Cell biology chapter 8

KDEL sequence

A four amino acid sequence (Lys-Asp-Glu-Leu) attached to the C-terminus of ER resident proteins

GTP

A nucleotide composed of guanine, ribose, and three linked phosphate groups. It is incorporated into the growing RNA chain during synthesis of RNA and used as a source of energy during synthesis of proteins

Autoradiography

a procedure that locates radioactive substances in a slice of tissue; the radiation exposes a photographic emulsion or a piece of film that covers the tissue tissue sections containing radioactive isotopes are covered with a thin layer of photographic emulsion, which is exposed by radiation emanating from radioisotopes within the tissue. Sites in the cells containing radioactivity are revealed under the microscope by silver grains in the overlying emulsion

this occurs in endocrine cells that release hormones, in pancreatic acinar cells that release digestive enzymes, and in nerve cells that release neurotransmitters

regulated secretion

he endoplasmic reticulum is divided into two subcompartments,

the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER).

the RER lumen is packed with molecular ____________________________ that recognize and bind to unfolded or misfolded proteins and give them the opportunity to attain their correct (native) three‐dimensional structure

chaperones

Secretory activities of cells can be divided into two types

constitutive and regulated

Integral membrane proteins—other than those of mitochondria and chloroplasts—are also synthesized by ___________________________________ using the same machinery described for the synthesis of secretory and lysosomal proteins

co‐translational translocation

he synthesis of secretory, lysosomal, or plant vacuolar proteins usually takes place by

co‐translational translocation

endomembrane system consists of

in which the individual components function as part of a coordinated unit nuclear envelope, endoplasmic reticulum, golgi apparatus, lysosomes, vacuoles and the plasma membrane. does not include mitochondria or chloroplasts

Using autoradiography it was discovered that ________________________was to be the site of synthesis of secretory proteins

endoplasmic reticulum ER

The SRP‐ribosome‐nascent polypeptide complex is then recruited to the ER membrane through an interaction between the SRP and the SRP receptor on the ER membrane (step 2). The ribosome (with its nascent polypeptide) is then handed off from the SRP to the _________________________

translocon

Endoplasmic Reticulum (ER)

An extensive membranous network in eukaryotic cells, continuous with the outer nuclear membrane and composed of ribosome-studded (rough) and ribosome-free (smooth) regions.

Biosynthetic-secretory pathway

Pathway which includes the delivery of newly synthesized proteins and lipids from the ER to the Golgi apparatus, delivery from the Golgi apparatus to lysosomes or the plasma membrane, and the release of secretory products into the extracellular space

Whereas materials move out of the cell by the secretory pathway, the ___________________________ operates in the opposite direction. By following the __________________________________, materials move from the outer surface of the cell to compartments, such as endosomes and lysosomes, located within the cytoplasm

endocytic pathway

acinar cells

enzyme-secreting cells of the pancreas

2/3 of polypeptides are synthesized on ________________ ribosomes, that are not attached to the RER, and are subsequently released into the cytosol. This class includes what kind of protiens

free (a) proteins destined to remain in the cytosol (such as the enzymes of glycolysis and the proteins of the cytoskeleton), (b) peripheral proteins of the cytosolic surface of membranes (such as spectrins and ankyrins that are only weakly associated with the plasma membrane ' s cytosolic surface), (c) proteins that are transported to the nucleus (d) proteins to be incorporated into peroxisomes, chloroplasts, and mitochondria.

As will be evident in the following discussion, the composition of the ____________ (or cisternal) space inside the ER membranes is quite different from that of the surrounding cytosolic space

luminal

constitutive secretion

materials are transported in secretory vesicles from their sites of synthesis and discharged into the extracellular space in a continual manner performed by most cells: functions to maintain the extracellular matrix and plasma membrane

Transport vesicles move through the cytoplasm in a directed manner, often pulled by___________________ that operate on tracks formed by microtubules and microfilaments of the cytoskeleton

motor proteins

The RER, contains high levels of proteins involved in the movement of proteins into the ER lumen

nascent

once a polypeptide emerges from the ribosome, the hydrophobic signal sequence is recognized by a signal recognition particle (SRP) , which consists in mammalian cells of six distinct polypeptides and a RNA molecule, called the

7SL RNA.

nascent protein

A protein as it is being formed by a ribosome before it folds into its active shape

regulated secretion

Discharge of materials synthesized in the cell that have been stored in membrane-bound secretory granules in the peripheral regions of the cytoplasm, occurring in response to an appropriate stimulus

everal of the steps involved in the synthesis and trafficking of secretory proteins are regulated by the binding or hydrolysis of _______________

GTP

Co-translational translocation

Simultaneous transport of a secretory protein into the endoplasmic reticulum as the nascent protein is still bound to the ribosome and being elongated.

SER functions include

Synthesis of steroid hormones in the endocrine cells of the gonad and adrenal cortex. Detoxification in the liver of a wide variety of organic compounds, including barbiturates and ethanol, whose chronic use can lead to proliferation of the SER in liver cells Sequestering calcium ions within the cytoplasm of cells. The regulated release of Ca 2+ from the SER of skeletal and cardiac muscle cells (known as the sarcoplasmic reticulum in muscle cells) triggers contraction.

The translocon is

a protein channel embedded in the ER membrane through which the nascent polypeptide is able to move in its passage from the ribosome to the ER lumen.

the proteins that are synthesized on ribosomes attached to the cytosolic surface of the RER membranes and released into the ER lumen in a process called co‐translational translocation . These include protiens such as

a) secreted proteins, (b) integral membrane proteins (c) soluble proteins that reside within compartments of the endomembrane system, including the ER, Golgi complex, lysosomes, endosomes, vesicles, and plant vacuoles.

Approximately one‐third of the proteins encoded by a mammalian genome are synthesized on ribosomes attached to the cytosolic surface of the RER membranes and released into the ER lumen in a process called

co‐translational translocation .

GTP‐bound and GDP‐bound versions of a G protein have different conformations and thus have different abilities to bind other proteins. Because of this difference in binding properties, G proteins act like "molecular switches"; the GTP‐bound protein typically turns the process ___________, and hydrolysis of the bound GTP turns it _______________.

on off

GTP‐binding proteins (or G proteins) play key regulatory roles in many different cellular processes. 3 G proteins can be present in at least two alternate conformations,

one containing a bound GTP molecule and the other a bound GDP molecule

As the polypeptide enters the RER cisterna, a nascent polypeptide is acted on by a variety of enzymes located within either the membrane or the lumen of the RER. The N‐terminal portion containing the signal peptide is removed from most nascent polypeptides by a proteolytic enzyme, the signal _____________________________ Carbohydrates are added to the nascent protein by the enzyme _______________________________________ . Both the enzymes are integral membrane proteins associated with the translocon that act on nascent proteins as they enter the ER lumen.

peptidase. oligosaccharyltransferase

Some proteins enter the ER lumen with their cysteine residues in the reduced ( --SH) state, but they leave the compartment with many of these residues joined to one another as oxidized disulfides ( --SS ) . The formation (and rearrangement) of disulfide bonds is catalyzed by _________________________. Disulfide bonds play an important role in

protein disulfide isomerase ( PDI ) maintaining the stability of proteins that are present at the extracellular surface of the plasma membrane or secreted into the extracellular space.

the high degree of curvature of the SER tubules is induced and maintained by the presence of large numbers of membrane‐bending proteins, called ______________________ which are only present in small numbers at the edges of the flattened RER sheets

reticulons

In some of these cells, materials to be secreted are stored in large, densely packed, membrane‐bound

secretory granules

What determines the location in a cell where a protein is synthesized?

site of synthesis of a protein is determined by the sequence of amino acids in the N‐terminal portion of the polypeptide, which is the first part to emerge from the ribosome during protein synthesis. They suggested the following: 1. Secretory proteins contain a signal sequence at their N‐ terminus that directs the emerging polypeptide and ribosome to the ER membrane. 2. The polypeptide moves into the cisternal space of the ER through a protein‐lined, aqueous channel in the ER membrane. It was proposed that the polypeptide moves through the membrane as it is being synthesized, that is, co-translationally .

The rough ER is the________________________ of the biosynthetic pathway: It is the site of synthesis of the proteins, carbohydrate chains, and phospholipids that journey through the membranous compartments of the cell.

starting point

functions of the rough ER include the

synthesis of proteins and the synthesis of most of the lipids of a cell's membranes synthesis of secreted proteins, lysosomal proteins, and integral membrane proteins.

biosynthetic pathway

synthesis, modification, and transport of proteins

Consequently, membrane proteins must also be targeted to particular organelles, such as a lysosome or Golgi cisterna. These various types of cargo—secreted proteins, lysosomal enzymes, and membrane proteins—are routed to their appropriate cellular destinations by virtue of specific "addresses" or sorting signals that are encoded in

the amino acid sequence of the proteins or in the attached oligosaccharides.

Smooth Endoplasmic Reticulum (SER)

the region of the endoplasmic reticulum that has few or no ribosomes on its cytoplasmic surface and synthesizes carbohydrates, lipids, and steroid hormones; detoxifies chemicals like pesticides, preservatives, medications, and environmental pollutants, and stores calcium ions

Rough Endoplasmic Reticulum (RER)

the region of the endoplasmic reticulum that is studded with ribosomes and engages in protein modification