Cytoskeleton remodeling RalA regulation pathway
RalA regulation pathway v-Ral simian leukemia viral oncogene homolog A ( RalA ) belongs to a family ofsmall GTP-binding proteins (G-proteins) called monomeric G-proteins. The Ral subfamilyconsists of RalA and RalB proteins. RalA is localized at the cytoplasmic surface of the plasma membrane. It is atarget of posttranslational modification via attachment of lipid moieties, such asgeranyl, catalyzed by Geranylgeranyltransferase type I ( GGTase-I ). Theseposttranslational modifications affect localization and biological activity ofRalA . Like other G-proteins , RalA is found in two interconvertible forms, GDP-boundinactive and GTP-bound active. Conversion from the GDP- to GTP-bound form is catalyzed byGuanine nucleotide exchange factors (GEFs). Activity of GEF is often regulated by anupstream signal. GEF first interacts with the GDP-bound form and releases bound GDP. As aresult, a binary complex of a small G protein and GEF is formed. GEF in this complex issubsequently replaced by GTP resulting in formation of the GTP-bound small G protein. Three GEFs are known to interact with RalA. These are RalGDS,RGL, and RalGEF2. Two of them, RalGDS and RGL, have beenfound to be v-Ha-ras Harvey rat sarcoma viral oncogene homolog ( H-RAS ) proteineffectors , . They also bind RAP1A member of RAS oncogenefamily ( RAP-1A ), but biological role of these interactions is unclear , . RalGDS activity is affected by Formyl-Met-Leu-Phe receptor ( FPR ).RalGDS is localized to the cytosol and remains inactive in a complex formed withBeta-arrestins ( Beta-arrestin 1 and Beta-arrestin 2 ). In response toFPR stimulation, Beta-arrestin/ RallGDS protein complexes dissociate, andRalGDS translocates with Beta-arrestin from the cytosol to the plasma membrane.This leads to activation of the RalA effector pathway that affects cytoskeletalrearrangements . Conversion from GTP-bound form to GDP-bound form is a result of intrinsic GTPaseactivity of RalA. This activity is slow, and proteins called GTPase activatedproteins (GAPs) are known to stimulate it. The GAP proteins for RalA werecharacterized and partially purified. However, their genes have not been cloned yet. Aurora kinase ( Aurora -A) phosphorylates and activites RalA. Ral GTPases may also be involved in calcium/calmodulin-mediatedintracellular signaling pathways where RalA is activated by Ca(2+) viabinding with Calmodulin . Effectors for RalA RalBP1, Phospholipase D 1 ( PLD1), Filamin,and components of the exocyst implicate participation RalA in various cellprocesses . RalBP1 contains a RhoGAP homology domain that exhibits the GAP activity forRas-related C3 botulinum toxin substrate 1 ( Rac1 ) and Cell division cycle 42 (CDC42 ) proteins, thereby inhibiting Rac1/ CDC42 involved incytoskeleton remodeling . On the other hand, actin-binding proteinFilamin is an effector protein of RalA. Filamin crosslinks Actinfilaments into orthogonal networks and participates in the anchoring of membrane proteinsto the Actin cytoskeleton . RalA via RalBP1 interacts with the Mu-subunit of the heterotetramericCoat assembly protein complex 2 ( AP complex 2 medium (mu) chain ) and RALBP1associated Eps domain containing 1 and 2 ( REPS1 and REPS2 ) proteins.These proteins are involved in endocytosis and cell motility , , . Exocyst components Sec6, Sec6, Sec15B are direct effectors ofRalA , , . RalBP1 also interacts with the stress-responsive Heat shock factor 1 (HSF1 ) and regulates its activity . Another RalA protein effector, PLD1, is implicated in vesicletrafficking. PLD1 directly associates with RalA. However, RalA hasno effect on the activity of the PLD1. RalA is required for the stimulation ofPLD1 activity by the ADP-ribosylation factor 1 ( ARF1 ) . Thus the RalA signaling appears to regulate vesicle trafficking, cytoskeletonorganization, gene expression, and cell transformation.