Nuclear import of RNA polymerase II requires the conserved GPN‐loop GTPase XAB1/GPN1

Abstract

Over the past 3 decades, many efforts have been made to identify and characterize the factors that regulate the activity of RNA polymerase II (RNAPII), the enzyme that synthesizes all the mRNA and many small nuclear RNA in eukaryotes. Quite surprisingly, very little is known about the cell machinery that regulates the fate of RNAPII before or after transcription. In this study, we report that XAB1/GPN1, a member of a unique class of GTPases containing a conserved GPN‐loop, is involved in the biogenesis of RNAPII by regulating its nuclear import. XAB1/GPN1 is physically associated with RNAPII in the soluble cell fraction and is essential for cell growth. Accumulation of RNAPII in the cytoplasm of the cell was induced upon (i) depletion of XAB1/GPN1 by siRNA silencing, (ii) overexpression of a XAB1‐GFP dominant negative fusion protein, (iii) mutation of the GTP‐binding or GPN motifs of the XAB1/GPN1 homologue in yeast, NPA3, and (iv) treatment of cells with leptomycin B, an inhibitor of XPO1/CRM1‐dependent nuclear export. Yeast strains with mutations in the NPA3 gene showed hypersensitivity to benomyl, a specific inhibitor of microtubule assembly, suggesting a role for microtubules in RNAPII biogenesis. Together, these results indicate that XAB1/GPN1 shuttles between the cytoplasm and the nucleus to mediate nuclear import of RNAPII.

Translation initiation factor eIF1A rescues hygromycin B sensitivity caused by deleting the carboxy‐terminal tail in the GPN‐loop GTPase Npa3

“…GTPases play regulatory functions working as molecular switches (GDP-bound "off" and GTP-bound "on") within signaling pathways crucial to regulate diverse biological processes, including cell signaling, cell shape, and vesicular and non-vesicular transport, among others [4]. Npa3 is an essential protein required for the assembly and subsequent nuclear import of RNA polymerase II (RNAPII) [5][6][7][8][9]. The efforts to understand the molecular functions of the GPN proteins, including Npa3, have focused, almost exclusively, on their participation in RNAPII biogenesis and nuclear import [9][10][11][12].…”