Fungal Ice2p is in the same superfamily as SERINCs, restriction factors for HIV and other viruses

Abstract

Ice2p is an integral endoplasmic reticulum (ER) membrane protein in budding yeast S. cerevisiae named ICE because it is required for Inheritance of Cortical ER. 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. Alternately, Ice2p has been proposed to 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 proteins cytoplasmic face that contains multiple predicted amphipathic helices. Here we carried out a bioinformatic analysis to increase understanding of Ice2p. Firstly, regarding topology, we found that diverse members of the fungal Ice2 family have ten transmembrane helices, which places the long loop on the exofacial face of Ice2p, where it cannot form inter-organelle bridges. Secondly, we identified Ice2 as a full-length homologue of SERINC (serine incorporator), a family of proteins with ten transmembrane helices found universally in eukaryotes. Since SERINCs are potent restriction factors for HIV and other viruses, study of Ice2p may reveal functions or mechanisms that shed light on viral restriction by SERINCs.