Endoplasmic Reticulum PowerPoint Presentation

Endoplasmic Reticulum PowerPoint Presentation. Polypeptides are synthesized at two distinct sites within the cell: 1. On ribosomes attached to the cytosolic surface of the RER membranes (1/3rd polypeptides) . These include (a) secreted proteins, (b) integral membrane proteins, and (c) soluble proteins that reside within compartments of the endomembrane system, including the ER, Golgi complex, lysosomes, endosomes, vesicles, and plant vacuoles. 2 On “free” ribosomes: These are subsequently released into the cytosol. This class includes (a) proteins destined to remain in the cytosol (such as the enzymes of glycolysis and the proteins of the cytoskeleton), (b) peripheral proteins of the cytosolic surface of membranes (such as spectrins and ankyrins that are only weakly associated with the plasma membrane’s cytosolic surface), (c) proteins that are transported to the nucleus and (d) proteins to be incorporated into peroxisomes, chloroplasts, and mitochondria. Proteins in the latter two groups (c) & (d) are synthesized to completion in the cytosol and then imported posttranslationally into the appropriate organelle across its boundary membrane(s). What determines the location in a cell where a protein is synthesized? • Secretory proteins contain a signal sequence at their Nterminus that directs the emerging polypeptide and ribosome to the ER membrane. • The polypeptide moves into the cisternal space of the ER through a protein-lined, aqueous channel in the ER membrane. It was proposed that the polypeptide moves through the membrane as it is being synthesized, that is, cotranslationally. We offer online tutoring, help with assignments and essay writing for all Majors with a guaranteed pass. For assistance Contact Tutor Lucas: