm. Very large dense bodies (3-5 micron) occurred in many cells. These possibly represent intracellular pools of released secretory materials, as they were occasionally seen in continuity with the luminal contents. Structures whose appearance was intermediate between the very large dense bodies and lipofuscin granules were common, suggesting crinophagic activity. There was an apparent decrease in numbers of polysomes and in the extent of the Golgi apparatus. These fine structural changes are consistent with impairments with advanced age in synthesis and posttranslational processing of secretory products by affected GCT cells. In addition to cell-to-cell variation in any one male, there was also interanimal variation in the degree and extent of these senescent changes.

Estrogen influence on maturational pathway of murine mammary tumor virus: an immunoelectron microscopy study

Modifications induced by estrogens on hormone-independent murine mammary tumor (MMT) and its main etiological agent, the MMT virus (MMTV), are reported. High doses of estrogens released continuously from silastic capsules delay significantly the development of transplanted tumors into syngeneic hosts. Neoplastic cells present a striking cytoplasmic vacuolization and changes in the MMTV differentiation pattern. Mature virions are detected budding into cytoplasmic vacuoles instead of the extracellular space as in spontaneous and untreated transplanted tumors. This phenomenon is reversed after estrogen withdrawal at the first sign of tumor development. Application of electron microscope immunocytochemistry with colloidal gold-protein A complex and multiple monospecific antibodies reveals several interesting features. In spontaneous and untreated tumor grafts, structural viral proteins p14 and p25 appear in both intracytoplasmic capsids and mature extracellular viruses. By contrast glycoprotein gp55 labels only the envelope of mature virus. In estrogen-treated tumors this antigenic pattern is modified and the gp55 is detected in those atypical virions maturing into the intracytoplasmic vacuoles. These observations led to the conclusions that the delay in the development of hormone-independent mammary tumors caused by estrogen is due to an abnormal maturational viral process and that estrogens induce alterations of polarity in the translocation process of viral envelope glycoproteins.

Mitochondrial protein import

Most mitochondrial proteins are synthesized as precursor proteins on cytosolic polysomes and are subsequently imported into mitochondria. Many precursors carry amino-terminal presequences which contain information for their targeting to mitochondria. In several cases, targeting and sorting information is also contained in non-amino-terminal portions of the precursor protein. Nucleoside triphosphates are required to keep precursors in an import-competent (unfolded) conformation. The precursors bind to specific receptor proteins on the mitochondrial surface and interact with a general insertion protein (GIP) in the outer membrane. The initial interaction of the precursor with the inner membrane requires the mitochondrial membrane potential (delta psi) and occurs at contact sites between outer and inner membranes. Completion of translocation into the inner membrane or matrix is independent of delta psi. The presequences are cleaved off by the processing peptidase in the mitochondrial matrix. In several cases, a second proteolytic processing event is performed in either the matrix or in the intermembrane space. Other modifications can occur such as the addition of prosthetic groups (e.g., heme or Fe/S clusters). Some precursors of proteins of the intermembrane space or the outer surface of the inner membrane are retranslocated from the matrix space across the inner membrane to their functional destination ('conservative sorting'). Finally, many proteins are assembled in multi-subunit complexes. Exceptions to this general import pathway are known. Precursors of outer membrane proteins are transported directly into the outer membrane in a receptor-dependent manner. The precursor of cytochrome c is directly translocated across the outer membrane and thereby reaches the intermembrane space. In addition to the general sequence of events which occurs during mitochondrial protein import, current research focuses on the molecules themselves that are involved in these processes.

The lipid environment of the nicotinic acetylcholine receptor in native and reconstituted membranes

Detailed knowledge of the membrane framework surrounding the nicotinic acetylcholine receptor (AChR) is key to an understanding of its structure, dynamics, and function. Recent theoretical models discuss the structural relationship between the AChR and the lipid bilayer. Independent experimental data on the composition, metabolism, and dynamics of the AChR lipid environment are analyzed in the first part of the review. The composition of the lipids in which the transmembrane AChR chains are inserted bears considerable resemblance among species, perhaps providing this evolutionarily conserved protein with an adequate milieu for its optimal functioning. The effects of lipids on the latter are discussed in the second part of the review. The third part focuses on the information gained on the dynamics of AChR and lipids in the membrane, a section that also covers the physical properties and interactions between the protein, its immediate annulus, and the bulk lipid bilayer.

The insulin receptor: structure and function

Promising progress in understanding the molecular basis of insulin action has been achieved by demonstrating that the insulin receptor is an insulin-sensitive tyrosine kinase. Here we discuss the structure of this receptor kinase and compare it with receptors for related growth factors. We review the known modes to regulate the receptor kinase activity, either through its autophosphorylation (on tyrosine residues) or through its phosphorylation by other kinases (on serine and threonine residues). We discuss the role of the receptor kinase activity in hormone signal transduction in light of results indicating a reduced kinase activity in insulin-resistant states. Finally, studies to identify natural substrates for the insulin receptor kinase are presented. The possible physiological role of these phosphorylated substrates in mediating insulin action is evaluated.

LH action in the Leydig cell: modulation by angiotensin II and corticotropin releasing hormone, and regulation of P450(17) alpha mRNA

Luteinizing hormone is the major regulator of Leydig cell differentiation and steroidogenic function. A number of hormones produced by the Leydig cell (e.g. estrogen, angiotensin, CRF, vasopressin) and the tubular compartment (inhibin, TGF beta), can influence both acute and long-term actions of LH. Conversely, hormones produced in the Leydig cells modulate tubular function (e.g. androgen, beta-endorphin, oxytocin). The LH stimulatory event can be negatively influenced by the action of angiotensin II through the guanyl nucleotide inhibitory unit of adenylate cyclase. We have recently discovered an action of corticotrophin releasing hormone through specific high-affinity low-capacity receptors in the Leydig cells which involves a pertussis toxin insensitive guanyl nucleotide regulatory unit with interaction between signalling pathways and resulting inhibition of LH induced cAMP generation and consequently of steroidogenesis. In contrast to other tissues the CRF receptor in the Leydig cells did not couple to Gs. CRF action is exerted through direct or indirect action of protein kinase C, at the level of the catalytic subunit of adenylate cyclase. Physiological increases in endogenous LH cause positive regulation of membrane receptors and steroidogenesis, while major elevations in circulating gonadotropin can induce down-regulation of LH receptors and desensitization of steroid responses in the adult cell. Gonadotropin-induced desensitization in adult rat tests include an estrogen mediated steroidogenic lesion of the microsomal enzymes 17 alpha-hydroxylase/17,20-desmolase. For further understanding of the regulation of this key enzyme of the androgen pathway the rat P450(17) alpha cDNA was cloned and sequenced. This cDNA expressed in COS-1 cells 17 alpha-hydroxylase/17,20-desmolase activities. From the deduced amino acid sequence, two transmembrane regions were identified, a signal peptide for insertion in the ER, and a 2nd transmembrane region separated from the first by 122 amino acids. The carboxy terminal non-transmembrane region possesses 4 hydrophobic clefts, of which cleft II would contain the putative steroid binding site for both hydroxylase and lyase activities. The rat cDNA was employed to evaluate the hormonal regulation of mRNA levels in adult and fetal Leydig cells. Low dose hCG treatment caused an early increase in mRNA levels followed by a return to control values at later times, while with higher desensitizing doses the initial increase in mRNA was followed by a marked reduction in mRNA at 24 h and a small recovery at 48 h. Fetal rat Leydig cells treated with E2 showed a 70% decrease in P450 mRNA levels, and testosterone production closely followed the changes in mRNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Rat testis P-450(17)alpha cDNA: the deduced amino acid sequence, expression and secondary structural configuration

A complete amino acid sequence for rat testis P-450(17)alpha was deduced from nucleotide analysis of a cDNA clone isolated from a rat Leydig cell cDNA library. This DNA clone, containing initiation and termination codons and a polyA tail, translated a polypeptide in COS-1 cells that expressed both 17 alpha-hydroxylase and 17,20 lyase activities. It exhibited significant similarity to the nucleotide and deduced amino acid sequences of the bovine and human cytochrome P-450(17)alpha, particularly with respect to the highly conserved regions and secondary structure. The P-450(17)alpha appears to be anchored to the membrane of the endoplasmic reticulum through two transmembrane regions, specifically the N terminal insertion peptide and the stop-transfer sequence. Hydropathic analysis indicates that the remainder of the C terminus is associated with the membrane through four hydrophobic clefts, including the putative steroid binding site.

Budding of small vesicles from the rough-surfaced endoplasmic reticulum in secretory ameloblasts of rat molar tooth germs

The budding of small vesicles from the rough-surfaced endoplasmic reticulum (rER) was examined in the secretory ameloblast of rat molar tooth germs by ordinary fixation or prolonged osmium fixation. The budding of small vesicles from the rER was observed not only at the special region (transitional region) of the rER system, which abutted on the cis-face of the Golgi apparatus, but also at other regions of the rER in the secretory ameloblast. Small vesicles (presumed to be transitional vesicles) were adjacent to the rER, which also showed budding of vesicles. After prolonged osmium treatment, osmium deposits appeared in small vesicles, as well as in the cisternae of the cis saccule of the Golgi apparatus. Small vesicles containing osmium deposits were located at various regions of the cell, including the cis-face of the Golgi apparatus. These findings indicate that the budding of small vesicles from the rER is not restricted to the transitional region of the rER system of the secretory ameloblast, but is found at various regions of the cell. This indicates that newly synthesized proteins may be transferred from the rER cisternae to the transitional vesicles not only in the transitional region of the rER system adjacent to the Golgi apparatus, but also in other regions of the secretory ameloblast.

Topogenic analysis of the human immunodeficiency virus type 1 envelope glycoprotein, gp160, in microsomal membranes

The orientation in cellular membranes of the 856 amino acid envelope glycoprotein precursor, gp160, of human immunodeficiency virus type 1 was investigated in vitro. Variants of the env gene were transcribed using the bacteriophage SP6 promoter, translated using a rabbit reticulocyte lysate, and translocated into canine pancreatic microsomal membranes. Immunoprecipitation studies of gp160 variants using antibodies specific for various gp160-derived polypeptides provided evidence that the external (cell surface) domain of gp160 begins at the mature amino terminus of the protein and continues through amino acid 665. A stop-transfer sequence (transmembrane domain) was identified in a hydrophobic region COOH-terminal to amino acid 665 and NH2-terminal to amino acid 732. Protease protection experiments demonstrated that gp160 possesses a single cytoplasmic domain COOH-terminal to residue 707. Membrane extraction studies using carbonate buffer provided evidence that the 29 amino acid hydrophobic domain (residues 512-541) of gp160 was unable to serve as a stop-transfer sequence. Finally, we propose that the cytoplasmic tail of gp160 forms a secondary association with the microsomal membranes.

Primary structure of the cytochrome P450 lanosterol 14 alpha-demethylase gene from Candida tropicalis

We report the nucleotide sequence of the gene and flanking DNA for the cytochrome P450 lanosterol 14 alpha-demethylase (14DM) from the yeast Candida tropicalis ATCC750. An open reading frame (ORF) of 528 codons encoding a 60.9-kD protein is identified. This ORF includes a characteristic heme-binding domain, HR2, common to all P450 proteins. This protein and the 14DM from Saccharomyces cerevisiae share 66.5% identical and 23.1% conservatively replaced amino acids in a 516-amino-acid alignment, and thus are orthologous forms of the P450LIA1 gene. Conversely, C. tropicalis 14DM shares relatively little sequence similarity with P450alk, the predominant P450 protein present when this organism is grown on n-alkanes. Sequence information of these three yeast P450s will be useful for structure-function analyses in the future.

Insertion of a multispanning membrane protein occurs sequentially and requires only one signal sequence

To study the insertion of multispanning membrane proteins into the endoplasmic reticulum, we constructed novel proteins on the cDNA level by repeating, up to four times, the internal signal-anchor domain of the asialoglycoprotein receptor H1. Upon in vitro translation in the presence of microsomes, these polypeptides are indeed inserted as polytopic membrane proteins. The first hydrophobic domain functions as a signal and the second as a stop-transfer sequence, while the third initiates a second translocation process, halted again by the fourth. We were able to demonstrate that insertion occurs sequentially, starting with the first apolar segment from the amino terminus. By replacing the original signal-anchor domains by a mutant sequence not recognized by signal recognition particle (SRP), it was shown that only the first hydrophobic domain needs to be a signal sequence and that the second translocation event does not require SRP.

Sequence of contactin, a 130-kD glycoprotein concentrated in areas of interneuronal contact, defines a new member of the immunoglobulin supergene family in the nervous system

The primary amino acid sequence of contactin, a neuronal cell surface glycoprotein of 130 kD that is isolated in association with components of the cytoskeleton (Ranscht, B., D. J. Moss, and C. Thomas. 1984. J. Cell Biol. 99:1803-1813), was deduced from the nucleotide sequence of cDNA clones and is reported here. The cDNA sequence contains an open reading frame for a 1,071-amino acid transmembrane protein with 962 extracellular and 89 cytoplasmic amino acids. In its extracellular portion, the polypeptide features six type 1 and two type 2 repeats. The six amino-terminal type 1 repeats (I-VI) each consist of 81-99 amino acids and contain two cysteine residues that are in the right context to form globular domains as described for molecules with immunoglobulin structure. Within the proposed globular region, contactin shares 31% identical amino acids with the neural cell adhesion molecule NCAM. The two type 2 repeats (I-II) are each composed of 100 amino acids and lack cysteine residues. They are 20-31% identical to fibronectin type III repeats. Both the structural similarity of contactin to molecules of the immunoglobulin supergene family, in particular the amino acid sequence resemblance to NCAM, and its relationship to fibronectin indicate that contactin could be involved in some aspect of cellular adhesion. This suggestion is further strengthened by its localization in neuropil containing axon fascicles and synapses.

Prepupal differentiation of Drosophila imaginal discs: identification of four genes whose transcripts accumulate in response to a pulse of 20-hydroxyecdysone

We have isolated and initially characterized a novel set of four genes expressed during the prepupal differentiation of imaginal discs of Drosophila melanogaster. These four ecdysone-dependent genes are named EDG-42A, EDG-64CD, EDG-78E and EDG-84A-1 based on their respective chromosomal locations. Their expression is like that expected for genes encoding proteins that participate in the formation of the pupal cuticle. Transcripts complementary to these genes accumulate in imaginal discs during an 18-hr in vitro culture period that begins with a 6-hr pulse of 20-hydroxyecdysone (20-HE). Transcripts for three of these genes were not detected in imaginal discs following culture in the absence or the continuous presence of 20-HE (1 microgram/ml). Transcripts corresponding to EDG-64CD exhibit delayed accumulation in the continuous presence of 20-HE. Transcripts corresponding to three of the genes are only detected in the prepupal stage of development. Only EDG-64CD is complementary to transcripts present at other stages of development. One of the genes, EDG-78E, encodes a pupal cuticle protein. This is the first reported isolation of a set of steroid hormone-responsive genes that require first the presence, then removal of hormone for transcript accumulation.

Processing, secretion, and biological properties of a novel growth factor of the fibroblast growth factor family with oncogenic potential

We recently reported that the protein encoded in a novel human oncogene isolated from Kaposi sarcoma DNA was a growth factor with significant homology to basic and acidic fibroblast growth factors (FGFs). To study the properties of this growth factor (referred to as K-FGF) and the mechanism by which the K-fgf oncogene transforms cells, we have studied the production and processing of K-FGF in COS-1 cells transfected with a plasmid encoding the K-fgf cDNA. The results show that, unlike basic and acidic FGFs, the K-FGF protein is cleaved after a signal peptide, glycosylated, and efficiently secreted as a mature protein of 176 or 175 amino acids. Inhibition of glycosylation impaired secretion, and the stability of the secreted K-FGF was greatly enhanced by the presence of heparin in the cultured medium. We have used the conditioned medium from transfected COS-1 cells to test K-FGF biological activity. Similar to basic FGF, the K-FGF protein was mitogenic for fibroblasts and endothelial cells and induced the growth of NIH 3T3 mouse cells in serum-free medium. Accordingly, K-fgf-transformed NIH 3T3 cells grew in serum-free medium, consistent with an autocrine mechanism of growth. We have also expressed the protein encoded in the K-fgf protooncogene in COS-1 cells, and it was indistinguishable in its molecular weight, glycosylation, secretion, and biological activity from K-FGF. Taken together, these results suggest that the mechanism of activation of this oncogene is due to overexpression rather than to mutations in the coding sequences.

Protein translocation across membranes

Many newly synthesized proteins must be translocated across a membrane to reach their final destinations. Translocation requires a signal on the protein itself, a loose conformation of the protein, energy, and receptor-like components in the cytosol and on the target membrane.

Comparison of the bovine herpesvirus 1 gI gene and the herpes simplex virus type 1 gB gene

In a previous report, we localized the gene for a 130-kilodalton envelope glycoprotein (gI) of bovine herpesvirus 1 (BHV-1) to a 3.6-kilobase HpaI-KpnI restriction endonuclease fragment from the long unique region of the BHV-1 genome (map position 0.405 to 0.432) and showed that a herpes simplex virus 1 (HSV-1) glycoprotein B (gB) probe uniquely hybridized to this BHV-1 restriction fragment. Here we present the complete nucleotide sequence of the BHV-1 gI gene and the predicted 932-amino-acid sequence of the gI primary translation product. Comparison with the published nucleotide sequence of the HSV-1 (KOS) gB gene (D. J. Bzik, B. A. Fox, N. A. DeLuca, and S. Person, Virology 133:301-314, 1984) reveals a similarity of 56.3% at the nucleotide level and 45.9% at the amino acid level. Upstream of the proposed gI coding region are potential mRNA transcriptional promoter elements including a TATA box and multiple Sp1 binding sites (GC boxes). Downstream of the gI coding region are two sequence elements associated with mRNA cleavage and polyadenylation (AATAAA and a GT-rich region roughly 30 nucleotides further downstream). Like HSV-1 gB, the predicted gI amino acid sequence exhibits two broad hydrophobic regions likely to represent a transient amino-terminal signal sequence and a transmembrane anchor domain (near the carboxyl terminus). Additional features shared with gB include 6 potential N-linked glycosylation sites and 10 highly conserved cysteine residues in the gI extracellular domain. Two regions of nonsimilarity between gI and gB are a centrally located 22-amino-acid region of gI for which there is essentially no gB counterpart and the transient amino-terminal leaders which differ in both size and sequence. The hydrophobic signal sequence of the gI leader, unlike that of gB, is preceded by an unusually large region of predominantly hydrophilic amino acids. The unusual length of the gI leader may result from an overlap between that portion of the gI coding region and a potential upstream coding region.

Molecular cloning of a cDNA coding biliary glycoprotein I: primary structure of a glycoprotein immunologically crossreactive with carcinoembryonic antigen

We have isolated and sequenced four overlapping cDNA clones from a normal adult human colon library, which together gave the entire nucleotide sequence for biliary glycoprotein I (BGP I). BGP I is a member of the carcinoembryonic antigen (CEA) gene family, which is a subfamily in the immunoglobulin gene superfamily. The deduced amino acid sequence of the combined clones for BGP I revealed a 34-residue leader sequence followed by a 108-residue N-terminal domain, a 178-residue immunoglobulin-like domain, a 108-residue region specific to BGP I, a 24-residue transmembrane domain, and a 35-residue cytoplasmic domain. The nucleotide sequence of BGP I exhibited greater than 80% identity with CEA and nonspecific crossreacting antigen (NCA) in the leader peptide, N-terminal domain, and immunoglobulin-like domain. The BGP I-specific domain, designated A', was 56.7% and 55.8% identical at the nucleotide level and 42.6% and 39.6% identical at the amino acid level to the immunoglobulin-like domain of NCA and the first immunoglobulin-like domain of CEA, respectively. Beyond nucleotide position 1375 the 3' region of the BGP I cDNA was found to be specific for BGP I. Hybridization of a probe from this region to electrophoretic blots of RNAs from different human tissues showed a predominant 2.8-kilobase (kb) message accompanied by weaker bands 4.1 and 2.1 kb in size. The same probe gave a single band in Southern blot analysis of restricted total human DNA. Using a coding region probe from the BGP I domain A', we observed 4.1- and 2.1-kb messages. Lack of the 2.8-kb band suggested that different forms of BGP I may be generated by posttranscriptional modification of the same gene. We propose that BGP I diverged from NCA by acquiring an immunoglobulin-like domain substantially different from the domains found in NCA or CEA and also a new cytoplasmic domain. The latter feature should result in a substantially different membrane anchorage mechanism of BGP I compared to CEA, which lacks the cytoplasmic domain and is anchored via a phosphatidylinositol-glycan structure. Protein structural analysis of BGP I isolated from human bile revealed a blocked N terminus, 129 amino acids of internal sequence that are in agreement with the translated cDNA sequence, and five glycosylation sites in the peptides sequenced.

cDNA cloning and expression of murine macrophage colony-stimulating factor from L929 cells

A 4-kilobase and a 2-kilobase cDNA clone encoding a murine macrophage colony-stimulating factor have been isolated. Except for 2 amino acid residue differences, these two clones encode the same 520 amino acid residue protein, which is preceded by a 32-amino acid residue signal peptide. The two clones, whose molecular masses correspond to the two transcripts observed in murine L929 fibroblasts, contain 3' untranslated regions that are markedly different in sequence and length. Both clones can be expressed in COS cells and the recombinant protein is active in a mouse bone marrow colony assay.

Molecular cloning and sequence analysis of cDNA encoding human prostatic acid phosphatase

lambda gt11 clones encoding human prostatic acid phosphatase (PAP) (EC 3.1.3.2) were isolated from human prostatic cDNA libraries by immunoscreening with polyclonal antisera. Sequence data obtained from several overlapping clones indicated that the composite cDNAs contained the complete coding region for PAP, which encodes a 354-residue protein with a calculated molecular mass of 41,126 Da. In the 5'-end, the cDNA codes for a signal peptide of 32 amino acids. Direct protein sequencing of the amino-terminus of the mature protein and its proteolytic fragments confirmed the identity of the predicted protein sequence. PAP has no apparent sequence homology to other known proteins. However, both the cDNA clones coding for human placental alkaline phosphatase and PAP have an alu-type repetitive sequence about 900 nucleotides downstream from the coding region in the 3'-untranslated region. Two of our cDNA clones differed from others at the 5'-ends. RNA blot analysis indicated mRNA of 3.3 kb. We are continuing to study whether acid phosphatases form a gene family as do alkaline phosphatases.

Membrane guanylate cyclase is a cell-surface receptor with homology to protein kinases

Guanylate cyclase has been strongly implicated as a cell-surface receptor on spermatozoa for a chemotactic peptide, and on various other cells as a receptor for atrial natriuretic peptides. Resact (Cys-Val-Thr-Gly-Ala-Pro-Gly-Cys-Val-Gly-Gly-Gly-Arg-Leu-NH2), the chemotactic peptide released by sea urchin Arbacia punctulata eggs, is specifically crosslinked to A. punctulata spermatozoan guanylate cyclase. After the binding of the peptide the state of guanylate cyclase phosphorylation modulates enzyme activity. We report here that the deduced amino-acid sequence of the spermatozoan membrane form of guanylate cyclase predicts an intrinsic membrane protein of 986 amino acids with an amino-terminal signal sequence. A single transmembrane domain separates the protein into putative extracellular and cytoplasmic-catalytic domains. The cytoplasmic carboxyl-terminal 95 amino acids contain 20% serine, the likely regulatory sites for phosphorylation. Unexpectedly, the enzyme is homologous to the protein kinase family.

Tyrosine sulphation is not required for microvillar expression of intestinal aminopeptidase N

The effect of 2,6-dichloro-4-nitrophenol (DCNP), an inhibitor of phenol sulphotransferases (EC 2.8.2.-), on the biosynthesis of aminopeptidase N (EC 3.4.11.2) was studied in organ-cultured pig intestinal mucosal explants. At 50 microM DCNP did not affect protein synthesis but it decreased incorporation of [35S]sulphate into aminopeptidase N and other major microvillar hydrolases by 70-85% compared with controls, indicating an inhibition of their post-translational tyrosine sulphation. In labelling experiments with [35S]methionine from 0.5 to 5 h, DCNP was tested for its possible influence on synthesis, processing and microvillar expression of aminopeptidase N, but no effect on any of these parameters could be detected. It can therefore be concluded that tyrosine sulphation is not required (for instance as a sorting signal) for the targeting of newly synthesized enzymes to the microvillar membrane.

A specific sorting signal is not required for the polarized secretion of newly synthesized proteins from cultured intestinal epithelial cells

Caco-2 cells, derived from human colon, have the morphological, functional, and biochemical properties of small intestinal epithelial cells. After infection with enveloped viruses, influenza virions assembled at the apical plasma membrane while vesicular stomatitis virus (VSV) particles appeared exclusively at the basolateral membrane, similar to the pattern observed in virus-infected Madin-Darby canine kidney (MDCK). When grown in Millicell filter chamber devices and labeled with [35S]methionine, Caco-2 monolayers released all of their radiolabeled secretory products preferentially into the basal chamber. Among the proteins identified were apolipoproteins AI and E, transferrin, and alpha-fetoprotein. No proteins were observed to be secreted preferentially from the apical cell surface. The lysosomal enzyme beta-hexosaminidase was also secreted primarily from the basolateral surface of the cells in the presence or absence of lysosomotropic drugs or tunicamycin, which inhibit the targetting of lysosomal enzymes to lysosomes. Neither of these drug treatments significantly affected the polarized secretion of other nonlysosomal proteins. In addition, growth hormone (GH), which is released in a nonpolar fashion from MDCK cells, was secreted exclusively from the basolateral membrane after transfection of Caco-2 cells with GH cDNA in a pSV2-based expression vector. Similar results were obtained in transient expression experiments and after selection of permanently transformed Caco-2 cells expressing GH. Since both beta-hexosaminidase and GH would be expected to lack sorting signals for polarized exocytosis in epithelial cells, these results indicate that in intestinal cells, proteins transported via the basolateral secretory pathway need not have specific sorting signals.

Signals for the incorporation and orientation of cytochrome P450 in the endoplasmic reticulum membrane

Cytochrome P450b is an integral membrane protein of the rat hepatocyte endoplasmic reticulum (ER) which is cotranslationally inserted into the membrane but remains largely exposed on its cytoplasmic surface. The extreme hydrophobicity of the amino-terminal portion of P450b suggests that it not only serves to initiate the cotranslational insertion of the nascent polypeptide but that it also halts translocation of downstream portions into the lumen of the ER and anchors the mature protein in the membrane. In an in vitro system, we studied the cotranslational insertion into ER membranes of the normal P450b polypeptide and of various deletion variants and chimeric proteins that contain portion of P450b linked to segments of pregrowth hormone or bovine opsin. The results directly established that the amino-terminal 20 residues of P450b function as a combined insertion-halt-transfer signal. Evidence was also obtained that suggests that during the early stages of insertion, this signal enters the membrane in a loop configuration since, when the amino-terminal hydrophobic segment was placed immediately before a signal peptide cleavage site, cleavage by the luminally located signal peptidase took place. After entering the membrane, the P450b signal, however, appeared to be capable of reorienting within the membrane since a bovine opsin peptide segment linked to the amino terminus of the signal became translocated into the microsomal lumen. It was also found that, in addition to the amino-terminal combined insertion-halt-transfer signal, only one other segment within the P450b polypeptide, located between residues 167 and 185, could serve as a halt-transfer signal and membrane-anchoring domain. This segment was shown to prevent translocation of downstream sequences when the amino-terminal combined signal was replaced by the conventional cleavable insertion signal of a secretory protein.

Sucrase-isomaltase deficiency in humans. Different mutations disrupt intracellular transport, processing, and function of an intestinal brush border enzyme

Eight cases of congenital sucrase-isomaltase deficiency were studied at the subcellular and protein level with monoclonal antibodies against sucrase-isomaltase. At least three phenotypes were revealed: one in which sucrase-isomaltase protein accumulated intracellularly probably in the endoplasmic reticulum, as a membrane-associated high-mannose precursor, one in which the intracellular transport of the enzyme was apparently blocked in the Golgi apparatus, and one in which catalytically altered enzyme was transported to the cell surface. All patients expressed electrophoretically normal or near normal high-mannose sucrase-isomaltase. The results suggest that different, probably small, mutations in the sucrase-isomaltase gene lead to the synthesis of transport-incompetent or functionally altered enzyme which results in congenital sucrose intolerance.

The Kluyveromyces lactis KEX1 gene encodes a subtilisin-type serine proteinase

KEX1 is a chromosomal gene required for the production of the killer toxin encoded by the linear DNA plasmid pGKL-1 of Kluyveromyces lactis. The nucleotide sequence of the cloned KEX1 gene has been determined. The deduced structure of the KEX1 protein, 700 amino acids long, indicated that it contained an internal domain with a striking homology to the sequences of the subtilisin-type proteinases, and a probable transmembrane domain near the carboxyl terminus. The results confirm the hypothesis that the product of the gene KEX1 of K. lactis is a proteinase involved in the processing of the toxin precursor.