FIG. 4.

S303 is strongly phosphorylated in the wild-type and S307A mutant HSF1. To obtain two-dimensional tryptic phosphopeptide maps, wild-type (WT), S303A, S307A, or S303/7A HSF1 was transfected into K562 cells, after which the cells were in vivo labeled with [³²P]orthophosphate and exposed to a heat shock (42°C, 1 h). The labeled HSF1 was digested with trypsin and resolved by two-dimensional mapping. The white arrow indicates phosphopeptide phosphorylated on both S303 and S307. The black arrow indicates phosphopeptides phosphorylated either on S303 or on S307. The starting points are indicated as “x.”