Lecture Protein sorting at the cytoplasm.pdf

Protein sorting at the cytoplasm Lecture BIOL2020 The cytoplasm way of sorting proteins Sorting signals for protein translocation into organelles are linear signal sequences Part 1: into the nucleus The nuclear envelope consists of two concentric membranes, which are perforated by nuclear po...

Protein sorting at the cytoplasm Lecture BIOL2020 The cytoplasm way of sorting proteins Sorting signals for protein translocation into organelles are linear signal sequences Part 1: into the nucleus The nuclear envelope consists of two concentric membranes, which are perforated by nuclear pore complexes nuclear pore complexes (NPCs) perforate the nuclear envelope in all eukaryotes. Each NPC is composed of a set of approximately 30 different proteins, or nucleoporins. Unstructured proteins at the inner ring form a mesh cytoplasm ● This mesh acts as a sieve that restricts the diffusion of large macromolecules while allowing smaller molecules to pass nucleus Unstructured proteins regions lack a well-structured three-dimensional fold. Some proteins are entirely unstructured. Proteins with a nuclear localization sequence (NLS) are directed into the nucleus ● ● ● Adding a NLS to a cytoplasmic protein moves it to the nucleus Nuclear localization signals have flanking basic clusters Proteins with a nuclear export sequence (NES) leave the nucleus The import receptors (importins) are soluble cytosolic proteins that contain multiple low-affinity binding sites for the FG repeats found in the unstructured domains of several nucleoporins. the importin–cargo complex locally dissolves the gel-like mesh and can diffuse into and within the NPC pore The concentration of Ran bound to GTP provides energy and directionality For importins, Ran-GTP in the nucleus promotes cargo dissociation Importins are the receptor for NLS containing proteins. Exportins are the NES receptors For exportins, Ran-GTP in the nucleus promotes cargo binding, rather than promoting cargo dissociation as in the case of importins. a cytosolic GTPase-activating protein (GAP) triggers GTP hydrolysis Ran-GDP, and a nuclear guanine nucleotide exchange factor (GEF) promotes the exchange of GDP for GTP For exportins, Ran-GTP in the nucleus promotes cargo binding, rather than promoting cargo dissociation as in the case of importins. importin-beta The α subunit of importin binds the nuclear localization signal The β subunit binds the unstructured chains. importin-alpha https://pdb101.rcsb.org/motm/85 the cargo nucleoplasmin In the cytoplasm, a GTP molecule in Ran (shown in bright red) is hydrolyzed and the Ran dissociates, leaving importin-beta ready to carry the next cargo protein inside GTP Ran importin-beta Exportins bind to both the export signal, either directly or via an adaptor, and to NPC proteins to guide their cargo to the cytosol. exportin nucleoporin Cargo Many proteins are known to have both NESs and NLSs and thus shuttle constantly between the nucleus and the cytosol Part 2: Into the mitochondria Most mitochondrial proteins are synthesized in the cytoplasm and then translocated into the mitochondria Mitochondrial and plastid proteins have sequence signals The TIM and TOM systems move polypeptide chains through the two membranes of the mitochondria ● ● ● Translocase of the outer membrane = TOM Translocase of the inner membrane = TIM For the for chloroplasts : TIC and TOC HSP-70 chaperones interact with mitochondrial proteins at the cytoplasm and prevent them from folding Hsp-70 chaperones Target Sequence