Exportin Crm1 is important for Swi6 nuclear shuttling and MBF transcription activation in Saccharomyces cerevisiae

Kenneth D Belanger,Naveen Hussain,Karyn G Belanger,Amy N Russo,William T Yewdell,Matthew F Barber,Susan J Geier,Emily K Damuth,Mark A Pettit

doi:10.1186/s12860-022-00409-6

Abstract

BackgroundSwi6 acts as a transcription factor in budding yeast, functioning in two different heterodimeric complexes, SBF and MBF, that activate the expression of distinct but overlapping sets of genes. Swi6 undergoes regulated changes in nucleocytoplasmic localization throughout the cell cycle that correlate with changes in gene expression. This study investigates how nucleocytoplasmic transport by multiple transport factors may influence specific Swi6 activities.ResultsHere we show that the exportin Crm1 is important for Swi6 nuclear export and activity. Loss of a putative Crm1 NES or inhibition of Crm1 activity results in changes in nucleocytoplasmic Swi6 localization. Alteration of the Crm1 NES in Swi6 results in decreased MBF-mediated gene expression, but does not affect SBF reporter expression, suggesting that export of Swi6 by Crm1 regulates a subset of Swi6 transcription activation activity. Finally, alteration of the putative Crm1 NES in Swi6 results in cells that are larger than wild type, and this increase in cell size is exacerbated by deletion of Msn5.ConclusionsThese data provide evidence that Swi6 has at least two different exportins, Crm1 and Msn5, each of which interacts with a distinct nuclear export signal. We identify a putative nuclear export signal for Crm1 within Swi6, and observe that export by Crm1 or Msn5 independently influences Swi6-regulated expression of a different subset of Swi6-controlled genes. These findings provide new insights into the complex regulation of Swi6 transcription activation activity and the role of nucleocytoplasmic shuttling in regulated gene expression.

Highlights

Swi6 acts as a transcription factor in budding yeast, functioning in two different heterodimeric complexes, SBF and MBF, that activate the expression of distinct but overlapping sets of genes
This nuclear accumulation decreases during G2 and M-phase and the protein becomes largely cytoplasmic [41]. While these observations suggest that Swi6 protein shuttles in and out of the nucleus in a cell cycle-dependent manner, such changes in localization could be observed due to changes in nuclear import rates combined with localized Swi6 protein degradation
We identify a putative nuclear export signal for Crm1 within Swi6 that is necessary for efficient nuclear export in a cell cycledependent pattern

Summary

Introduction

Swi acts as a transcription factor in budding yeast, functioning in two different heterodimeric complexes, SBF and MBF, that activate the expression of distinct but overlapping sets of genes. Of the hundreds of yeast genes that undergo changes in expression during the G 1-S transition, the vast majority fall at least partially under the regulation of two heterodimeric transcription factors, MBF and SBF [14]. MBF (MluI cell-cycle box binding factor) and SBF (Swi4/6 cell-cycle box binding factor) are both composed of Swi plus one other protein, Mbp and Swi, respectively ([15, 16], reviewed in [17]) In their respective complexes with Swi, Swi and Mbp function as the sequence-specific DNA binding component, while Swi itself functions as the transcription activating component [16, 18, 19]. SBF and MBF regulate the expression of distinct but overlapping sets of genes with MBF primarily controlling genes involved in DNA replication and repair while SBF upregulates genes involved in cell growth, cell wall organization, and the timing of progression through START ([14, 20]; reviewed in [21])

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Conclusion