Plate 1.4 The Cell A. Cell organelles(epithelial cell) Golgi complex Nucleus Chromatin vacuole Nucleolus B. Cell structure(epithelial cell)in electron micrograph Vacuole Tight junction Cell border Mitochondria Golgi complex Basal membrane
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10 The Cell(continued the interaction of a number of proteins float freely in the ound to the within a ribonucleoprotein complex called the cytosolic side of the endoplasmic reticulum, spliceosome Introns usually make up the lions described below. Each ribosome is made up of ozens of proteins associated with a number ey make up 95% of the nucleotide chain of of structural RNA molecules called ribosomal agulation factor VIll, which contains 25 in- RNA(rRNA). The two subunits of the ribosome g trons. mRNA can also be modified (e.g. are first transcribed from numerous rRNA ters the cytosol (CIc). Nuclear pores are for protein synthesis(translation )(C2). Syn 25MDa)located within the nuclear en- ence of specific tRNA molecules(at least one velope. They allow large molecules such as for each of the 21 proteinogenous amino transcription factors, RNa polymerases or cy- acids). In this case, the target amino acid is smic steroid hormone receptors to pa ound to the c-c-A end of the tRNa molecule into the nucleus, nuclear molecules such as (same in all tRNAs), and the corresponding an- mRNA and tRNa to pass out of the nucleus, and ticodon that recognizes the mRNA codon is lo- other molecules such as ribosomal proteins to cated at the other end (E). Each ribosome travel both ways. The(ATP-dependent) pas- has two tRNA binding sites: one for the last in- of a molecule in either direction cannot corporated amino acid and another for the one occur without the help of a specific signal that beside it(not shown in E). Protein synthes guides the molecule into the gins when the start codon is read and ends mentioned 5 cap is responsible for the exit of once the stop codon has been reached. The ri- IRNA from the nucleus. and one or two bosome then breaks down into its two sub- specific sequences of a few(mostly cationic) units and releases the mRNA (C2). Ribo- ino acids are required as the signal for the somes can add approximately 10-20 amino of proteins into the nucleus. These acids per second. However, since an mRNA equences form part of the peptide chain of strand is usually translated simultaneously by such nuclear proteins and probably create a many ribosomes(polyribosomes or polysomes) peptide loop on the proteins surface. In the at different sites, a protein is synthesized much case of the cytoplasmic receptor for glucocor. faster than its mRNA In the bone marrow. for icoids(p 280), the nudear localization sig. example, a total of around 5 x 10 hemoglobin al is masked by a chaperone protein(heat copies containing 574 amino acids each are shock protein 90, hsp90) in the absence of the produced per second glucocorticoid, and is released only after the The endoplasmic reticulum(ER, -C, F) ormone binds, thereby freeing hsp90 from plays a central role in the synthesis of proteins he receptor. The "activated"receptor then and lipids it also serves as an intracellular Ca2+ reaches the cell nucleus, where it binds to store(p 17 A). The ER consists of a net-like specific DNA sequences and controls specific system of interconnected branched channels and flat cavities bounded by a membrane. the sts of two mem- enclosed spaces(cisterns)make up around 10% branes (-two phospholipid bilayers) that of the cell volume, and the membrane com- merge at the nuclear pores. The two mem- to 70% of the membrane mass of a anes consist of different materials. The ex- cell. Ribosomes can attach to the cytosol ernal membrane is continuous with the men ce of parts of the ER, forming a rough endo- which is described below (F) The mRNA exported from the nucleus brane proteins (G)for the plasma mem- travels to the ribosomes(C1), which either brane, endoplasmi
101 Fundamentals and Cell Physiology the interaction of a number of proteins within a ribonucleoprotein complex called the spliceosome. Introns usually make up the lion’s share of pre-mRNA molecules. For example, they make up 95% of the nucleotide chain of coagulation factor VIII, which contains 25 introns. mRNA can also be modified (e.g., through methylation) during the course of posttranscriptional modification. RNA now exits the nucleus through nuclear pores (around 4000 per nucleus) and enters the cytosol ( C1c). Nuclear pores are high-molecular-weight protein complexes (125 MDa) located within the nuclear envelope. They allow large molecules such as transcription factors, RNA polymerases or cytoplasmic steroid hormone receptors to pass into the nucleus, nuclear molecules such as mRNA and tRNA to pass out of the nucleus, and other molecules such as ribosomal proteins to travel both ways. The (ATP-dependent) passage of a molecule in either direction cannot occur without the help of a specific signal that guides the molecule into the pore. The abovementioned 5′ cap is responsible for the exit of mRNA from the nucleus, and one or two specific sequences of a few (mostly cationic) amino acids are required as the signal for the entry of proteins into the nucleus. These sequences form part of the peptide chain of such nuclear proteins and probably create a peptide loop on the protein’s surface. In the case of the cytoplasmic receptor for glucocorticoids ( p. 280), the nuclear localization signal is masked by a chaperone protein (heat shock protein 90, hsp90) in the absence of the glucocorticoid, and is released only after the hormone binds, thereby freeing hsp90 from the receptor. The “activated” receptor then reaches the cell nucleus, where it binds to specific DNA sequences and controls specific genes. The nuclear envelope consists of two membranes (= two phospholipid bilayers) that merge at the nuclear pores. The two membranes consist of different materials. The external membrane is continuous with the membrane of the endoplasmic reticulum (ER), which is described below (F). The mRNA exported from the nucleus travels to the ribosomes ( C1), which either float freely in the cytosol or are bound to the cytosolic side of the endoplasmic reticulum, as described below. Each ribosome is made up of dozens of proteins associated with a number of structural RNA molecules called ribosomal RNA (rRNA). The two subunits of the ribosome are first transcribed from numerous rRNA genes in the nucleolus, then separately exit the cell nucleus through the nuclear pores. Assembled together to form a ribosome, they now comprise the biochemical “machinery” for protein synthesis (translation) ( C2). Synthesis of a peptide chain also requires the presence of specific tRNA molecules (at least one for each of the 21 proteinogenous amino acids). In this case, the target amino acid is bound to the C–C–A end of the tRNA molecule (same in all tRNAs), and the corresponding anticodon that recognizes the mRNA codon is located at the other end ( E). Each ribosome has two tRNA binding sites: one for the last incorporated amino acid and another for the one beside it (not shown in E). Protein synthesis begins when the start codon is read and ends once the stop codon has been reached. The ribosome then breaks down into its two subunits and releases the mRNA ( C2). Ribosomes can add approximately 10–20 amino acids per second. However, since an mRNA strand is usually translated simultaneously by many ribosomes (polyribosomes or polysomes) at different sites, a protein is synthesized much faster than its mRNA. In the bone marrow, for example, a total of around 5 1014 hemoglobin copies containing 574 amino acids each are produced per second. The endoplasmic reticulum (ER, C, F) plays a central role in the synthesis of proteins and lipids; it also serves as an intracellular Ca2+ store ( p. 17 A). The ER consists of a net-like system of interconnected branched channels and flat cavities bounded by a membrane. The enclosed spaces (cisterns) make up around 10% of the cell volume, and the membrane comprises up to 70% of the membrane mass of a cell. Ribosomes can attach to the cytosolic surface of parts of the ER, forming a rough endoplasmic reticulum (RER). These ribosomes synthesize export proteins as well as transmembrane proteins ( G) for the plasma membrane, endoplasmic reticulum, Golgi apparaThe Cell (continued) Translation disorders, virus pathogenicity, tumorigenesis
Plate 1.5 The Cell ll 11 C. Transcription and translation Genomic dnA Nucleus RNA polymerase factors and 2 Translation in ribosome ranscription RNA R bosome mRNA mRNA export breakdown Ribosomes→ D. Transcription and splicing E. Protei ein coding in DNA and RNA 45-67 DNA Genomi Transcription and Splicing Export from nucleus Exon cIntron mRNA ALUUUACGAIGAI Readina direction 顶88 Splicing mRNA B88 Growth of peptide chain
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12 The Cell(continued) syn- Regulation of gene expression takes place thesis(at the amino end) by such ribosomes on the level of transcription (Cla), RNA (still unattached)induces a signal sequence dification(C1b), mRNA ex which a signal recognition particle(SRP)in the RNA degradation (C1d), translation (Cle), mporarily halted and(b)the ribosome(me- degradation(Fg) diated by the SRP and a SRP receptor)attaches a ribosome receptor on the ER membrane. site of oxidation of carbohydrates and lipids to After that, synthesis continues In synthesis of CO2 and H20 and associated O2 expenditure export protein, a translocator protein conveys The Krebs cycle(citric acid cycle ), respiratory hain and related ATP synthesis also occur in is completed. Synthesis of membrane mitochondria Cells intensely active in meta- proteins is interrupted several times(depend- bolic and transport activities are rich in mito- e ing on the number of me ane-spanning chondria-e.g, hepatocytes, intestinal cells domains (G2)by translocator protein clo- and renal epithelial cells. Mitochondria are en- ure, and the corresponding(hydrophobic) closed in a double membrane consisting of a sequence is pushed into the pho membrane. The smooth endoplasmic brane. The latter is deeply infolded, forming a m(SER)contains no ribosomes and is series of projections (cristae): it also has im- the production site of lipids (e.g, for lipo- portant transport functions (p. 17 B). Mit proteins, -p 256 ff. )and other substances. chondria probably evolved as a result of sym- The ER membrane containing the synthesized biosis between aerobic bacteria and anaer embrane proteins or export proteins forms cells(symbiosis hypothesis). The mitochondrial sicles which are transported to the Golgi ap- DNA(mtDNA) of bacterial origin and the paratus. double membrane of mitochondria are relicts The Golgi complex or Golgi apparatus(F) of their ancient history. Mitochondria also has sequentially linked functional compart- contain ribosomes which synthesize all pro- ments for further processing of products from teins encoded by mtDNA. the endoplasmic reticulum. It consists of a cis- Lysosomes are vesicles (f, g) that arise Golgi network (entry side facing the ER). from the Er (via the golgi acked flattened cisternae( Golgi stacks )and a involved in the intracellular digestion of ma ans-Golgi network(sorting and distribution). romolecules. These are taken up into the cell Functions of the golgi complex: either by endocytosis (e. g, uptake of albumin polysaccharide synthesis; otake of bacteria by macrophage fication), e.g. glycosylation of membrane pro- -p 94 ff. ) They may also originate from the eins on certain amino acids(in part in the Er) degradation of a cell's own organelles(auto- that are later borne as glycocalyces on the ex- phagia, e.g. of mitochondria )delivered inside rnal cell surface(see below)and y-carboxy. autophagoson B, F). A portion of the er lation of glutamate residues(p. 102): docytosed membrane material recycles (e.g, phosphorylation of sugars of glycoproteins receptor recycling in receptor-mediated en- 28). Early and late below ) are intermediate stages in this vesicular trans- packaging"of proteins meant for export port. Late endosomes and lysosomes contain nto secretory vesicles(secretory granules ) the acidic hydrolases(proteases, nucleases, li- es,glycosidases, phosphatases, etc. th racellular space(see p. 248. for example). are active only under acidic Hence, the Golgi apparatus represents a membrane contains an H-ATPase that creates nd distribution nter for proteins and lipids received from the the lysosomes and assorted transport proteins
121 Fundamentals and Cell Physiology tus, lysosomes, etc. The start of protein synthesis (at the amino end) by such ribosomes (still unattached) induces a signal sequence to which a signal recognition particle (SRP) in the cytosol attaches. As a result, (a) synthesis is temporarily halted and (b) the ribosome (mediated by the SRP and a SRP receptor) attaches to a ribosome receptor on the ER membrane. After that, synthesis continues. In synthesis of export protein, a translocator protein conveys the peptide chain to the cisternal space once synthesis is completed. Synthesis of membrane proteins is interrupted several times (depending on the number of membrane-spanning domains ( G2) by translocator protein closure, and the corresponding (hydrophobic) peptide sequence is pushed into the phospholipid membrane. The smooth endoplasmic reticulum (SER) contains no ribosomes and is the production site of lipids (e.g., for lipoproteins, p. 256 ff.) and other substances. The ER membrane containing the synthesized membrane proteins or export proteins forms vesicles which are transported to the Golgi apparatus. The Golgi complex or Golgi apparatus ( F) has sequentially linked functional compartments for further processing of products from the endoplasmic reticulum. It consists of a cisGolgi network (entry side facing the ER), stacked flattened cisternae (Golgi stacks) and a trans-Golgi network (sorting and distribution). Functions of the Golgi complex: ◆ polysaccharide synthesis; ◆ protein processing (posttranslational modification), e.g., glycosylation of membrane proteins on certain amino acids (in part in the ER) that are later borne as glycocalyces on the external cell surface (see below) and γ-carboxylation of glutamate residues (p. 102); ◆ phosphorylation of sugars of glycoproteins (e.g., to mannose-6-phosphate, as described below); ◆ “packaging” of proteins meant for export into secretory vesicles (secretory granules), the contents of which are exocytosed into the extracellular space (see p. 248, for example). Hence, the Golgi apparatus represents a central modification, sorting and distribution centerfor proteins and lipids received from the endoplasmic reticulum. Regulation of gene expression takes place on the level of transcription ( C1a), RNA modification ( C1b), mRNA export ( C1c), RNA degradation ( C1d), translation ( C1e), modification and sorting ( F,f), and protein degradation ( F,g). The mitochondria ( A, B; p. 17 B) are the site of oxidation of carbohydrates and lipids to CO2 and H2O and associated O2 expenditure. The Krebs cycle (citric acid cycle), respiratory chain and related ATP synthesis also occur in mitochondria. Cells intensely active in metabolic and transport activities are rich in mitochondria—e.g., hepatocytes, intestinal cells, and renal epithelial cells. Mitochondria are enclosed in a double membrane consisting of a smooth outer membrane and an inner membrane. The latter is deeply infolded, forming a series of projections (cristae); it also has important transport functions ( p. 17 B). Mitochondria probably evolved as a result of symbiosis between aerobic bacteria and anaerobic cells (symbiosis hypothesis). The mitochondrial DNA (mtDNA) of bacterial origin and the double membrane of mitochondria are relicts of their ancient history. Mitochondria also contain ribosomes which synthesize all proteins encoded by mtDNA. Lysosomes are vesicles ( F, g) that arise from the ER (via the Golgi apparatus) and are involved in the intracellular digestion of macromolecules. These are taken up into the cell either by endocytosis (e.g., uptake of albumin into the renal tubules; p. 158) or by phagocytosis (e.g., uptake of bacteria by macrophages; p. 94 ff.). They may also originate from the degradation of a cell’s own organelles (autophagia, e.g., of mitochondria) delivered inside autophagosomes ( B, F). A portion of the endocytosed membrane material recycles (e.g., receptor recycling in receptor-mediated endocytosis; p. 28). Early and late endosomes are intermediate stages in this vesicular transport. Late endosomes and lysosomes contain acidic hydrolases (proteases, nucleases, lipases, glycosidases, phosphatases, etc., that are active only under acidic conditions). The membrane contains an H+ -ATPase that creates an acidic (pH 5) interior environment within the lysosomes and assorted transport proteins that (a) release the products of digestion (e.g., The Cell (continued) Bacterial defense, acute pancreatitis, cystinosis
Plate 1. 6 The Cell Ill 13 orting, recyding, an Transcripts ribosomes ER-boune Protein and lipid modificat Golgi stacks Breakdown Sorti Protein breakdow cretory endosome Cytosol Phagocytosis rotein space Clathrin Constitutive secretion
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