YEAST FLOCCULATION - LECTIN SYNTHESIS AND ACTIVATION

Yeast flocculation involves binding of surface lectins to carbohydrate receptors on neighbouring cell walls. Brewing strains of Saccharomyce s cerevisiae normally become flocculent in the stationary phase of gro wth. This paper presents evidence that lectins are synthesized in expo nential phase, inserted into the cell wall, and activated later at the time of flocculation onset. Cycloheximide failed to prevent flocculat ion unless it was added in early growth; with later additions progessi vely larger degrees of flocculation occurred. Flocculation onset was d elayed by cycloheximide but was otherwise cycloheximide insensitive. P reflocculent cells could be artificially activated to full flocculatio n by heat. Artificial activation of samples from growing yeast culture s confirmed the progressive synthesis of lectins throughout exponentia l growth. Pronase E treatment of whole cells prior to heating prevente d any activation of flocculation. It was concluded that lectins were s ynthesized continuously from an early stage of growth and rapidly inse rted into the cell wall (accessible by pronase E), where they remained inactive for up to 14 h, before being activated at flocculation onset by an as-yet unknown mechanism. It was found that lectin synthesis an d activation occurred in all brewing strains tested.