High Expression Levels of Cell Wall Protein Cwp2p Decrease the Turbidity of Fresh Lager Beer by Reducing the Size of Haze Particles

Abstract

Cwp2p is one of the major mannoproteins of the yeast cell wall. In this study, the CWP2 gene was placed under the constitutive TDH3 promoter and introduced into a lager production strain. When the obtained CWP2-enhanced strain was subjected to a fermentation trial, the resultant beer was less turbid and contained haze particles that were smaller than those in the control beer with the parental strain. Enhanced expression of CWP2 in the transformant strain resulted in a reduction in the size of haze particles, similar to a case in which the parental strain was propagated aerobically for the fermentation trial. The aerobically prepared parental pitching yeast cells exhibited a higher level of CWP2 mRNA expression during fermentation. These results suggest that the aerobic propagation of pitching yeast exerts a haze-reduction effect by changing the expression profiles of specific cell wall mannoproteins, including Cwp2p. Immunoblot analysis detected the green fluorescent protein-cell wall protein (GFP-Cwp2p) fusion protein in fresh beer when expressed in a lager brewing strain, indicating that a glycosylphosphatidylinositol (GPI)-anchored cell wall protein, such as Cwp2p, can be released into a solution under physiological conditions. The expression level of the aerobiosis-specific mannoproteins (e.g., Cwp2p) appeared to be a key factor in the determination of haze particle size and, accordingly, beer turbidity.