Awa1 and hpf1

by Vivienne Baillie Gerritsen

Share it on Twitter, Facebook or Google+

When we raise a glass of wine, rarely do we give a thought to what has been involved in its making. Yet a wine's hue, its taste, its aroma, its sparkle and even the nature of its haze are given the same attention a mother would to her newborn. Many of the qualities of a wine are the doings not only of proteins inherent to the grapes, rice or any other product used to make it, but also to proteins which belong to yeast strains that are added for fermentation, and hence the production of alcohol. Two such proteins are AWA1 and HPF1. AWA1 is involved in the production of foam as the Japanese rice wine - sake - is brewed, and HPF1 in the production of haze in white wine. And both proteins belong to the cell wall of different strains of Saccharomyces cerevisiae.

In the process of brewing sake, fermentation produces huge amounts of foam - the level of which is used as an indicator of the fermenting progress. Brewing, however, could be made an easier task if there was less froth, so researchers set out to find out what was making it in the first place. It turned out that the culprit is a protein that has been baptised AWA1, from 'awa' meaning 'froth' in Japanese. It is thought that one end of AWA1 sticks out of the yeast cell wall and goes fishing for the carbon dioxide bubbles which are a direct product of fermentation, thus forming the froth.

Likewise, in the making of grape wine, haze frequently appears. Wines with no haze are generally regarded as wines of greater quality than those that are turbid. Consequently, the level of haze has become an indicator of quality. In grape wine, haze is caused by the aggregation of unknown wine components and grape proteins. Certain strains of S. cerevisiae actually decrease haze thanks to haze protective factors, or HPFs. In particular, HPF1 reduces haze by preventing the grape proteins from forming large aggregates with wine components. As a result, the wine is less turbid.

Haze and froth are two qualities that have great commercial significance. Besides using AWA1 to identify different sake yeast strains in order to select those that make less froth, strains that are good at producing it can be introduced into the process of brewing beer where froth spells excellence. Similarly, haze protective factors such as HPF1 could replace current methods that are used to get rid of haze but which frequently modify characteristics that lessen the quality of the wine, by removing aroma components. Certainly, yeast has spent many a year with humans, not only for the benefit of their senses but also for that of social cohesion.

UniProt cross references
Cell wall protein AWA1, Saccharomyces cerevisiae (Baker's yeast), Q8TGE1
Haze protective factor 1, Saccharomyces cerevisiae (Baker's yeast), Q05164

Also read the Protein Spotlight article: Of froth and haze
