Identification of the evolutionarily conserved nuclear envelope proteins Lem2 and MicLem2 in Tetrahymena thermophila

Masaaki Iwamoto; Yasuhiro Fukuda; Hiroko Osakada; Chie Mori; Yasushi Hiraoka; Tokuko Haraguchi

doi:10.1016/j.gene.2019.100006

Identification of the evolutionarily conserved nuclear envelope proteins Lem2 and MicLem2 in Tetrahymena thermophila

Gene. 2019:721S:100006. doi: 10.1016/j.gene.2019.100006. Epub 2019 Jan 22.

Authors

Masaaki Iwamoto¹, Yasuhiro Fukuda², Hiroko Osakada¹, Chie Mori¹, Yasushi Hiraoka³, Tokuko Haraguchi⁴

Affiliations

¹ Advanced ICT Research Institute Kobe, National Institute of Information and Communications Technology, Kobe 651-2492, Japan.
² Graduate School of Agricultural Science, Tohoku University, Osaki, 989-6711, Japan.
³ Advanced ICT Research Institute Kobe, National Institute of Information and Communications Technology, Kobe 651-2492, Japan; Graduate School of Frontier Biosciences, Osaka University, Suita 565-0871, Japan.
⁴ Advanced ICT Research Institute Kobe, National Institute of Information and Communications Technology, Kobe 651-2492, Japan; Graduate School of Frontier Biosciences, Osaka University, Suita 565-0871, Japan. Electronic address: tokuko@nict.go.jp.

PMID: 34530986
DOI: 10.1016/j.gene.2019.100006

Abstract

Lem2 family proteins, i.e. the LAP2-Emerin-MAN1 (LEM) domain-containing nuclear envelope proteins, are well-conserved from yeasts to humans, both of which belong to the Opisthokonta supergroup. However, whether their homologs are present in other eukaryotic phylogenies remains unclear. In this study, we identified two Lem2 homolog proteins, which we named as Lem2 and MicLem2, in a ciliate Tetrahymena thermophila belonging to the SAR supergroup. Lem2 was localized to the nuclear envelope of the macronucleus (MAC) and micronucleus (MIC), while MicLem2 was exclusively localized to the nuclear envelope of the MIC. Immunoelectron microscopy revealed that Lem2 in T. thermophila was localized to both the inner and outer nuclear envelopes of the MAC and MIC, while MicLem2 was mostly localized to the nuclear pores of the MIC. Molecular domain analysis using GFP-fused protein showed that the N-terminal and luminal domains, including the transmembrane segments, are responsible for nuclear envelope localization. During sexual reproduction, enrichment of Lem2 occurred in the nuclear envelopes of the MAC and MIC to be degraded, while MicLem2 was enriched in the nuclear envelope of the MIC that escaped degradation. These findings suggest the unique characteristics of Tetrahymena Lem2 proteins. Our findings provide insight into the evolutionary divergence of nuclear envelope proteins.

Keywords: HeH domain; LEM domain; MSC domain; Man1; Man1-Src1p-C-terminal domain; Nuclear dimorphism; Nuclear envelope; Protist.