Important Trends in UCP3 Investigation

Abstract

Membrane uncoupling protein 3 (UCP3), a member of the mitochondrial uncoupling protein family, was discovered in 1997. UCP3's properties, such as its high homology to other mitochondrial carriers, especially to UCP2, its short lifetime and low specificity of UCP3 antibodies, have hindered progress in understanding its biological function and transport mechanism over decades. The abundance of UCP3 is highest in murine brown adipose tissue (BAT, 15.0 pmol/mg protein), compared to heart (2.7 pmol/mg protein) and the gastrocnemius muscle (1.7 pmol/mg protein), but it is still 400-fold lower than the abundance of UCP1, a biomarker for BAT. Investigation of UCP3 reconstituted in planar bilayer membranes revealed that it transports protons only when activated by fatty acids (FA). Although purine nucleotides (PN) inhibit UCP3-mediated transport, the molecular mechanism differs from that of UCP1. It remains a conundrum that two homologous proton-transporting proteins exist within the same tissue. Recently, we proposed that UCP3 abundance directly correlates with the degree of FA β-oxidation in cell metabolism. Further development in this field implies that UCP3 may have dual function in transporting substrates, which have yet to be identified, alongside protons. Evaluation of the literature with respect to UCP3 is a complex task because (i) UCP3 features are often extrapolated from its "twin" UCP2 without additional proof, and (ii) the specificity of antibodies against UCP3 used in studies is rarely evaluated. In this review, we primarily focus on recent findings obtained for UCP3 in biological and biomimetic systems.

Keywords: cell metabolism; fatty acid beta-oxidation; mitochondria; proton transport; uncoupling protein expression pattern.

FIGURE 1

Coupling and uncoupling in mitochondria of brown adipose tissue. A section of a brown fat mitochondrion with outer mitochondrial membrane (OMM), intermembrane space (IMS) and cristae of the inner mitochondrial membrane (IMM) is shown. The complexes of the electron transport chain (ETC, beige) shuttle protons (dark blue) across the IMM and create a proton gradient, which conserved energy drives the ATP synthesis by the ATP synthase (F₀F₁, brown) in the cristae. Uncoupling protein 1 (UCP1, light blue), which is largely present in brown fat mitochondria IMM short-circuits the coupling of ECT and F₀F₁ by mediating a proton leak and dissipating the conserved energy as heat. The homologous UCP3 (green) with a similar proton transport activity is also present in the IMM but at much lower amount and its biological function is still unknown.

FIGURE 2

Human UCP3 primary sequence characteristics. (A) Multiple sequence alignment of hUCP1, hUCP3S, hUCP3L, mUCP1, and mUCP3. Amino acid sequences of human UCP1 (NP_068605.1), mouse UCP1 (NP_033489.1), human UCP3 short isoform (NP_073714.1), human UCP3 long isoform, and mouse UCP3 (NP_033490.1) were compared with respect to homology using “Multiple sequence alignment with hierarchical clustering” (Corpet, 1988). Red, dark red and black colored residues indicate homologous, similar and different residues between the proteins, respectively. (B) Simple scheme of the structure of the human UCP3 long isoform based on its homology to ANT and ANT crystallographic structure (Pebay-Peyroula et al., 2003).

FIGURE 3

New concept for the expression of uncoupling proteins.

FIGURE 4

Proton transport mechanisms.

FIGURE 5

Mechanisms of UCP-PN interaction and inhibition. (A) PN inhibition mechanism for UCP1. The α-, β-, and γ-phosphate of PNs bind to R277, R183, and R84, respectively. R84 does not interact with the β-phosphate of diphosphate-PNs. The three P-R bonds additively contribute to maximum inhibition but interact independently. None of them is essential for inhibition or PN binding. (B) Mechanism of UCP3 inhibition by PNs. R184 and R84 bind to the α-and β-phosphate of PNs. Interaction between R184 and the PN α-phosphate is essential for protein inhibition and may induce a conformational change. Interaction of R84 with the β-phosphate increases binding strength. Instead of R278, another residue is proposed to be a part of the UCP3 PN-binding-pocket, which binds to the γ-phosphate of PN.

FIGURE 6

Dual function of mitochondrial carrier proteins.