FIGURE 2.

Overall view of the SifA-SKIP(PH) complex. A, ribbon diagram of the SifA-SKIP(PH) complex, viewed in two orientations rotated by 90°. The SifA N- (residues 21–136) and C-terminal (residues 137–328) are shown in yellow and orange, respectively, and the SKIP(PH) (residues 772–876) is in green. The SifA conserved motifs WE(I/M)XXFF, which is important for translocation (13), and WXXXE, which has been proposed to mimic activated GTPase (16), are highlighted in pink and cyan, respectively. The Leu¹³⁰ position buried at the complex interface is displayed in red. B, molecular surface of the complex (left), color-coded, and oriented as in the left view of panel A, with the surface buried at the complex interface shown in green. Close-up view (right) of key residues involved in the binding interface. C, mapping sequence conservation in the SifB homolog onto the molecular surface of SifA (yellow and orange for the N- and C-terminal domain), oriented as in the left view of panel A, with non-conserved side chains from the N- and C-terminal domain shown in pink and magenta, respectively. Small patches of non-conserved surface regions (pink) are located in the N-terminal domain and within the binding interface (green) while large patches of non-conserved surface regions (magenta) are clustered within the C-terminal domain. D, surface electrostatic potential map of SifA, oriented as in the right view of panel A, showing a dominant electronegative potential except for a large patch of electropositive potential clustered near the translocation motif in the N-terminal domain. Electrostatic surface potentials are contoured at −3/+3 kT/e electrostatic units (k, Boltzmann constant; T, temperature in Kelvin; e, electronic charge), where red describes a negative and blue a positive potential. The figures were generated with PyMOL (DeLano Scientific (2004), San Carlos, CA), and panel D was generated with the APBS plug-in for PyMOL.