Practice question

Unidirectional accumulation of proteins against concentration gradients through nuclear pores cannot arise from gate selectivity alone because FG nucleoporin mesh permits diffusion in both directions. Direction emerges from asymmetric distribution of Ran regulators generating chemical potential. Nuclear RCC1 bound to chromatin continuously exchanges GDP for GTP using nuclear GTP pool, while cytoplasmic RanGAP1 SUMOylated and tethered to RanBP2/Nup358 plus soluble RanBP1 hydrolyzes GTP outside, creating gradient high inside low outside, more than hundred-fold difference. Importins bind NLS cargo tightly where Ran-GTP low in cytosol and release where Ran-GTP high in nucleus; exportins like CRM1 behave oppositely, binding leucine-rich NES only when Ran-GTP high in nucleus and releasing after hydrolysis in cytosol. Continuous GTP turnover fuels recycling of Ran and receptors, analogous to ion pump. NTF2 import of Ran-GDP sustains supply. Collapse of gradient upon temperature-sensitive RCC1 loss causes equilibration of import cargo, while actin filaments, ATP motors or disulfide bond formation do not provide polarity, directly demonstrating Ran-GTP gradient as primary source of directionality for compartmentalized proteome maintenance and nucleocytoplasmic identity during interphase and stress response.