The uncoupling protein from brown adipose tissue mitochondria. The environment of the tryptophan residues as revealed by quenching of the intrinsic fluorescence

Modeling the transmembrane arrangement of the uncoupling protein UCP1 and topological considerations of the nucleotide-binding site

The uncoupling protein from brown adipose tissue (UCP1) is a mitochondrial proton transporter whose activity is inhibited by purine nucleotides. UCP1, like the other members of the mitochondrial transporter superfamily, is an homodimer and each subunit contains six transmembrane segments. In an attempt to understand the structural elements that are important for nucleotide binding, a model for the transmembrane arrangement of UCP1 has been built by computational methods. Biochemical and sequence analysis considerations are taken as constraints. The main features of the model include the following: (i) the six transmembrane alpha-helices (TMHs) associate to form an antiparallel helix bundle; (ii) TMHs have an amphiphilic nature and thus the hydrophobic and variable residues face the lipid bilayer; (iii) matrix loops do not penetrate in the core of the bundle; and (iv) the polar core constitutes the translocation pathway. Photoaffinity labeling and mutagenesis studies have identified several UCP1 regions that interact with the nucleotide. We present a model where the nucleotide binds deep inside the bundle core. The purine ring interacts with the matrix loops while the polyphosphate chain is stabilized through interactions with essential Arg residues in the TMH and whose side chains face the core of the helix bundle.

Design, synthesis, and characterization of a novel hemoprotein

Here we describe a synthetic protein (6H7H) designed to bind four heme groups via bis-histidine axial ligation. The hemes are designed to bind perpendicular to another in an orientation that mimics the relative geometry of the two heme a groups in the active site of cytochrome c oxidase. Our newly developed protein-design program, called CORE, was implemented in the design of this novel hemoprotein. Heme titration studies resolved four distinct K(D) values (K(D1) = 80 nM, K(D2) = 18 nM, K(D3) > or = 3 mM, K(D4) < or = 570 nM, with K(D3) x K(D4) = 1700); positive cooperativity in binding between the first and second heme, as well as substantial positive cooperativity between the third and forth heme, was observed. Chemical and thermal denaturation studies reveal a stable protein with native-like properties. Visible circular dichroism spectroscopy of holo-6H7H indicates excitonic coupling between heme groups. Further electrochemical and spectroscopic characterization of the holo-protein support a structure that is consistent with the predefined target structure.

Structural and functional study of a conserved region in the uncoupling protein UCP1: the three matrix loops are involved in the control of transport

It has been reported that the region 261-269 of the uncoupling protein from brown adipose tissue mitochondria, UCP1, has an important role in the control of its proton translocating activity. Thus the deletion of residues Phe267-Lys268-Gly269 leads to the loss of the nucleotide regulation of the protein, while the complete deletion of the segment leads to the formation of a pore. The region displays sequence homology with the DNA-binding domain of the estrogen receptor. The present report analyzes the structure, by NMR and circular dichroism, of a 20 amino acid residue peptide containing the residues of interest. We demonstrate that residues 263-268 adopt an alpha-helical structure. The helix is at the N-terminal end of the sixth transmembrane domain. The functional significance of this helix has been examined by site-directed mutagenesis of the protein expressed recombinantly in yeasts. Alterations in the structure or orientation of the region leads to an impairment of the regulation, by nucleotides and fatty acids, of the transport activity. UCP1 is one member of the family formed by the carriers of the mitochondrial inner membrane. The family is characterized by a tripartite structure with three repeated segments of about 100 amino acid residues. Two of the mutations have also been performed in the first and second matrix loops and the effect on UCP1 function is very similar. We conclude that the three matrix loops contribute to the formation of the gating domain in UCP1 and propose that they form a hydrophobic pocket that accommodates the purine moiety of the bound nucleotide.

A fluorescence study of single tryptophan-containing mutants of enzyme IImtl of the Escherichia coli phosphoenolpyruvate-dependent mannitol transport system

The fluorescence properties of six different single Trp mutants of the mannitol-specific transporter of Escherichia coli were studied in order to derive structural information at different locations in the enzyme. The use of pure detergent and special protein purification protocols was essential for reliable fluorescence spectra, as judged from tyrosine-like fluorescence in a tryptophan-minus mutant (Robillard et al., 1996). The steady-state fluorescence spectra of EIImtl mutants with single tryptophan residues at positions 30, 42, 109, 117, 320, and 384 provided information concerning the polarity of the environment and the effects of mannitol binding at these positions. Tryptophan positions 42, 109, and 117 with emission maxima ranging from 337 to 340 nm are relatively polar, and position 384 with an emission maximum at 346 nm is highly polar, whereas position 30 is highly apolar with a maximum at 324 nm. The fluorescence characteristics of tryptophan 30 suggest a buried position in a hydrophobic part of the enzyme, which is confirmed by the low Stern-Volmer quenching constant for I- quenching. Positions 109 and 117 show the highest quenching constants, indicating the most exposed positions, whereas positions 320 and 42 are moderately quenched, by I-. The tryptophan residue at position 384 is, even in the absence of externally added quencher, very strongly quenched, possibly by the carboxylate from aspartate 384 or by a tyrosinate at position 458 which is nearby in the folded protein (AB et al., in preparation; van Montfort et al., in preparation). The observed emission maxima and accessibilities of the tryptophans at the different positions are consistent with the predicted topology of the enzyme (Sugiyama et al., 1991). When mannitol is bound to wild-type EIImtl, an increase in fluorescence emission intensity was observed (Wood, 1988) which can now be attributed primarily to increased fluorescence intensity of the tryptophan at position 30.

Fluorescence quenching at interfaces and the permeation of acrylamide and iodide across phospholipid bilayers

Studies of fluorescence quenching in membrane proteins are complicated by the fact that the barrier effect of the bilayer towards the quenchers is not known with precision. Our studies show that (a) both acrylamide and iodide can permeate the membrane at comparable rates, (b) when quenchers are added externally to a vesicle suspension, the apparent Stern-Volmer quenching constants for the same fluorophores are lower in the inner than in the outer aqueous compartments, and (c) at least some non-polar fluorophores embedded in the bilayer are quenched by iodide, but not by acrylamide.