Fig. 6. Ubiquitin internalization information is not a tyrosine- or di-leucine-based signal but is carried in the three-dimensional structure of the polypeptide. (A) Sequence of ubiquitin. Shown in bold and underlined are residues that may be part of well-characterized plasma membrane protein internalization motifs. (B and C) α–factor internalization assays were performed on cells expressing different Ste2p and Ste2p–Ub mutants (see Figure 1). Cells expressing these receptors were grown to log phase in SD minimal medium and assayed as described in Figure 2A. All curves represent the average of at least three independent experiments and error bars represent the standard deviations. (B) Ste2p-340–Ub (LHY713, □); Ste2p-337R,340Stop (LHY848, ⋄); Ste2p-340-DQQRLI (LHY1518, •). (C) Ste2p-376–Ub3xR (LHY1127, □); Ste2p-4xR,378Stop (LHY319, ⋄); Ste2p-376–UbV26G (LHY1157, ▪); Ste2p-376–UbL43A (LHY1162, •).