D

A) The ribosome making the polypeptide chain approaches the cell membrane. The polypeptide with its signal sequence binds to the signal recognition protein. B) The signal recognition protein recognizes the translocase and binds to it, allowing the polypeptide chain to begin its journey through the membrane. C) After the signal sequence exits the translocase, leader peptidase cuts the polypeptide chain, liberating the signal peptide. D) Final folding of the protein occurs outside the cell.

is synthesized and follows the signal sequence into and through the membrane via the translocase. This is known as cotranslational export, since the protein is exported as it is made. The signal sequence is cut off by the leader peptidase after translocation (Fig. 8.27).

There are approximately 500 translocases per E. coli cell. Each cell exports about 1 x 106 proteins from the cytoplasm prior to dividing. In a cell that doubles in 20 minutes, 100 proteins are exported per minute per translocase. Protein export is 10fold faster than protein synthesis. So the demand for a growing protein chain will allow the translocase to be ready for a new chain as fast as the ribosome can make it. Note that in gram-negative bacteria such as E. coli, most of these exported proteins are structural components of the outer membrane that are being made constantly, rather than enzymes being excreted outside the cell for digestive purposes.

In eukaryotes, cotranslational export occurs across the membranes of the endo-plasmic reticulum. In multi-cellular eukaryotes proteins involved in digestion, such as amylases and proteases, must be exported. So must proteins located in blood and other body fluids, such as antibodies, albumins and circulating peptide hormones. When the animal genes for preproinsulin or ovalbumin are put into E. coli, correct export across the cell membrane occurs and cleavage of the signal sequence by the E. coli leader peptidase happens at the correct position. Conversely, yeast cells correctly process and excrete bacterial b-lactamase. Thus, the export machinery is highly conserved between diverse organisms.

cotranslational export Export of a protein across a membrane while it is still being synthesized by a ribosome leader peptidase Enzyme that removes the leader sequence after protein export

Protein released and partially folded

Folded protein

FIGURE 8.28 Chaperonins Act by Two General Mechanisms

Unfolded protein

Protein released and partially folded

Unfolded protein

Folded protein

Folded protein

FIGURE 8.28 Chaperonins Act by Two General Mechanisms

A) Chap eronins of the Hsp70 type act during protein formation by binding to hydrophobic patches of the protein. Once chaperonins are released, the protein automatically folds. B) Large chaperonins, such as GroE, act after translation by sequestering misfolded protein in a central cavity. Freed from the influences of other molecules in the cytoplasm, the protein will fold correctly.

Chaperonins are proteins that promote the correct folding of other proteins.

Molecular Chaperones Oversee Protein Folding

Molecular chaperones, or chaperonins, are proteins that oversee the correct folding of other proteins. Many chaperonins are called heat shock proteins (HSPs), as their levels increase at high temperature (see Ch. 9). Chaperonins may be divided into two main classes: those that prevent premature folding and those that attempt to rectify mis-folding. Obviously, chaperonins cannot "know" the correct 3-D structure for several other proteins. Mechanistically, they act to prevent incorrect folding, rather than actively creating a correct structure.

During bacterial protein export, the secretory chaperonin SecB keeps the polypeptide chain from folding up prematurely. Secreted proteins must travel through a narrow translocase channel and so must remain unfolded until they reach the other side of the membrane. The Hsp70 set of chaperonins tends to bind to newly made or highly uncoiled proteins (Fig. 8.28).

chaperone Sometimes "molecular chaperone";same as chaperonin chaperonin Protein that oversees the correct folding of other proteins heat shock protein (HSP) Protein induced in response to high temperature. Many heat shock proteins are chaperonins

Protein Synthesis Occurs in Mitochondria and Chloroplasts 225

The more complex GroE (= Hsp60/Hsp10) chaperonin machine is involved in attempting to refold damaged or misfolded proteins. When polypeptide chains unfold, they expose hydrophobic regions that are normally clustered in the center of the folded protein. Left to themselves, many proteins could refold. However, inside a cell, there is a high concentration of protein. Consequently, exposed hydrophobic regions from multiple proteins bind to each other and the proteins aggregate together. The GroE chaperonin machine forms a cavity in which a single polypeptide can refold on its own, protected from interactions with other polypeptide chains.

Protein synthesis in mitochondria and chloroplasts resembles that of bacteria in many respects.

Protein Synthesis Occurs in Mitochondria and Chloroplasts

Mitochondria and chloroplasts are thought to be of prokaryotic origin. The symbiotic hypothesis of organelle origins argues that symbiotic prokaryotes evolved into organelles by specializing in energy production and progressively losing their genetic independence (see Ch. 20 for further details). Both mitochondria and chloroplasts contain circular DNA that encodes some of their own genes and they divide by binary fission. They contain their own ribosomes and make some of their own proteins. Organelle ribosomes resemble the ribosomes of bacteria rather than the ribosomes of the eukaryotic cytoplasm. The initiation and elongation factors of organelles are also bacterial in nature. Nonetheless, there are differences in composition between organelle and bacterial ribosomes, as shown in Table 8.03.

Components of Cytoplasmic, Organelle and Bacterial Ribosomes

Location

Subunits

Ribosomal RNA

Proteins

Animal Cytoplasm

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