Other Ways of Protein Sorting - Assignment Example

Other ways of protein sorting Fill in the missing words. Transcript from lecture s and Lehninger Ch27 Page 1063 1. Some sugars act as sorting signals Glycosylation: addition of sugars to form antigens on cell surfaces- E.g. Red cell blood groups i.e. Mannose-6-phosphate is the sorting signal for lysosomes containing > 50 hydrolytic enzymes. Proteins bind to a mannose-6-phosphate receptor in the Golgi. 2. Nuclear Targeting Involves the movement of macromolecules molecules through nuclear pores to the cytoplasm.

Construction of the Ribosome: Exportation of RNA molecules synthesized in the nucleus to the cytosol. Ribosomal protein synthesis (of the 60 & 40 s subunits) occurs in the cytosol and are imported back into the nucleus. They are then constructed in the nucleus into a complete ribosome. Nuclear Proteins Nuclear proteins like DNA and RNA polymerases, histones etc. are synthesized in the cytosol and imported into the nucleus. Dispersal of nuclear proteins occurs during cell division and has to be re-imported after the nuclear envelope is re-established.

To allow repeated nuclear importation the signal sequence that target to the nucleus is not cleaved off. This is termed the nuclear localization sequence NLS. This sequence may be located anywhere on the primary sequence. Note the in aa position with different nuclear proteins. Some lie within the Signal Sequence section (aa 3-36) some do not. Nuclear importation involves alpha and beta importin and a GTPase called Ran. Alpha and beta importin produce a hetrodimer. They act as a soluble receptor for proteins targeted to the nucleus.

The alpha subunit binds to the NLS of the nuclear protein in the cytosol. The whole complex then docks at a nuclear pore and translocates across the pore via an energy driven process requiring Ran GTPase. During translocation the importin subunits separate, importin beta dissociates from importin alpha and the NLS bound to the alpha importin dissociates. The alpha and beta importin are then recycled and are transported through the nuclear pore into the cytosol.