Split Ub measures the proximity between Sec63p and membrane-associated proteins in vivo. Sec63CRUp containing cells expressing Nub, Nua, and Nug constructs of Sec62p (A), Sec61p (B), Ssh1p (C), Bos1p (D), Ste14p (E), Sed5p (F), Sso1p (G), Snc1p (H), Tom22p(I), Vam3p (J), Tpi1p (K), and Guk1p (L) were spotted (105 and 103 cells) on selective media lacking uracil (A–M) and leucine and histidine (A and D) or leucine and tryptophan (B, C, and E–M) to select for the presence of the Cub and Nub constructs. (M) Sec63CRUp-containing cells bearing either the empty plasmid, Nub-, Nua-, -Nug-Sec22p or Nub-, Nua-, Nug-Sec61p were spotted (105, 104, 103 cells) on plates lacking uracil. Cells were grown for 4 d.