Lec16_05_24_04 - Chapter 16 Moving Proteins into Membranes...

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Unformatted text preview: Chapter 16 Moving Proteins into Membranes and Organelles Protein targeting is defined as process of delivering newly synthesized proteins into proper destinations. Two types: Targeting to to membrane of an intracellular organelle soon After synthesis by translation in ribosome. Membrane proteins on the other hand are inserted into the lipid bilayer of membrane. Proteins can be sorted to the endoplasmic reticulum, mitochrondrion, chloroplast, peroxisome and nucleus by general process shown in Fig 16-1. Proteins Targeted to the ER membrane enter the secretory pathway. Soluble proteins and membrane proteins may go to PM or lysosomes, Targeting to ER generally involves nascent proteins still in process of being synthesized. Proteins targeted to the golgi, lysosome, or PM may get there by first budding from one organelle and fusing from other. This distinct process covered in Chapter 17. Chapter 16 deals with proteins targeted via signaling sequences. Each organelle carries a set of receptor proteins that bind only to specific kinds of signal sequences assuring specificity of targeting. The protein containing a cognate sequence that interacts with its receptor the protein chain is transferred to some type of translocation channel allowing p;rotien to pass through membrane bilayer. The one directionality of this process is mediated by hydrolysis of ATP. These signaling sequences are eventually removed from mature protein by proteases. Four questions define the protein targeting events:- What is nature of signal sequence and differences among them?- What is the cognate receptor for a signal sequence?- What is the structure of translocation channel allowing transfer of protein across bilayer?- What is source of energy that drives unidirectional transfer to occur? Same secretory pathway for synthesizing and sorting secreted proteins in the ER, golgi, and lysosomes. Many different proteins are secreted including extracellular matrix, digestive enzymes, etc. However, pancreatic cells and other secretory cells are models for studying pathways as they contain organelles of the secrectory pathway in great abundance. Rough ER is region that receives proteins entering the secretory pathway and membranes are densely studded with ribosomes (Fig 16-2). When broken apart through homogenization ER brakes up into rough microsomes and retain same orientation as in cell. The experiment of Fig 16-3 depict pulse labeling and treatment with protease that shows secretory proteins synthesized on ribosome and then bound to cytosolic face of ER where they become localized to ER vesicles during synthesis. Hydrophobic N-terminal signal sequence targets nascent secretory proteins to ER....
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Lec16_05_24_04 - Chapter 16 Moving Proteins into Membranes...

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