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.
Highlights
1-chimaerin belongs to the chimaerin family of GTPase-activating proteins (GAPs) and is encoded by the CHN2 gene, which encodes the 2- and 3-chimaerin isoforms
Chimaerins are a family of GTPase-activating proteins (GAPs)6 that negatively regulate Rac, a small GTPase that plays important roles in control of cell morphology and locomotion in normal and cancer cells. 1-chimaerin is a member of the chimaerin family that is coded by CHN2, a gene that codes for 2-chimaerin and the recently identified 3-chimaerin isoform [1,2,3]
1-chimaerin is one of the two main transcripts encoded by the CHN2 gene, and it is generated by an alternative transcription start site located upstream of exon 7 [1, 2] (Fig. 1A)
Summary
1-chimaerin belongs to the chimaerin family of GTPase-activating proteins (GAPs) and is encoded by the CHN2 gene, which encodes the 2- and 3-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. Chimaerins are a family of GTPase-activating proteins (GAPs) that negatively regulate Rac, a small GTPase that plays important roles in control of cell morphology and locomotion in normal and cancer cells. A 1-chimaerin GAP variant regulates nuclear Rac and lipids that result in Rac activation at discrete cellular locations in response to specific signaling inputs [16, 17]. We demonstrated that 1-⌬7p-chimaerin acts as a negative regulator of Rac in this cell compartment and identified a functional role of this chimaerin variant in regulation of the cell cycle
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