The first half of the ScFtsY N-terminal sequence, ScFtsY11-24, did not target recombinant EGFP to the membrane as efficiently as the full N-terminal sequence ScFtsY11-39. This finding suggests that the entire ScFtsY N-terminal sequence may be required to obtain the full membrane-targeting efficiency. In contrast, EcFtsY1-14 did not target EGFP to the membrane; this result demonstrated that our genetic manipulation and addition of the linker sequence did not produce the observed membrane-targeting HDAC inhibitor effect. The high efficiency with which the ScFtsY N-terminus targeted EGFP to the
membrane and the high membrane-binding affinity revealed by the carbonate treatment experiments indicated that the ScFtsY N-terminus bound the membrane tightly. This tight binding suggested that the ScFtsY N-terminus
might have inserted into the membrane, as opposed to the superficial attachment to the membrane that has been observed with the EcFtsY N-terminus (Braig et al., 2009). It was reported that by using the thiol-specific, membrane-impermeable probe maleimide-polyethylene glycol (Mal-PEG), membrane insertion structures can be distinguished from structures that are only peripherally associated (Braig et al., 2009). Under oxidative conditions, Mal-PEG forms disulfide bridges between accessible cysteine residues of a given protein and increases check details the mass of the protein, which leads to a mobility shift detectable by SDS-PAGE. If the cysteine residues were inserted into the membrane, Mal-PEG would not be able to access them. The N-terminal Interleukin-3 receptor sequence of ScFtsY does not contain any cysteine residue, but EGFP contains two cysteine residues. The cysteine residues in EGFP were mutated to their most similar residue, threonine, and this mutated EGFP was linked to ScFtsY11-39 using the
LPGPELPGPE linker. The resulting construct was labeled ScFtsY11-39m. Next, the 3rd, 13th, 22nd, 32nd, and 39th residues in ScFtsY11-39m were mutated to cysteines to create the five following constructs: ScFtsY11-39mI3C, ScFtsY11-39mI13C, ScFtsY11-39mV22C, ScFtsY11-39mG32C, and ScFtsY11-39mE39C; each of these constructs has one single cysteine residue (Fig. 3). The 32nd and 39th position in ScFtsY11-39m were located in the linker sequence. The expression of the single cysteine constructs was verified using Western blot. In addition, we confirmed that these amino acid substitutions did not interfere with their membrane association. Their carbonate resistance was also not impaired (Fig. 3). The single cysteine constructs were first incubated with Mal-PEG in membrane-free conditions (Fig. 4, lane 1–3). In these conditions, the cysteine residues were exposed, and Mal-PEG was able to react with them. Two bands of mutant proteins appeared consistently: one at 27 kDa and another at 40 kDa (Fig. 4, lane 1). The single cysteine constructs has a molecular weight of 27 kDa.