Nucleus

nucleus

• membrane-bound organelle

• likely evolved from rough ER - nuclear envelope is continuous with ER and forms part of it

nuclear envelope

• consists of inner and outer membrane - associated with ER and has ribosomes on its surface

• separates nuclear and cytoplasmic compartments

nuclear pore complexes

• mediate movement of nuclear proteins into nucleus and mRNA into cytoplasm

• central channel filled with fluid-like phase - allows diffusion of small molecules

• cytoplasmic face - ring like structure, cytoplasmic filaments extend from cytoplasmic side of NPC into cytosol

• nucleoplasmic face - nuclear pore basket, distal ends of filaments are joined by a terminal ring

• nuclear pore complexes made up of nucleoporins - 3 types:

1. structural - form scaffold of nuclear pores, Y complex - set of 7 structural nucleoporins, 16 copies of Y complex form the basic structural scaffold of the pore

2. membrane. - connect inner and outer regions of the membrane through highly curved region

3. FG - line channel of NPC, associated with nuclear basket and cytoplasmic filaments - contain multiple hydrophobic repeats and are highly dynamic

nuclear transport

• prospective nuclear protein linked to nuclear localisation signal

• nuclear localisation signal directs selective transport into nucleus

• nuclear transport receptors recognise proteins with nuclear localisation signals and bind to NLS, mediate interaction of protein within receptor-pore complex

• nuclear transport receptor binds to FG-repeats on nucleoporins in NPC and complex moves through NPC via random diffusion

• RanGTP binds to import factor on nucleoplasmic side of membrane - causes conformational change so cargo is released into nucleoplasm

• nuclear transport receptor travels back to cytoplasmic side of membrane so it is recycled

transport receptors

• known as karyopherins or importins/exportins

• importin alpha binds to cargo and then to importin beta, importin beta binds to NPC and transports protein through

• CRM1 - major protein export factor

• TAP - export factor involved in mRNA export

Ran system

• controls direction of transport

• Ran - Ras-related nuclear small GTPase - molecular switch

• RanGAP - interacts with Ran on cytoplasmic side of NPC, this stimulates hydrolysis of GTP to GDP so that Ran changes conformation and dissociates from importin

• RanGEF - causes release of GDP from Ran so a GTP molecule can rebind on nucleoplasmic side of NPC, means Ran can be recycled

Protein nuclear import

• cargo binds to receptor via NLS

• receptor moves through NPC

• RanGTP binds to receptor - causes conformational change so cargo released

• Receptor-RanGTP exported

• RanGAP interacts with Ran so it is stimulated to hydrolyse GTP - undergoes conformational change and dissociates from receptor

• Free receptor binds to another cargo

• RanGEF interacts with Ran so that GDP molecule removed and replaced with a GTP molecule