Lem2 and Lnp1 maintain the membrane boundary between the nuclear envelope and endoplasmic reticulum

Abstract

The nuclear envelope (NE) continues to the endoplasmic reticulum (ER). Proper partitioning of NE and ER is crucial for cellular activity, but the key factors maintaining the boundary between NE and ER remain to be elucidated. Here we show that the conserved membrane proteins Lem2 and Lnp1 cooperatively play a crucial role in maintaining the NE-ER membrane boundary in fission yeast Schizosaccharomyces pombe. Cells lacking both Lem2 and Lnp1 caused severe growth defects associated with aberrant expansion of the NE/ER membranes, abnormal leakage of nuclear proteins, and abnormal formation of vacuolar-like structures in the nucleus. Overexpression of the ER membrane protein Apq12 rescued the growth defect associated with membrane disorder caused by the loss of Lem2 and Lnp1. Genetic analysis showed that Apq12 had overlapping functions with Lnp1. We propose that a membrane protein network with Lem2 and Lnp1 acts as a critical factor to maintain the NE-ER boundary.

High-throughput identification of nuclear envelope protein interactions inSchizosaccharomyces pombeusing an arrayed membrane yeast-two hybrid library

“…Lastly, we examined the Lem2 interacting prey for factors potentially associated with NPC quality control at the NE. In S. pombe , Lem2 is required for maintaining NE morphology and regulating membrane flow from the ER into the NE to control nuclear size (47–49). However, a direct role for Lem2 in NPC quality control has not been shown in S. pombe , and Lem2 is not known to interact directly with any components of the NPC or the ESCRT (endosomal sorting complexes required for transport) machinery that facilitate NE/NPC quality control in other organisms.…”

Nuclear Envelope Proteins Modulating the Heterochromatin Formation and Functions in Fission Yeast

“…Lnp1 localizes at the three-way junction of tubular ER network in humans and S. cerevisiae [137][138][139][140][141]. Double deletion of Lem2 and Lnp1 disturbs the partitioning between NE and ER, and consequently causes severe membrane disorder and growth defects [142]. Moreover, Lem2 and Lnp1 act as barriers to the membrane flow between the ER and Golgi and contribute to the control of the nuclear size [143].…”

“…Loss of Lem2, in combination with loss of Bqt4 or Lnp1, causes severe nuclear protein leakage owing to NE holes formed [142,144]. Recent studies indicate that Lem2 seals an NE hole in cooperation with Cmp7 (homologue of human CHMP7) and endosomal sorting complex required for transport-III (ESCRT-III) [148][149][150].…”

“…Thus, an intriguing possibility is that FG repeat proteins and Swi6, which also form liquid droplets [80], create a peripheral compartment through phase separation, which is required for the silencing of heterochromatin. Recently, other interesting studies have demonstrated that lipid metabolism is involved in modulating heterochromatin states and chromatin functions [142,164]. The involvement of the lipid components of NE and ER in chromatin functions need to be elucidated.…”

High-Throughput Identification of Nuclear Envelope Protein Interactions inSchizosaccharomyces pombeUsing an Arrayed Membrane Yeast-Two Hybrid Library

“…Unlike Ima1 and Cut11, which transiently localize to the SPB during mitosis, Lem2 is found at the SPB throughout interphase and it is present at the INM during the entire yeast cell cycle (Hiraoka et al 2011). Lem2 and Man1 play at least two roles at the INM: heterochromatin tethering (Steglich et al 2012;Banday et al 2016;Barrales et al 2016;Tange et al 2016;Pieper et al 2020) and regulation of NE composition, integrity and structure (Hiraoka et al 2011;Gonzalez et al 2012;Gu et al 2017;Yang et al 2017;Kume et al 2019;Hirano et al 2020).…”