Identification of a truncated β1-chimaerin variant that inactivates nuclear Rac1

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.

Rac-GEF/Rac Signaling and Metastatic Dissemination in Lung Cancer

“…As indicated above, the multidomain nature of these exchange factors is key for determining their subcellular localization via specific mechanisms in each case. The recent identification of Rac-GAP-mediated nuclear Rac1 inactivation in lung cancer cells also supports the compartmentalization of the Rac-GDP/Rac-GTP cycle (Casado-Medrano et al, 2020).…”

SmgGDS: An Emerging Master Regulator of Prenylation and Trafficking by Small GTPases in the Ras and Rho Families

“…Several functions of nuclear Rac1 have been described, including controlling nuclear shape ( Navarro-Lerida et al, 2015 ), stimulating rRNA synthesis ( Justilien et al, 2017 ), promoting the cell cycle ( Michaelson et al, 2008 ), inducing neoplastic transformation ( Huff et al, 2013 ), and enhancing malignancy ( Huff et al, 2013 ; Navarro-Lerida et al, 2015 ; Justilien et al, 2017 ). Rac1 is activated in the nucleus by the GEF ECT2 ( Huff et al, 2013 ; Justilien et al, 2017 ), and it is inactivated by a nuclear variant of β1-chimaerin ( Casado-Medrano et al, 2020 ). The binding of prenylated Rac1 to SmgGDS-558 provides a way for prenylated Rac1 to enter the nucleus and participate in these signaling pathways.…”

“…Some small GTPases are known to be activated at sites other than membranes. For example, the Ras family members Rap1A and Rap1B ( Mitra et al, 2003 ; Goto et al, 2010 ; Ntantie et al, 2013 ; Griffin et al, 2018 ) and the Rho family members RhoA ( Dubash et al, 2011 ; Staus et al, 2014 ; Gayle et al, 2015 ) and Rac1 ( Lanning et al, 2003 ; Michaelson et al, 2008 ; Huff et al, 2013 ; Navarro-Lerida et al, 2015 ; Justilien et al, 2017 ; Casado-Medrano et al, 2020 ) can enter the nucleus where they can be activated by nuclear GEFs and participate in nuclear signaling pathways. These findings suggest that prenylation that promotes membrane anchoring is not always required for Ras and Rho family members to become activated.…”