Functional analysis of the methyltransferase SMYD in the single-cell model organism Tetrahymena thermophila

Lysine methylation of histones and non-histones plays a pivotal role in diverse cellular processes.The SMYD(SET and MYND domain)family methyltransferases can methylate various histone and non-histone substrates in mammalian systems,implicated in HSP90 methylation,myofilament organization,cancer inhibition,and gene transcription regulation.To resolve controversies concerning SMYD's substrates and functions,we studied SMYD1(TTHERM_00578660),the only homologue of SMYD in the unicellular eukaryote Tetrahymena thermophila.We epitope-tagged SMYD1,and analyzed its localization and interactome.We also characterized △SMYD1 cells,focusing on the replication and transcription phenotype.Our results show that:(1)SMYD1 is present in both cytoplasm and transcriptionally active macronucleus and shuttles between cytoplasm and macronucleus,suggesting its potential association with both histone and non-histone substrates;(2)SMYD1 is involved in DNA replication and regulates transcription of metabolism-related genes;(3)HSP90 is a potential substrate for SMYD1 and it may regulate target selection of HSP90,leading to pleiotropic effects in both the cytoplasm and the nucleus.