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Beta1PIX, the PAK-interacting exchange factor, requires localization via a coiled-coil region to promote microvillus-like structures and membrane ruffles

Glaxo-IMCB Group, Institute of Molecular and Cell Biology, 30 Medical Drive, Singapore 117609.
Journal of Cell Science (Impact Factor: 5.33). 01/2002; 114(Pt 23):4239-51.
Source: PubMed

ABSTRACT PIX is a Rho-family guanine nucleotide exchange factor that binds PAK. We previously described two isoforms of PIX that differ in their N termini. Here, we report the identification of a new splice variant of betaPIX, designated beta2PIX, that is the dominant species in brain and that lacks the region of approximately 120 residues with predicted coiled-coil structure at the C terminus of beta1PIX. Instead, beta2PIX contains a serine-rich C terminus. To determine whether these splice variants differ in their cellular function, we studied the effect of expressing these proteins in HeLa cells. We found that the coiled-coil region plays a key role in the localization of beta1PIX to the cell periphery and is also responsible for PIX dimerization. Overexpression of beta1, but not beta2PIX, drives formation of membrane ruffles and microvillus-like structures (via activation of Rac1 and Cdc42, respectively), indicating that its function requires localized activation of these GTPases. Thus, beta1PIX, like other RhoGEFs, exerts specific morphological functions that are dependent on its intracellular location and are mediated by its C-terminal dimerization domain.

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    • "Besides the cc-domain, the pleckstrin homology (PH) domain of PIX proteins has also been reported to play a role in intracellular targeting. As the PH domain of aPIX and bPIX share only 72% amino acid homology (Koh et al., 2001), they might be directed to different target sites within the same cell. Importantly, aPIX and bPIX may also form heterodimers via their cc-domains (Rosenberger et al., 2003). "
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    • "Further, Bagrodia et al. (1998) identified βPix (named p85Cool-1) and a smaller alterative splice variant (p50Cool-1) as two proteins that facilitated interactions between PAK and DBL homology (DH) and pleckstrin homology (PH) domains. Finally, Koh et al. (2001) reported an isoform of βPix designated β 2 Pix; that isoform contained a serine-rich region not found in the original βPix protein (which is now designated as β 1 Pix-a, Kim et al., 2000) nor the β 1 Pix-b and β 1 Pix-c isoforms (Oh et al., 1997; Kim et al., 2000). The structure and functional domains of β 1 Pix are provided in Figure 1. "
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    • "β-PIX is a multidomain protein comprised of an SH3 domain, a DH-PH Rac/Cdc42 GEF domain, a GIT1-binding region, and a proline-rich region (Fig. 3) (Rosenberger and Kutsche, 2006). In the brain, two major β-PIX isoforms are expressed that differ only at their C-termini; β1-PIX possesses a coiled-coil dimerization domain and a PDZ-binding motif, whereas β2-PIX contains a serine-rich region (Koh et al., 2001). These two β-PIX isoforms are present at high levels during development, and they continue to be expressed in the adult in restricted brain regions such as the hippocampus and cerebellum (Kim et al., 2000). "
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