Academic journal article Genetics

Kel1p Mediates Yeast Cell Fusion through a Fus2p- and Cdc42p-Dependent Mechanism

Academic journal article Genetics

Kel1p Mediates Yeast Cell Fusion through a Fus2p- and Cdc42p-Dependent Mechanism

Article excerpt

CELL fusion is an essential and ubiquitous process in eukaryotic organisms, with many examples of cell fusion events throughout embryogenesis and development. In mammals, these include sperm-egg fusion during fertilization (Wassarman and Litscher 2008), placental trophoblast fusion during pregnancy (Huppertz and Borges 2008), and the fusion of myoblasts to form myofibers during skeletal muscle development (Kim et al. 2015). Blocks in placental trophoblast fusion have been correlated with preeclampsia during pregnancy (Gauster et al. 2009). Despite the importance of these events, little is known about the molecular mechanisms that control cell fusion.

During mating of the budding yeast, Saccharomyces cerevisiae, two haploid cells of opposite mating types fuse to form a diploid zygote, making this organism an excellent model to study cell fusion (Ydenberg and Rose 2008; Merlini et al. 2013). The yeast mating pathway begins with pheromone recognition and subsequent activation of a wellcharacterized kinase cascade. The results of the cascade are the activation of mating-specific genes, cell-cycle arrest, and polarized growth along the pheromone gradient toward the mating partner. The formation of the mating projection causes the cell to become pear shaped, a form commonly called a "shmoo," and the tip comes into contact with its partner cell, forming the zone of cell fusion. As such, the shmoo tip constitutes a localization hub for many proteins necessary for cell fusion (Ydenberg and Rose 2008).

Genetic studies have identified four shmoo-tip-localized proteins (Fus1p, Fus2p, Rvs161p, and Prm1p) likely to play direct roles in the fusion pathway. FUS1, FUS2, and PRM1 are all pheromone-induced genes and are required in at least one of two mating cells to produce a diploid. Rvs161p is a BAR domain protein related to amphiphysin that plays a role in endocytosis in mitotic cells by stabilizing curved membranes (Crouzet et al. 1991; Friesen et al. 2006). Therefore, RVS161 is expressed in mitotic cells, but strongly induced by pheromone (Brizzio et al. 1998). Fus2p and Rvs161p form a complex, which is transported to the shmoo tip in an actin- and Myo2p-dependent manner (Brizzio et al. 1998; Paterson et al. 2008; Sheltzer and Rose 2009), and anchored at the cortex in a mechanism requiring both actin and Fus1p (Paterson et al. 2008). Mutations in FUS1, FUS2, and RVS161 block the removal of cell wall material between two mating cells (Trueheart et al. 1987; Trueheart and Fink 1989; Brizzio et al. 1998). Deletion of both FUS1 and FUS2 causes a more severe mating phenotype than either single deletion alone, suggesting that FUS1 and FUS2 have some nonoverlapping functions (Trueheart et al. 1987; Gammie et al. 1998). Prm1p acts after cell wall removal, to facilitate plasma membrane fusion (Heiman and Walter 2000).

One of the key morphological events observed before fusion occurs is the clustering of vesicles at the center of the zone of cell fusion. These vesicles are smaller than mitotic vesicles, suggesting that they are mating specific (Baba et al. 1989). The vesicles remain closely associated with the residual cell wall after fusion (Gammie et al. 1998). Cells lacking FUS1 fail to localize the vesicles to the center of the zone of cell fusion, while fus2 and rvs161 mutants localize the vesicles normally, suggesting that the Fus2p-Rvs161p complex acts after vesicle clustering, and thus later than Fus1p (Trueheart et al. 1987; Gammie et al. 1998). Fus2p is thought to regulate the fusion of the vesicles to the plasma membrane, releasing hydrolases that break down the cell wall between mating cells (Gammie et al. 1998; Paterson et al. 2008).

Fus2p contains a Dbl-homology domain, similar to Rho-type GTP exchange factors (GEFs). The Dbl-homology domain is required for Fus2p function, suggesting that Fus2p acts with a Rho-GTPase. In support of this hypothesis, Fus2p binds to Cdc42p, and alleles of cdc42 defective for Fus2p binding exhibit a cell fusion defect (Ydenberg et al. …

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