2. Describe how a lysosomes is made Diagram 5: Endomembrane system Rough endopla

Question

2. Describe how a lysosomes is made Diagram 5: Endomembrane system Rough endoplasmic reticulumNucleus Nuclear envelope Nuclear pore Ribosomes Smooth endoplasmic reticulum Secretory Vesicle Lysosome Golgi apparatus Plasma membrane Describe the structures in the above diagram, and discuss their functions. Da What is the Endomembrane System, what does it do, and why is it important? Describe how a protein destined to be secreted via exocytosis is made. Describe how a protein destined to be a transmembranal protein is made. Describe how a Lysosome is made.

Explanation / Answer

1. The transmembrane proteins have a unique orientation in the membrane which is established during biosynthesis on the endoplasmic reticulum (ER) membrane. We can start our discussion with single-pass transmembrane proteins that have a cytosolic C-terminus and an exoplasmic N-terminus. This class of proteins possess an N-terminal signal sequence that targets them to the ER and an internal stop-transfer membrane-anchor sequence for the membrane-spanning ? helix.The membrane-spanning segments of the polypeptide chains contact the hydrophobic environment and are largely composed of non-polar amino acids. But peptide bonds are polar in nature and because water is absent in the environment, all peptide bonds in the bilayer form hydrogen bonds with one another. Thus the polypeptide chain suitably forms a regular ? helix as it crosses the bilayer. The ER signal sequence on a nascent polypeptide chain is cleaved while the chain is still being synthesized, and the new N-terminus of the growing polypeptide pass through the ER membrane into the lumen. The internal sequence prevents further extrusion of the nascent chain into the ER lumen. As the synthesis continues, the sequence then moves laterally through the proteins that line the multiprotein complex through which a nascent secretory protein enters the ER lumen as it is being synthesized (translocon) and becomes anchored in the membrane. Hence this hydrophobic sequence functions as both a stop-transfer and membrane-anchor sequence. The C-terminus of the nascent chain remains in the cytosol and is not transferred across the ER membrane. For proteins with N-terminal cytosolic region and C-terminal ER lumen region a single internal hydrophobic signal-anchor sequence functions as both an ER signal sequence and membrane-anchor sequence. This sequence directs insertion of the nascent chain into the ER membrane so that the N-terminus of the signal sequence faces the cytosol. For multipass transmembrane protein each membrane-spanning ? helix act as a topogenic sequence. As the nascent chain is inserted, the second hydrophobic ? helix acts as a stop-transfer membrane-anchor sequence, preventing further extrusion of the nascent chain through the translocon. Thus after synthesis of the first two transmembrane ? helices, both ends of the nascent chain are in the cytosol and a loop faces the lumen. C terminus continues into the cytosol.

2. Lysosomes are formed by the fusion of vesicles from the golgi complex with endosomes with several hydrolytic enzymes inside. The enzymes are produced in the rough endoplasmic reticulum, tagged for lysosomes by the addition of mannose-6-phopahte (M6P) and delivered into the Golgi apparatus via transport vesicles. Cells take up external materials via endocytosis and once inside the cytoplasm, those external materials remain captured in membranous vesicular bodies, called endosomes. The cellular membrane components of this endosome are recycled back to the plasma membrane. The endosome pH decreases and it is now called late endosome. Meanwhile, the lysosomal vesicles fuse with a late endosome and due to the acidic nature, M6P gets removed and the hydrolytic enzymes become free in it. This fusion turns the late endosome into a mature lysosome, capable of digesting materials via endocytosis.  

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