Ribose-binding protein (RBP), one of the periplasmic binding proteins in E. coli, is involved in the active transport of ribose and chemotaxis. RBP is exported to the periplasm with the help of the secretion apparatus including SecA and SecYEG. Although it was reported that RBP is translocated in a Ffh dependent manner in vivo, there are still some controversies as to what factors are involved in the targeting of RBP, especially about the involvement of SecB. To elucidate the exact targeting mechanism of RBP to the membrane, studies with an in vitro translocation system are required.
In the present investigation, the translocation of RBP was studied using the in vitro translation/translocation system. It was found that RBP is translocated into heterologous eukaryotic microsomes co-translationally as well as post-translationally. However, RBP is translocated into E. coli inverted membrane vesicles(IMV) only post-translationally. During the translocation across the IMV membrane, degradated forms of RBP with the molar mass of 14 and 16 KDa were produced. Various factors such as phenylmethylsulfonyl fluoride(PMSF), GTP and proton motive force affected the translocation of RBP. The increase of RBP translocation induced by GTP suggests that the Ffh, which have GTPase activity, is involved in the targeting. In the presence of PMSF, no degradated products were observed but instead the mature form of RBP appeared. Moreover, the efficiency of RBP translocation was increased by PMSF. An unknown protease might be involved in the in vitro translocation of RBP. It was also found that SecB enhanced the post-translational translocation of RBP. It appears that RBP is targeted via both Ffh and SecB. It was also found that the RBP translocation is inhibited by an unknown protease.