Title: Identification of a truncated β1-chimaerin variant that inactivates nuclear Rac1.
Authors: Casado-Medrano, Victoria; Barrio-Real, Laura; Gutiérrez-Miranda, Laura; González-Sarmiento, Rogelio; Velasco, Eladio A; Kazanietz, Marcelo G; Caloca, María J
Published In J Biol Chem, (2020 01 31)
Abstract: β1-chimaerin belongs to the chimaerin family of GTPase-activating proteins (GAPs) and is encoded by the CHN2 gene, which also encodes the β2- and β3-chimaerin isoforms. All chimaerin isoforms have a C1 domain that binds diacylglycerol as well as tumor-promoting phorbol esters and a catalytic GAP domain that inactivates the small GTPase Rac. Nuclear Rac has emerged as a key regulator of various cell functions, including cell division, and has a pathological role by promoting tumorigenesis and metastasis. However, how nuclear Rac is regulated has not been fully addressed. Here, using several approaches, including siRNA-mediated gene silencing, confocal microscopy, and subcellular fractionation, we identified a nuclear variant of β1-chimaerin, β1-Δ7p-chimaerin, that participates in the regulation of nuclear Rac1. We show that β1-Δ7p-chimaerin is a truncated variant generated by alternative splicing at a cryptic splice site in exon 7. We found that, unlike other chimaerin isoforms, β1-Δ7p-chimaerin lacks a functional C1 domain and is not regulated by diacylglycerol. We found that β1-Δ7p-chimaerin localizes to the nucleus via a nuclear localization signal in its N terminus. We also identified a key nuclear export signal in β1-chimaerin that is absent in β1-Δ7p-chimaerin, causing nuclear retention of this truncated variant. Functionally analyses revealed that β1-Δ7p-chimaerin inactivates nuclear Rac and negatively regulates the cell cycle. Our results provide important insights into the diversity of chimaerin Rac-GAP regulation and function and highlight a potential mechanism of nuclear Rac inactivation that may play significant roles in pathologies such as cancer.
PubMed ID: 31871052
MeSH Terms: Alternative Splicing; Amino Acid Motifs/genetics; Animals; COS Cells; Cell Cycle/genetics; Cell Line, Tumor; Cell Nucleus/metabolism*; Chimerin Proteins/genetics*; Chimerin Proteins/metabolism*; Chlorocebus aethiops; Diglycerides/metabolism; Exons/genetics; Gene Silencing; Humans; Protein Domains/genetics; Protein Isoforms/metabolism; RNA, Small Interfering; Sequence Deletion; rac1 GTP-Binding Protein/genetics; rac1 GTP-Binding Protein/metabolism*