Solutes modify a conformational transition in a membrane transport protein

Abstract

The bacterial outer-membrane vitamin B(12) transporter, BtuB, undergoes a dramatic order-to-disorder transition in its N-terminal energy-coupling motif (Ton box) upon substrate binding. Here, site-directed spin labeling (SDSL) is used to show that a range of solutes prevents this conformational change when ligand is bound to BtuB, resulting in a more ordered Ton box structure. For each solute examined, the data indicate that solutes effectively block this conformational transition through an osmotic mechanism. The molecular weight dependence of this solute effect has been examined for a series of polyethylene glycols, and a sharp molecular weight cutoff is observed. This cutoff indicates that solutes are preferentially excluded from a cavity within the protein as well as the protein surface. Furthermore, the sensitivity of the conformational change to solution osmolality is consistent with a structural model predicted by SDSL. When the Ton box is unfolded by detergents or mutations (rather than by ligand binding), solutes, such as polyethylene glycols and salts, also induce a more structured compacted conformation. These results suggest that conformational changes in this class of outer membrane transporters, which involve modest energy differences and changes in hydration, may be modulated by a range of solutes, including solutes typically used in protein crystallization.

FIGURE 1

Models for BtuB. (a) Crystal structure of BtuB in the absence of substrate (vitamin B₁₂) (Protein Data Bank ID 1NQE), and (b) model for BtuB in the presence of substrate, where the configuration of the Ton box has been modified from that in the crystal structure to be consistent with the results of SDSL (3). In this case, SDSL indicates that the N-terminus, including the Ton box, unfolds to position 15 or 16. (c) Crystal structure of BtuB in the presence of substrate (Protein Data Bank ID 1NQH) (7). The Ton box includes residues 6–12. The portion of the structure highlighted in stick form represents residues 6–17. (1) A model for the conformational change that takes place under physiological conditions when substrate binds BtuB. (2) The conformational change that takes place under conditions of high osmolality upon the addition of substrate. (3) A conformational change that takes place upon the addition of osmolyte in the presence of substrate.

FIGURE 2

MTSL spin label is attached to a cysteine side chain to produce the spin-labeled side chain R1.

FIGURE 3

Normalized X-band EPR spectra of purified reconstituted BtuB labeled in the Ton box at position 10 (V10R1): (a) in POPC without substrate, and (b) in POPC with substrate (vitamin B₁₂) added. The addition of substrate induces an unfolding in the Ton box. (c) In the soaking solution used for protein crystallization with substrate added. Each spectrum is a composite arising from a highly mobile R1 population (downward-facing arrow), corresponding to the unfolded state and a motionally restricted R1 population, corresponding to the folded or docked state (upward-facing arrow). Spectra are 100 gauss scans.

FIGURE 4

Series of EPR spectra from V10R1 in the presence of substrate with the addition of 0% to 30% w/v PEG 400 to a buffer solution of 10 mM HEPES, 130 mM NaCl, pH 6.5. The decrease in amplitude reflects the conversion of the Ton box from an undocked to a docked (or trapped state). Also shown are plots of ln(K) versus osmolality obtained for V10R1 when either PEG 3350 (•) or PEG 400 (○) is used as the solute. The different slopes reflect different thermodynamic hydration volume changes probed by these solutes (see Table 1).

FIGURE 5

Renderings of the solvent-exposed surface (Connolly surface) on the periplasmic end of BtuB for the hatch region (residues 6–135) of BtuB. Surface area for residues 6–15 is shown in green; surface area for residues 16–135 is shown in yellow. (a) The BtuB crystal structure in the Ca²⁺- and B₁₂-bound state (Protein Data Bank ID 1NQH), and (b) a model of BtuB in the Ca²⁺- and substrate-bound state where the conformation of the Ton box is consistent with the results of SDSL (see Fig. 1 b) and unfolded from residues 6–15.

FIGURE 6

When the Ton box is disturbed, it may be refolded by solutes. EPR spectrum of (a) T7R1 in POPC, (b) T7R1/V10P in POPC, and (c) T7R1/V10P in the crystallization soaking buffer. EPR spectra for N13R1 in (d) POPC and (e) the crystallization soaking buffer. EPR spectra of V10R1 in (f) POPC/OG (1:17) mixed micelles and (g) POPC/OG mixed micelles plus 30% PEG 3350. Spectra are 100 gauss scans.