Sandbox 122: Difference between revisions

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Wayne Decatur, Student, Allison Granberry, Marisa L. VanBrakle

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 An importin, such as Kapβ2, binds to a specific cargo by recognition of an NLS and carries the cargo through the NPC by interacting with intrinsically disordered Nups called FG-Nups. FG-Nups line the passageway of the NPC and contain repeats of phenylalanine and glycine. These unstructured FG-Nups form a low-density cloud within the central channel extending from the cytoplasm to the nucleoplasm. The cloud acts as an effective exclusion filter for those particles that do not contain FG repeat binding sites. This is referred to as the zone of selectivity.
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 ==='''How does Kapβ2 identify its cargo?'''===
 '''Structure of Kapβ2'''
+Kapβ2 is a superhelix comprised of 20 HEAT repeats (the name HEAT derives from Huntington, Elongation factor 3 A subunit of protein phosphatase 2A and Tor1 kinase), each of which consists of two anti-parallel helices. The electrostatic potential of the internal surface of Kapβ2 superhelix at the C-terminal arch is negative.
-Kapβ2 is a superhelix comprised of 20 HEAT repeats, each of which consists of two anti-parallel helices.
+HEAT repeats 9-13 and 14-18 form the binding site of Kapβ2 cargos while repeats 1-8 constitute the Ran GTPase binding site.3 Ran GTPase, a small 216-residue protein, is found more frequently in the nucleus and enables cargos to be released from KapB2.
-HEAT repeats 9-13 and 14-18 form the binding site of the cargo while repeats 1-8 constitute the Ran binding site.
-'''Kapβ2 Cargo Binding and Conformational Change'''
-The NLS, located on the cargo, binds to the C-terminal arch of Kapβ2.
+Kapβ2 Cargo Binding and Conformational Change4
+The NLSs located on Kapβ2 cargos are named the PY- NLS and they bind to the C-terminal arch of Kapβ2.
-Recognition of the NLS by Kapβ2 follows certain guidelines:
+Recognition of the PY-NLS by Kapβ2 follows certain guidelines:
+(i) PY-NLS, when not bound to Kapβ2, lacks a secondary structure.
-(i) NLS, when not bound to Kapβ2, lacks a secondary structure.
+(ii) PY-NLS has an overall positive charge allowing for electrostatic compatibility with Kapβ2.
+(iii) General sequence for the PY-NLS is either a hydrophobic or basic motif at the N- terminus and a R-X2-5-P-Y motif at the C- terminus.
-(ii) NLS has an overall positive charge allowing for electrostatic compatibility with Kapβ2.
+The PY-NLS of 2H4M contains a hydrophobic rather than a basic N- terminal motif. Hydrophobic interactions at the N- terminal motif of the PY- NLS include: Phe273, Gly274, Pro275 and Met276 of the NLS with Trp730 and Ile773 of Kapβ2. Interactions at the C- terminal R-X2-5-P-Y motif of the NLS include: Arg284 of the NLS with Glu509 and Asp543 of Kapβ2; Pro288 and Tyr289 of the NLS with Ala380, Ala 381, Ala 384, Leu419, Ile457, Trp460 and Arg464 of Kapβ2.
-(iii) General sequence for the NLS is either hydrophobic with a central basic motif or rich with basic residues and lacking hydrophobic residues. The NLS of 2H4M follows the former motif as opposed to the latter.
+Upon bin'''ding Kapβ2, the NLS gains structure, conforms to and makes contact with the internal surface of the Kapβ2 C-terminal arch.
+'''How does Kapβ2 pass through the NPC?'''
+Once the cargo binds to Kapβ2, the complex travels through the NPC by interactions with the FG-Nups.
-Upon binding conformational change occurs in which the NLS gains structure and conforms to the shape of the C-terminal arch of Kapβ2.
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