8.A.169. The Fungal Inheritance of Cortical ER protein 2 (ICE2) Family
ICE2 homologs are integral membrane proteins of ~ 500 aas and ~12 TMSs. They may be involved in cytokinesis that assemble early in the cell cycle as a patch at the incipient bud site of yeast and form a ring approximate 15 min before bud emergence, which transforms into an hour-glass shaped collar of cortical filaments that spans both sides of the mother-bud neck (Mino et al. 1998). This collar persists until just before cytokinesis, when it splits into two rings that occupy opposite sides of the neck. Septins at the bud neck serve as a structural scaffold that recruits different components involved in diverse processes at specific stages during the cell cycle. Many proteins bind asymmetrically to the septin collar. Septin assembly is regulated by protein kinases GIN4 and/or CLA4 and may act by recruiting MYO1 and HOF1, involved in septation, to the site of cleavage. Septins are also involved in cell morphogenesis, bud site selection, chitin deposition, cell cycle regulation, cell compartmentalization and spore wall formation (Mino et al. 1998).
Ice2p is an integral endoplasmic reticulum (ER) membrane protein in budding yeast, S. cerevisiae, named ICE because it is required for Inheritance of the cortical ER. Ice2p is involved in an ER metabolic branch-point that regulates the flux of lipid either to be stored in lipid droplets or to be used as membrane components (Alli-Balogun and Levine 2021). Alternatively, Ice2p may act as a tether that physically bridges the ER at contact sites with both lipid droplets and the plasma membrane via a long loop on the protein's cytoplasmic face that contains multiple predicted amphipathic helices. Alli-Balogun and Levine 2021 carried out bioinformatic analyses revealing that diverse members of the fungal Ice2 family have 10 TMSs, which places the long loop on the exofacial face of Ice2p, where it cannot form inter-organelle bridges. Ice2p is a full-length homolog of SERINC (serine incorporator), a family of proteins with 10 TMSs found universally in eukaryotes. Since SERINCs are potent restriction factors for HIV and other viruses, the study of Ice2p may reveal functions or mechanisms that shed light on viral restriction by SERINCs.