Novel Mechanism of the Co-regulation of Nuclear Transport of SmgGDS and Rac1

Abstract

The armadillo protein SmgGDS promotes guanine nucleotide exchange by small GTPases containing a C-terminal polybasic region (PBR), such as Rac1 and RhoA. Because the PBR resembles a nuclear localization signal (NLS) sequence, we investigated the nuclear transport of SmgGDS with Rac1 or RhoA. We show that the Rac1 PBR has significant NLS activity when it is fused to green fluorescent protein (GFP) or in the context of full-length Rac1. In contrast, the RhoA PBR has very poor NLS activity when it is fused to GFP or in the context of full-length RhoA. The nuclear accumulation of both Rac1 and SmgGDS is enhanced by Rac1 activation and diminished by mutation of the Rac1 PBR. Conversely, SmgGDS nuclear accumulation is diminished by interactions with RhoA. An SmgGDS nuclear export signal sequence that we identified promotes SmgGDS nuclear export. These results suggest that SmgGDS⅐ Rac1 complexes accumulate in the nucleus because the Rac1 PBR has NLS activity and because Rac1 supplies the appropriate GTP-dependent signal. In contrast, SmgGDS⅐RhoA complexes accumulate in the cytoplasm because the RhoA PBR does not have NLS activity. This model may be applicable to other armadillo proteins in addition to SmgGDS, because we demonstrate that activated Rac1 and RhoA also provide stimulatory and inhibitory signals, respectively, for the nuclear accumulation of p120 catenin. These results indicate that small GTPases with a PBR can regulate the nuclear transport of armadillo proteins.Armadillo (ARM) 1 family proteins that contain multiple copies of the ϳ42-amino acid (aa) ARM motif include SmgGDS, p120 catenin (p120ctn), ␤-catenin, plakoglobin, APC, karyopherin ␣ (also known as importin ␣), and several other proteins (reviewed in Refs. 1-3). Nucleocytoplasmic shuttling by many ARM proteins allows them to regulate events in different cellular compartments, including gene transcription and cell adhesion (reviewed in Refs. 2-7). ARM proteins enter the nucleus by different mechanisms (reviewed in Refs. 2-7). Karyopherin ␣ enters the nucleus when it associates with proteins containing a nuclear localization signal (NLS) sequence consisting of a series of adjacent lysines or arginines (reviewed in Ref. 3). The NLS sequence is believed to anchor within the long surface groove formed by the multiple ARM repeats of karyopherin ␣, promoting the nuclear import of both the NLS-containing protein and karyopherin ␣ (3, 5). APC possesses two NLS sequences and may enter the nucleus by associating with karyopherin ␣ or related proteins (4, 7). The mechanisms by which other ARM proteins enter the nucleus are less clear, because some of these proteins neither possess classic NLS sequences, nor have they been reported to associate with NLS-containing proteins.Several ARM proteins interact with the Rho family of small GTPases (8 -15) or with guanine nucleotide exchange factors (GEFs) for these GTPases (16,17). SmgGDS promotes guanine nucleotide exchange by small GTPases containing a C-terminal polybasic region (PBR), which is a series of adjacent l...

Cross-talk between Rac1 GTPase and dysregulated Wnt signaling pathway leads to cellular redistribution of β-catenin and TCF/LEF-mediated transcriptional activation

“…However, a recent report challenges this view, demonstrating the presence of Rac1 in the nuclear compartment of cells (Lanning et al, 2003). Our data support the nuclear localization of Rac1.…”

“…Rac1 was recently identified to have a functional nuclear localization signal sequence present at the Cterminus denoted the polybasic region (PBR), found to enhance the nuclear accumulation of protein complexes containing SmgGDS and Rac1 (Lanning et al, 2003). Nuclear expression of V12Rac1PBRQ mutant was significantly decreased in HCT116 cells (Figure 6).…”

“…Nuclear localization of Rac1 likely occurs via a signal sequence present at its C-terminus denoted the polybasic region (PBR). This region was recently shown to enhance the nuclear accumulation of Rac1-mediated activation of the canonical Wnt Pathway S Esufali and B Bapat protein complexes containing armadillo domain-containing proteins such as SmgGDS and p120-catenin and Rac1 (Lanning et al, 2003). We hypothesized that Rac1 may regulate nuclear accumulation of b-catenin, also an armadillo-domain containing protein, in a similar manner.…”

MicroRNA miR-124 regulates neurite outgrowth during neuronal differentiation

“…Localization of Rac1 to the membrane and cytoplasm is important for Rac1 activity and function in promoting lamellipodia formation [75][76][77]. Nuclear accumulation of Rac1 has been shown to affect stability of Rac1 and eventually lead to Rac1 protein degradation [77,82,83]. Both the disruption of lamellipodia and localization of Rac1 to the nucleus in P19 cells expressing miR-124 suggest that miR-124 may indirectly downregulate the activity of Rac1 in the cytoplasm.…”

Rho GDP dissociation inhibitor α interacts with estrogen receptor α and influences estrogen responsiveness

“…A careful examination of RhoGDIa amino acid sequence failed to identify any nuclear localization signal, suggesting that another protein may assist in the shuttling of RhoGDIa between the cytoplasm and the nucleus. It seems plausible that cdc42 isoform1, a RhoGTPase family member with a polybasic region in its C-terminal end that can function as a nuclear localization signal (Lanning et al 2003, Williams 2003, might perform this function since we identified both isoforms 1 and 2 of cdc42 in our agarose gel purification experiments as ERa-associated proteins (Schultz-Norton et al 2007). Alternatively, RhoGDIa could be accompanied by a protein such as 14-3-3, which binds and helps to redistribute an array of signaling proteins (Fu et al 2000, Kino et al 2003, Diviani et al 2004.…”