The nuclear pore complex (NPC) is both the major conduit for nucleocytoplasmic 
trafficking and a platform for organizing macromolecules at the nuclear 
envelope. We report that yeast Esc1, a non-NPC nuclear envelope protein, is 
required both for proper assembly of the nuclear basket, a structure extending 
into the nucleus from the NPC, and for normal NPC localization of the Ulp1 SUMO 
protease. In esc1Delta cells, Ulp1 and nuclear basket components Nup60 and Mlp1 
no longer distribute broadly around the nuclear periphery, but co-localize in a 
small number of dense-staining perinuclear foci. Loss of Esc1 (or Nup60) alters 
SUMO conjugate accumulation and enhances ulp1 mutant defects. Similar to 
previous findings with Mlp1, both Esc1 and Ulp1 help retain unspliced pre-mRNAs 
in the nucleus. Therefore, these proteins are essential for proper nuclear 
basket function, which includes mRNA surveillance and regulation of SUMO protein 
dynamics. The results raise the possibility that NPC-localized protein 
desumoylation may be a key regulatory event preventing inappropriate pre-mRNA 
export.