Molecular Fundamentals - Protein Sorting and Modifications

what are the functions of post-translational modifications?

• controls protein activation

• guides protein transport

• alters protein-protein or protein-DNA interactions

• targets proteins for degradation

what is proteolytic cleavage?

enzyme-mediated removal of amino acids residues in a protein

the following are examples of what?

• removal of start methionine

• removal of signal sequence

• removal of a certain amino acid sequence to form an active protein

proteolytic cleavage

many hormones are synthesized as what?

one large protein that is cleaved into separate smaller active peptides

what are the enzymes responsible for adding a phosphate group to a protein?

protein kinases

what are the enzymes responsible for removing phosphate groups?

phosphatases

what are the three amino acids that the majority of phosphorylation events occur on?

serine, threonine, or tyrosine

what is the addition of oligosaccharides to proteins via a glycosidic bond to the hydroxyl (O-linked) or amine group (N-linked) of an amino acid side chain?

glycoslyation

what are the five types of post-transational modifications?

• proteolysis

• phosphorylation

• glycosylation

• ubiquination

• lipidation/prenylation

where does glycosylation occur?

in the ER and Golgi

most glycosylated proteins are either what?

secreted from the cell or embedded in the plasma membrane

acetylation of histones can be used to do what?

regulate gene expression

what happens when lysines are acetylated?

the chromatin structure is relaxed and the DNA is available for transcription

what happens to histones in the absence of lysine acetylation?

histones remain closely linked and transcription is repressed

methylation of histones can also do what?

regulate gene expression

what are the methods of addition of fatty acids and lipids to proteins?

myristoylation, palmitoylation, prenylation, farnesyl, and geranylgerany

what describes hydroxylation?

addition of hydroxyl group to an amino acid

mutations in the gene encoding the lysyl hydroxylase leads to a form of what?

the disease Ehlers-Danlos syndrome

Carboxyl groups are added to glutamate residues by what?

vitamin K-dependent carboxylation reaction

what is an important modification for blood clotting factors?

carboxylation

what is ubiquitination (or ubiquitylation)?

the addition of the small polypeptide ubiquitin to a protein via a lysine residue to target proteins for degradation in the proteasome

what directs the transport of proteins?

its amino acid sequence

where are secretory proteins, resident ER proteins, and lysosomal enzymes are synthesized, modified, and sorted where?

in the ER and modified and sorted through the golgi apparatus

proteins made in the cytosol must have what?

a mechanism for entering membrane-bound organelles, such as mitochondria and the nucleus

translocation into the ER is a co-translational process, which means?

it occurs while the protein is being synthesized

what proteins are transported into the ER?

• transmembrane proteins

• proteins found in the lumen of the ER or othe rmembrane-bound organelles

• secreted proteins

what does the ER signal sequence do?

recruits the ribosome to the ER

what is the signal-recognition particle (SRP)?

binds to the ER signal sequence and temporally stalls translation

what is the signal-recognition paarticel receptor (SRP-receptor)?

integral membrane receptor on the cytosol side of the ER membrane and brings SRP-ribosomes to the surface of the ER

the ER signal sequence in the translated protein is bound to what?

translocator

the ER signal sequence in translated protein is cleaved by what?

the signal peptidase enzyme

after translation of resident ER/secreted protein is complete, what happens?

the protein is folded in the lumen of the ER by chaperone proteins

the translation of resident ER/secreted proteins is used to do what?

synthesize proteins that remain in the ER lumen, are sent to other organelles, or are secreted

what is the major difference between making a transmembrane protein and making a soluble protein in the ER?

the presence of a hydrophobic stop-transfer sequence in the protein

what does the hydrophobic stop-transfer sequence in the transmembrane proteins do?

tells the translocator to stop embedding the protein in the membrane

what is the UPR (unfolded protein response)?

a cellular response to the accumulation of unfolded proteins in the ER

what activates transcription factors the increase the transcription of genes for chaperone proteins?

UPR (unfolded protein response)

if the increase in ER chaperone proteins doesn’t resolve the problem, what happens?

the cell may undergo programmed cell death (apoptosis)

what are examples of proteins transported into nucleus?

histones, transcription factors, polymerases

the transport of protein in and out of the nucleus occurs through what?

nuclear pore complex

transport in and out of the nucleus requires what?

energy from GTP hydrolysis

proteins that need to be transported into the nucleus contain what?

a nuclear localization signal (NLS)

nuclear localization signal is recognized by what?

nuclear import receptorst

the nuclear import receptors do what?

escort the protein through the nuclear pore complex

how are proteins transported through the outer membrane to enter the intermembrane space of the mitochondria?

a mitochondrial signal sequence in the protein is recognized by an import receptor on the outer membrane

the translocation of the protein into the mitochondria is accomplished throught what?

the activity of a translocator called the TOM complex, which is embedded in the outer membrane

where is the TOM complex embedded?

in the outer membrane of the mitochondria

proteins that are needed in the mitochondrial matrix must pass through what?

the TOM complex and the TIM complex

where is the TIM complex embedded?

in the inner membrane of the mitochondria

what powers the transport of proteins through the mitochondrial membranes?

ATP and the H+ gradient in the mitochondria

what are some of the functions of signal sequences?

• import into ER

• retention in lumen of ER

• import into mitochondria

• import into nucleus

• import into peroxisomes