Response of yeast protoplasts to their mating partners.

The mating process between two protoplasts or between a protoplast and a cell in the yeast Saccharomyces cerevisiae was manifested by a specific morphological response of only the cell partner. The cells produced projections, up to 5 micrometers long, to meet their protoplast partners. The protoplasts responded, after a period of nonspecific hernia-like growth, by ceasing to grow and assuming oval or spherical shapes. They never formed mating projections, apparently due to the absence of complete cell walls. Similarly to the cells, nuclear division in protoplasts was arrested and the nucleus migrated towards the plasma membrane at the site of protoplast-cell contact. Cytoplasmic microtubules were directed to this site, indicating the position of the spindle pole body (SPB) on the nucleus adjacent to the plasma membrane. Actin patches accumulated also in this region. These cytological features of the protoplasts were reminiscent of the reorganization of the cytoskeleton and nucleus characteristic of mating cells. This implies that the ability of protoplasts to produce and receive mating signals was unaffected by protoplasting. Fusion, however, was not initiated due to the absence of the complete cell wall in one of the partners. Thus, the cell wall appeared to be necessary for the expression of polarized growth during mating and for cell fusion.