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

beta1-chimaerin belongs to the chimaerin family of GTPase-activating proteins (GAPs) and is encoded by the CHN2 gene, which also encodes the beta2- and beta3-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 beta1-chimaerin, beta1-Delta7p-chimaerin, that participates in the regulation of nuclear Rac1. We show that beta1-Delta7p-chimaerin is a truncated variant generated by alternative splicing at a cryptic splice site in exon 7. We found that, unlike other chimaerin isoforms, beta1-Delta7p-chimaerin lacks a functional C1 domain and is not regulated by diacylglycerol. We found that beta1-Delta7p-chimaerin localizes to the nucleus via a nuclear localization signal in its N terminus. We also identified a key nuclear export signal in beta1-chimaerin that is absent in beta1-Delta7p-chimaerin, causing nuclear retention of this truncated variant. Functionally analyses revealed that beta1-Delta7p-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.