Prm1 Prevents Contact-Dependent Lysis of Yeast Mating Pairs

Abstract

Membrane fusion requires localized destabilization of two phospholipid bilayers, but unrestrained membrane destabilization could result in lysis. prm1 mutant yeast cells have a defect at the plasma membrane fusion stage of mating that typically results in the accumulation of prezygotes that have fingers of membranebound cytoplasm projecting from one cell of each pair into its mating partner in the direction of the osmotic gradient between the cells. However, some prm1 mating pairs fuse successfully whereas the two cells in other prm1 mating pairs simultaneously lyse. Lysis only occurs if both mating partners are prm1 mutants. Osmotic stabilization does not protect prm1 mating pairs from lysis, indicating that lysis is not caused by a cell wall defect. prm1 mating pairs without functional mitochondria still lyse, ruling out programmed cell death. No excess lysis was found after pheromone treatment of haploid prm1 cells, and lysis did not occur in mating pairs when prm1 was combined with the fus1 and fus2 mutations to block cell wall remodeling. Furthermore, short (<1 m) cytoplasmic microfingers indicating the completion of cell wall remodeling appeared immediately before lysis. In combination, these results demonstrate that plasma membrane contact is a prerequisite for lysis. Cytoplasmic microfingers are unlikely to cause lysis since most prm1 mating pairs with microfingers do not lyse, and microfingers were also detected before fusion in some wild-type mating pairs. The lysis of prm1 mutant mating pairs suggests that the Prm1 protein stabilizes the membrane fusion event of yeast mating.

Membrane Organization and Cell Fusion During Mating in Fission Yeast Requires Multipass Membrane Protein Prm1

“…It has been proposed that calcium would promote a plasma membrane repair mechanism that compensates the lack of Fig1p and Prm1p (Aguilar et al 2007). Although there is a fairly large body of information about S. cerevisiae Prm1p (Heiman and Walter 2000;Jin et al 2004;Aguilar et al 2007;Engel et al 2010; Olmo and Grote 2010a,b), the nature of its function remains unknown.S. cerevisiae and S. pombe are distantly related yeasts (Sipiczki 2000) and hence studies addressing mating in both organisms might help to understand the conserved and divergent functions of proteins involved in this developmental process (Merlini et al 2013).…”

Prm1 Functions as a Disulfide-linked Complex in Yeast Mating

“…We previously proposed that Prm1 coordinates the function of multiple fusion proteins to ensure that membrane merger proceeds to complete fusion rather than lysis (4,6). Dimerization could facilitate this function by allowing the Prm1 complex to interact simultaneously with two binding partners via identical binding sites.…”

“…6 and Table 1). Fusion was detected by monitoring the exchange of cytoplasmic GFP expressed in the MATa cells and cytoplasmic mCherry expressed in the MAT␣ cells (4,17). All single and combinatorial mutants containing C120S and/or C545S had a severe reduction in mating activity, but these mutants still retained some residual function when compared with the empty vector control.…”

“…Cell fusion is normal in WT ϫ prm1⌬ mating pairs. However, when two prm1⌬ mutants are mated, mating pairs assemble and properly remodel their cell walls to allow the two plasma membranes to come into contact, but the plasma membranes either remain in contact without fusing or simultaneously lyse (3,4).…”

“…Mating in the budding yeast Saccharomyces cerevisiae provides a genetically accessible model system to investigate cell fusion (2). Prm1 is a pheromone-regulated membrane glycoprotein with 14 potential N-glycosylation sites that promotes the plasma membrane fusion step of mating (3,4). It is targeted to its expected site of action at contact sites in mating pairs but is also found in intracellular vesicles and vacuoles.…”