Structural studies on three Vitis vinifera thaumatin-like proteins and their hazing potential in white wines

White wines generally contain a relatively low concentration of pathogenesis-related (PR) proteins of grape (Vitis vinifera), namely, thaumatin-like proteins (TLPs) and chitinases. These proteins play roles in the defence mechanism of plants against pathogens, and can cause protein haze in white wine unless removed prior to bottling. TLPs are, after the chitinases, the second most abundant proteins found in wines. It has been demonstrated that different isoforms of TLPs have different hazing potential and aggregation behaviour, but an explanation on why this occurs is lacking. In this work we present the elucidation of the molecular structures of three isoforms of grape TLPs purified from Sauvignon blanc grape juice. The three TLPs have very similar structures despite belonging to two different classes (F2/4JRU is a thaumatin-like protein while I/4L5H and H2/4MBT are VVTL1), and having different unfolding temperatures (56 vs. 62°C). Interestingly, protein F2/4JRU was found to be heat unstable thus forming haze, while this was not the case for I/4L5H. From the comparison of the three structures we attributed these differences in properties to the conformation of a single loop and the amino acid composition of its flanking regions. The presence of a disulfide bridge in this loop is probably the key for the unfolding/refolding behaviour of this area, and the fact that white wines are typically produced in reducing conditions and with SO2 added to prevent faults due to oxidation is likely to exacerbate haze formation. The availability of structural information on haze forming proteins could inform research into alternative wine stabilisation solutions as the development of target proteases for the degradation of haze-forming proteins, resulting in an important change in the winemaking stabilization practices.