Formation and maintenance of the double septin ring depend on the anillin homologue Bud4 and the Rho-GEF Bud3 (
Figure 2) [
19,
93,
206,
207,
208], which had been previously implicated in the axial budding pattern of haploid yeast cells [
101,
207,
209]. In the absence of Bud3 and Bud4 the septin collar disassembles completely at mitotic exit, without any obvious consequence on the kinetics of AMR constriction and cytokinesis [
206], in line with the notion that septin clearance after AMR assembly is not detrimental for cytokinesis. Both Bud3 and Bud4 localise as a double ring at the edges of the septin collar during mitosis and then remain associated with the split septin rings, suggesting that these proteins act as spatial cues to pre-pattern the septin double ring [
100,
101,
206,
207,
210,
211].
Septin ring splitting requires the activity of the Mitotic Exit Network (MEN;
Figure 2), a Hippo-like pathway triggered by a top GTPase (Tem1) that turns on two sequentially acting protein kinases (Cdc15 and Mob1-Dbf2) to ultimately promote the activation of the Cdc14 phosphatase (reviewed in [
212]). In turn, Cdc14 dephosphorylates and activates its upstream kinases Cdc15 and Mob1-Dbf2 in a positive feedback loop [
213,
214]. Cdc14 is the main CDK-counteracting phosphatase in budding yeast and its function, together with that of its upstream MEN regulators, is essential for mitotic exit and cytokinesis [
215,
216,
217,
218,
219,
220]. Mitotic exit elicits several processes, such as spindle disassembly and licensing of replication origins, and is an essential prerequisite for cytokinesis through dephosphorylation of specific mitotic CDK substrates [
221]. However, MEN promotes septin ring splitting also independently of its role in mitotic exit [
95], suggesting that one or more MEN targets prompt septin remodelling at cytokinesis. We have recently identified the cytokinetic protein Hof1 as one of such septin ring splitting regulators (
Figure 2) [
222]. Hof1 is involved in cytokinesis and is an established target of MEN [
112,
127,
223,
224]. It plays a major role in the control of actin polymerisation and bundling, partly by modulating formin activity [
225,
226,
227,
228,
229,
230]. Consistently, it has been implicated in polarised growth [
228,
231]. However, the cytokinetic function of Hof1 was mainly linked to activation of the chitin synthase Chs2, and
CHS2 overexpression or gain of function mutations can suppress the lethality of
hof1Δ mutants at high temperatures [
126,
232]. Our recent data indicate that
hof1 mutants also display defects in septin remodelling at cytokinesis [
222]. During mitosis, Hof1 associates with septins, forming two closely spaced rings at the edges of the septin collar [
133,
222,
223,
233].
In vitro, it can induce the formation of intertwined septin bundles, suggesting that during this cell cycle stage it may contribute to the robustness of the septin hourglass [
222,
229]. However, shortly before septin ring splitting Hof1 is displaced from the septin collar and relocates to the AMR, where it partially constricts alongside it [
133,
222,
223,
233]. This rapid translocation from septins to the AMR is triggered by Hof1 phosphorylation, primarily by the MEN kinase Dbf2, which disengages Hof1 from the septins allowing it to join the AMR [
222,
223,
224]. Phospho-mimicking
HOF1 mutant alleles can bypass the septin reorganisation defects seen in MEN mutants by displacing Hof1 from septins and enhancing its translocation to the AMR. Importantly, septin remodelling by Hof1 depends on its membrane-binding F-BAR domain, suggesting that a local membrane remodeling could underlie septin disassembly from the cleavage site and remodelling into the double ring [
222]. Since BAR domains can induce membrane curvature [
234,
235], Hof1 may bend the membrane at the division site to a radius that causes septin disassembly. Another possibility, stemming from the ability of BAR domains to cluster phosphoinositides [
236,
237], is that Hof1 may promote septin ring splitting by modifying the local composition of the plasma membrane. The non-essentiality of
HOF1, especially in some strain background, suggests that additional, as yet unidentified proteins participate to septin ring splitting alongside Hof1.