PX‐RICS, a novel splicing variant of RICS, is a main isoform expressed during neural development

Abstract

In our previous study, we identified RICS, a novel β β β β -catenin-interacting protein with the GAP activity toward Cdc42 and Rac1, and found that RICS plays an important role in the regulation of neural functions, including postsynaptic NMDA signaling and neurite outgrowth. Here we report the characterization of an N-terminal splicing variant of RICS, termed PX-RICS, which has additional phox homology (PX) and src homology 3 (SH3) domains in its N-terminal region. The PX domain of PX-RICS interacted specifically with phosphatidylinositol 3-phosphate [PtdIns(3)P], PtdIns(4)P and PtdIns(5)P. Consistent with this binding affinity, PX-RICS was found to be localized at the endoplasmic reticulum (ER), Golgi and endosomes. We also found that wild-type PX-RICS possessed much lower GAP activity than RICS, whereas a mutant form of PX-RICS whose PX domain lacks the binding ability to phosphoinositides (PIs) exhibited the GAP activity comparable to that of RICS. However, PX-RICS and RICS exhibited similar inhibitory effects on neurite elongation of Neuro-2a cells. Furthermore, we demonstrate that PX-RICS is a main isoform expressed during neural development. Our results suggest that PX-RICS is involved in early brain development including extension of axons and dendrites, and postnatal remodeling and fine-tuning of neural circuits.

The PX-RICS-14-3-3ζ/θ Complex Couples N-cadherin-β-Catenin with Dynein-Dynactin to Mediate Its Export from the Endoplasmic Reticulum

“…Antibodies-Rabbit polyclonal antibody to PX-RICS was generated as described previously (15). The commercially available antibodies used in this study were as follows.…”

“…RICS is expressed predominantly in neurons of the brain and localized to the growth cone and postsynaptic density, where it regulates neurite extension and presumably N-methyl-D-aspartate signaling (14). In contrast, PX-RICS is expressed in a wide variety of tissues and cell lines and localized to the ER and cis-Golgi (13,15). Our recent study has revealed that PX-RICS facilitates ER-to-Golgi transport of the N-cadherin-␤-catenin complex through its direct interaction with ␤-catenin, Cdc42, ␥-aminobutyric acid type A receptor-associated protein (GABARAP) and phosphatidylinositol 4-phosphate (PI4P) (13).…”

“…Our previous results suggest that PX-RICS-mediated ER-toGolgi transport of the N-cadherin-␤-catenin complex is dependent on multiple protein-protein (PX-RICS-GABARAP, -Cdc42, and -␤-catenin) and protein-lipid (PX-RICS-PI4P) interactions (13). The PX domain of PX-RICS has the highest binding affinity for PI4P (15), which is a predominant phosphoinositide in the ER and Golgi membranes and plays important roles in vesicular budding and/or fusion (31)(32)(33)(34). GABARAP is known to be associated with the ER membrane presumably through its hydrophobic phosphatidylethanolamine tail (35,36).…”

“…Classic cadherins (simply referred to cadherins hereafter) (5-9), however, are known to have no functional ER export motifs, and their efficient ER exit requires complex formation with ␤-catenin at the ER immediately after cadherin synthesis (10 -12). We have recently shown that this ␤-catenin-facilitated ER export of cadherins requires PX-RICS, a ␤-catenininteracting GTPase-activating protein (GAP) for Cdc42 (13).PX-RICS is an alternatively spliced isoform of RICS (RhoGAP involved in the ␤-catenin-N-cadherin and N-methyl-D-aspartate receptor signaling) and contains phox homology (PX) and Src homology 3 domains in its N-terminal region (14,15). RICS is expressed predominantly in neurons of the brain and localized to the growth cone and postsynaptic density, where it regulates neurite extension and presumably N-methyl-D-aspartate signaling (14).…”

“…PX-RICS is an alternatively spliced isoform of RICS (RhoGAP involved in the ␤-catenin-N-cadherin and N-methyl-D-aspartate receptor signaling) and contains phox homology (PX) and Src homology 3 domains in its N-terminal region (14,15). RICS is expressed predominantly in neurons of the brain and localized to the growth cone and postsynaptic density, where it regulates neurite extension and presumably N-methyl-D-aspartate signaling (14).…”