Lte1 contributes to Bfa1 localization rather than stimulating nucleotide exchange by Tem1

Monitoring spindle orientation: Spindle position checkpoint in charge

Every cell division in budding yeast is inherently asymmetric and counts on the correct positioning of the mitotic spindle along the mother-daughter polarity axis for faithful chromosome segregation. A surveillance mechanism named the spindle position checkpoint (SPOC), monitors the orientation of the mitotic spindle and prevents cells from exiting mitosis when the spindle fails to align along the mother-daughter axis. SPOC is essential for maintenance of ploidy in budding yeast and similar mechanisms might exist in higher eukaryotes to ensure faithful asymmetric cell division. Here, we review the current model of SPOC activation and highlight the importance of protein localization and phosphorylation for SPOC function.

The spindle position checkpoint is coordinated by the Elm1 kinase

Localization and activation of Elm1 at the bud neck coordinates SPC activity with mother–daughter polarity during cell division.

How dividing cells monitor the effective transmission of genomes during mitosis is poorly understood. Budding yeast use a signaling pathway known as the spindle position checkpoint (SPC) to ensure the arrival of one end of the mitotic spindle in the nascent daughter cell. An important question is how SPC activity is coordinated with mother–daughter polarity. We sought to identify factors at the bud neck, the junction between mother and bud, which contribute to checkpoint signaling. In this paper, we show that the protein kinase Elm1 is an obligate regulator of the SPC, and this function requires localization of Elm1 to the bud neck. Furthermore, we show that Elm1 promotes the activity of the checkpoint kinase Kin4. These findings reveal a novel function for Elm1 in the SPC and suggest how checkpoint activity may be linked to cellular organization.

Antagonistic roles of PP2A-Pab1 and Etd1 in the control of cytokinesis in fission yeast

In Schizosaccharomyces pombe, Etd1 is a positive regulator of the septation initiation network (SIN), a conserved GTPase-regulated kinase cascade that triggers cytokinesis. Here we show that a mutation in the pab1 gene, which encodes the B-regulatory subunit of the protein phosphatase 2A (PP2A), suppresses mutations in the etd1 gene. Etd1 is required for the function of the GTPase Spg1, a key regulator of SIN signaling. Interestingly, the loss of Pab1 function restored the activity of Spg1 in Etd1-deficient cells. This result suggests that PP2A-Pab1-mediated dephosphorylation inhibits Spg1, thus antagonizing Etd1 function. The loss of pab1 function also rescues the lethality of mutants of other genes in the SIN cascade such as mob1, sid1, and cdc11. Two-hybrid assays indicate that Pab1 physically interacts with Mob1, Sid1, Sid2, and Cdc11, suggesting that the phosphatase 2A B-subunit is a component of the SIN complex. Together, our results indicate that PP2A-Pab1 plays a novel role in cytokinesis, regulating SIN activity at different levels. Pab1 is also required to activate polarized cell growth. Thus, PP2A-Pab1 may be involved in coordinating polar growth and cytokinesis.

Spindle position is coordinated with cell-cycle progression through establishment of mitotic exit-activating and -inhibitory zones

How spatial information is translated into a chemical signal is a fundamental problem in all organisms. The spindle position checkpoint is a prime example of this problem. This checkpoint senses spindle position and, in budding yeast, inhibits the mitotic exit network (MEN), a signaling pathway that promotes exit from mitosis. We find that spindle position is sensed by a system composed of MEN-inhibitory and -activating zones and a sensor that moves between them. The MEN inhibitory zone is located in the mother cell, the MEN-activating zone in the bud, and the spindle pole body (SPB), where the components of the MEN reside, functions as the sensor. Only when an SPB escapes the MEN inhibitor Kin4 in the mother cell and moves into the bud where the MEN activator Lte1 resides can exit from mitosis occur. In this manner, spatial information is sensed and translated into a chemical signal.