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.
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Inheritance of cortical ER protein 2, ICE2, of 491 aas and 10 - 12 TMSs. It may be involved in septin rign assembly during yeast budding, but may also play a role in ER and Zn2+ homeostasis. ICE2 may also play roles in post-translation targeting of proteins to various membranes including the nuclear membrane. Its copy number (~660) in log phase growth is known (Ghaemmaghami et al. 2003). In addition to ICE2, Spa2p-Shs1p interactions may play an important role in cytokinesis (Mino et al. 1998). Fungal Ice2p is in the same superfamily as SERINCs (Alli-Balogun and Levine 2021). Ice2p is an integral endoplasmic reticulum (ER) membrane protein. Ice2p has also been reported to be 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, but it 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. 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. It is a full-length homolog of SERINC (serine incorporator), a family of proteins with 10 TMSs found universally in eukaryotes. SERINCs are potent restriction factors for HIV and other viruses in animals (Alli-Balogun and Levine 2021).
ICE2 of Saccharomyces cerevisiae
Uncharacterized protein of 682 aas and 11 or 12 TMSs in a 4 or 5 + 3 + 1 + 1 + 2 TMS arrangement.
UP of Saccharomycodes ludwigii
Uncharacterized protein of 417 aas and ~ 9 TMSs.
UP of Chaetomium globosum
ICE2 homolog of 426 aas and 10 TMSs in a probabe 3 + 4 + 3 TMS arrangement.
ICE2 of Rhodotorula toruloides
Uncharacterized protein of 418 aas and 10 probable TMSs in a 3 + 4 + 3 TMS arrangement.
UP of Cryptococcus wingfieldii
Uncharacterized protein of 741 aas and ~ 11 TMSs. It is related to ICE2 - an integral ER membrane protein with type-III transmembrane domains.
UP of Melanopsichium pennsylvanicum
Uncharacterized protein of 404 aas and 9 - 11 TMSs.
UP of Diversispora epigaea
Uncharacterized protein of 439 aas and ~ 10 TMSs. This protein may be distantly related to members of the NhaC family (TC# 2.A.35), and therefore may be a secondary carrier. If so, family ICE2 may move to TC subclass 2.A.
UP of Dimargaris cristalligena
Uncharacterized protein of 410 aas and ~11 TMSs, in a possible 7 + 2 + 2 TMS arrangement.
UP of Gryganskiella cystojenkinii
Uncharacterized protein of 380 aas and 10 TMSs.
UP of Paramicrosporidium saccamoebae
Uncharacterized protein with a large N-terminal hydrophilic domain, annotated as a glutamine amidotransferase domain, followed by a hydrophobic domain of 8 TMSs in an apparent 4 + 3 + 1 TMS arrangement. Only the transmembrane domains shows sequence similarity with other members of 8.A.169.
UP of Lentinula edodes (shiitake mushroom)