Cellular Departmentalization, Protein Targeting/Sorting/Trafficking, Membrane Transport, Nuclear Pores, NFAT Shuttling

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University of Massachusetts Amherst
Biochemistry & Molecular Bio.
David Gross

March 10 • Cell Departmentalization o euk & prok cells highly compartmentalized, allows for  separation of specific cellular functions  independent regulation of cellular processes o each compartment (organelle) must:  maintain specific identity  possess mechanisms to communicate w/ other cell parts (intracellular communication o cellular localization can be considered another level of control of gene expression & function • Organelles o biogenesis & maintenance presents cells w/ challenge  vast majority of proteins are encoded in nucleus & translated on cytoplasmic ribosomes  organellar proteins must be sorted to proper organelle, transported across organelle membrane, assembled into functional proteins • The signal hypothesis (Blobel G. & Dobberstein B. 1975) o hypothesis proposed to explain how proteins, which are synthesized on cytoplasmic ribosomes, are transported & assembled into appropriate cellular compartments (organelles) or location o hypothesis postulated tht new proteins contain built-in signals, address tags, that target proteins to,& leads them across, specific membranes through specialized channels o there’s a unique targeting signal for every organelle or cellular compartment • 3 categories of protein targeting/sorting/trafficking o gated transport – transport of individual polypepetides or protein complexes o transmembrane transport – same as gated. o vesicular transport – this is bulk transport of many different proteins that are usually folded into their 3D structure • Principles of gated & transmembrane transport of proteins o intrinsic targeting signal – the targeting signal is specific to the organelle or compartment o molecular chaperones associate w/ polypep to maintain solubility & prevent aggregation o targeting factor recognizes targeting signal & is specific to organelle or compartment o membrane receptors specific to organelle or compartment specifically recognize targeting factors & targeting signals o channel or pore acts as conduit for movement from cytoplasm to organelle interior o molec. chap. maintain solubility, can also provide thermodynamic driving force for transport across membrane, can function as molecular motors.  external chaperones distinct from internal chaperones (compartment) • nucleocytoplasmic transport o transport between cytoplasm & nucleus o gated transport o nuclear envelope  double membrane  contiguous w/ endoplasmic reticulum  punctuated w/ large nuclear pore complexes o major mechanism for regulation of gene expression via control of macromolecular shuttling btwn cytoplasm & nucleoplasm  signals sensed in cytoplasm result in transport of molecule into nucleus & activate or repress gene expression • nuclear pore complexes o have “fish cage” structure – the internal basket o embedded in nuclear envelope, but not transmembrane channel o these are true pores permeable to small molecules o larger molecules stopped by filaments, require assisted transport • nuclear localization signals o direct the import of proteins into nucleus from cytoplasm o can be located anywhere within primary sequence, but must be exposed @ protein surface in 3D structure o proteins transported as fully folded proteins & complexes • nuclear import o import mediated by important specific factors
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