NSP1: a yeast nuclear envelope protein localized at the nuclear pores exerts its essential function by its carboxy-terminal domain

NSP1 is located at the nuclear periphery in yeast and is essential for cell growth. Employing immunoelectron microscopy on yeast cells, we show that NSP1 is located at the nuclear pores. The molecular analysis of the NSP1 protein points to a two domain model: a nonessential domain (the first 603 amino acids) composed of repetitive sequences common to other nuclear proteins and an essential, carboxy-terminal domain (residues 604-823) mediating the vital function of NSP1. The NSP1 carboxy-terminal domain, which shows a heptad repeat organization, affected the correct location of two nuclear proteins: site-specific amino acid substitutions within a predicted alpha-helical structure of this domain caused a temperature-sensitive growth arrest at 37 degrees C and the appearance of NSP1 and NOP1, a nucleolar protein, in the cytosol.