2 nots - 1 Lecture 2 September 10, 2008 Intracellular...

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1 Lecture 2 September 10, 2008 Intracellular processing and transport of ECM molecules Molecular Organization of collagens Collagen Classification Fibrillar collagens: structure and assembly of type I collagen (prototype) Intracellular Processing A slide was presented showing an overview of intracellular protein trafficking (Fig 12-6, page 665, 4 th edition of Alberts), with a special emphasis on the secretory pathway. The synthesis of most secretory protein begins on free ribosomes in the cytosol. The presence of an N-terminal 15-30-residue (Signal Peptide, SP) serves to direct ribosomes to the ER membrane and to initiate the transport of the growing peptide (nascent peptide) across the ER membrane. Read pages 693-698 of Alberts. Key aspects of the lecture are present below. Features of the N-terminal peptide sequence: Contains one or more positively charged amino acid followed by a continuous stretch of 6-30 hydrophobic residues. The hydrophobic core is critical; substitution for even one charged residue can inhibit the ability of the protein to cross the ER membrane. Mechanism: Two key components in targeting the ribosomes to ER membranes: a signal recognition particle (SRP) and the SRP receptor located on the ER membrane. SRP is a cytosolic protein that binds transiently to the signal peptide (SP) sequence once approximately 70 amino acids have been synthesized. The SRP is a ribonucleoprotein complex composed of 6 discrete proteins with a 300-nu long RNA. See fig. 12-41 for their spatial organization and division of labor. The emphasis was on the signal peptide binding component, called P54 (based on its mw of 54 kDa). P54 contains a large number of clustered methionine (Met) residues whose hydrophobic side chains protrude outward and bind to the hydrophobic core of the SP. Protein synthesis is then arrested, preventing the translation of the entire protein before the ribosome has time reach the ER plasma membrane. Upon binding to the signal peptide, GDP, which is bound to the free SRP, is exchanged of GTP. The SPR-GTP complex binds and promotes the opening of an ER transmembrane protein
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This note was uploaded on 09/29/2009 for the course CSB csb327 taught by Professor Ringuitte during the Fall '08 term at University of Toronto- Toronto.

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2 nots - 1 Lecture 2 September 10, 2008 Intracellular...

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