Protein Biosynthesis -2

By

Amr S. Moustafa, M.D.; Ph.D.

Polyribosome (Polysomes)

Several ribosomes and one mRNA

Protein Targeting

Free Vs attached ribosomes

Localization signals

Regulation of Translation

The dephosphorylated eIF: Active form

Heme prevents eIF phosphorylation Heme stimulates translation

Proteins/mRNA interactions:

e.g., Ferritin and transferrin receptor synthesis

Posttranslational Modifications

Trimming

Covalent modifications:Phosphorylation Biotinylation

Glycosylation Farnesylation

Hydroxylation AcetylationCarboxylation

Ubiquitination

Posttranslational Modifications:

Trimming Removal of a part of the translated sequence Proteases protein activation Cellular site of cleavage:

ER and Golgi apparatus

Secretory vesicles: e.g., insulin

After secretion: e.g., collagen

At sites of action: zymogen e.g., trypsinogen

Posttranslational Modifications:

PhosphorylationAddition of phosphate to hydroxyl group of

Serine and threonineTyrosine

Reversible: Addition by kinasesRemoval by phosphatases

Increase or decrease the activity of a specific protein

e.g., Glycogen synthase and glycogen phosphorylase

Posttranslational Modifications:

Phosphorylation

Posttranslational Modifications:Glycosylation

Modifications of :proteins of plasma membraneproteins of lysosomesecretory proteins

Addition of carbohydrate chains to:Hydroxyl group of serine or threonine (O-linked)Amide group of asparagine (N-linked)

Site: Golgi apparatus and ER

Protein targeting: e.g., mannose-6-phosphate to lysosomal enzymes

Posttranslational Modifications:Glycosylation

Posttranslational Modifications:Hydroxylation

Proline and lysine

Endoplasmic reticulum

Prolyl or lysyl hydroxylases

e.g., Collagen

Posttranslational Modifications:Other Covalent Alterations - 1

Carboxylation:

γ-carboxylation of glutamic acid residue for maturation of vitamin K-dependent clotting factors

II, VII, IX and X

Carboxylation

Posttranslational Modifications:Other Covalent Alterations - 2

Biotinylation

Farnesylation

e.g., Acetyl Co-A Carboxylase

e.g., Protein anchoring to membranes

Posttranslational Modifications:Ubiquitination

Proteins:DefectiveRapid turnover

Ubiquitin: Small, highly conserved protein

Proteasome:Complex proteolytic systemATP-dependentCytosol