Question

Consider the synthesis of a hydrolytic enzyme that functions in the lumen of a lysosome.  Describe the pathway through which this protein is synthesized.  Start with the beginning of translation on a ribosome in the cytosol, and end as the protein becomes active in the lumen of the lysosome.  Emphasize the locations in the cell that the protein passes through as it is synthesized and the signals that control the localization of the protein

Answer 1

Proteins that become part of lysosomes are enzymes that are synthesized in the ribosomes attached to ER.

Ribosomes translate the protein and the protein molecule is released in to the ER lumen. The ER transports them to golgi through COPII coated transport vesicles. These COPII vesicles bud from regions on the ER called exit sites. These exit signals direct the proteins in to golgi apparatus. To exit from ER proteins must be properly folded and they also need to properly assembled if they are multi subunit proteins. Misfolded and misassembled proteins are degraded in cytoplasm. These vesicles that are budded off from ER fuse all of them to form vesicular tubular clusters which ultimately fuse with Golgi membranes and pass on the proteins in to golgi. With in the golgi apparatus, the enzymes mature . NUmber of enzymes to digest all kinds of substances are present in the lysosomes. So these enzymes are budded off as vesicles from trans-end of the golgi. Enzymes destined to be part of lysosomes are tagged with mannose-6-phosphate. These transport vesicles fuse with the endosome that is budded off from plasma membrane. The lysosomes are formed at the intersection between two path ways. One is secretory path way and another is endocytotic path way by which external materials are taken from cell surface by plasma membrane.