Differential protein expression by Shigella flexneri in intracellular and extracellular environments

Shigellae were intrinsically radiolabeled with [35S]methionine either extracellularly or while multiplying within infected HeLa cell monolayers. A complex pattern of suppression and induction of proteins was observed. Proteins of approximately 97, 62, 58, 50, 25, and 18 kilodaltons (kDa) were induced in Shigella flexneri isolated from infected monolayers. Proteins of 100, 85, 70, 64, and 55 kDa were suppressed under the same conditions but were seen in cells labeled in the tissue culture medium alone. Protein expression during the stages of attachment, invasion, and intracellular multiplication was examined by pulse-labeling. The 58-kDa protein was induced only during invasion, and the 62- and 25-kDa proteins were induced only during intracellular multiplication. Shift into a minimal medium with ion concentrations and pH mimicking intracellular conditions and endosomal pH resulted in the induction of the 97- and 58-kDa proteins, and reduction of the intracellular-like medium with 2-mercaptoethanol resulted in the induction of the 97-, 50-, and 25-kDa proteins and suppression of the 55-kDa protein. Radioimmunoprecipitations of shigellae grown in vitro and in vivo revealed differential expression of immunogenic proteins. Proteins corresponding in size to IpaB (62 kDa), IpaC (42 kDa), and IpaD (38 kDa) were lost during intracellular multiplication, whereas another protein corresponding to IpaA (80 kDa) was found to increase under the same conditions.