Junctional Rab13-binding protein (JRAB) regulates cell spreading via filamins

Prior research has shown that Rab13 and its effector JRAB regulate junctional development by modulating cell adhesion molecule transport and actin reorganization in epithelial cells. However, the specific role of JRAB in actin cytoskeletal dynamics remains unclear. Understanding how JRAB influences cytoskeletal rearrangement could provide new insights into cell morphology regulation. No prior work had resolved the molecular partners involved in JRAB-induced actin changes. This gap motivated further investigation into JRAB's mechanism of action in fibroblasts. The role of filamin in JRAB signaling was not previously established. This study aimed to clarify the pathways through which JRAB affects cell spreading. The findings may suggest novel regulatory mechanisms in cytoskeletal dynamics.

The aim of this study was to investigate how JRAB regulates actin cytoskeletal reorganization in fibroblasts. Researchers focused on identifying the proteins involved in JRAB-induced morphological changes. They used mutant JRAB proteins to observe effects on cell structure. The study sought to determine whether filamin isoforms mediate JRAB's effects. Previous work had not clarified the role of filamin in JRAB signaling. This paper attempts to address that uncertainty by examining coimmunoprecipitation patterns. The researchers also tested the impact of ASB2 on JRAB-induced cell spreading. These experiments aimed to reveal novel regulatory pathways in cytoskeletal dynamics.

The study used NIH3T3 fibroblasts expressing mutant JRAB proteins. Two mutant forms were tested: JRABΔCT and JRABΔCC. JRABΔCT adopts a constitutively closed state, while JRABΔCC remains open. Cellular morphology was observed following mutant expression. Researchers examined stress fiber formation and membrane ruffling. Coimmunoprecipitation was used to identify JRAB-interacting proteins. Filamin isoforms were detected in HA-JRABΔCC precipitates. ASB2 expression was introduced to assess effects on filamin degradation. These methods allowed the team to test the role of filamin in JRAB signaling.

Expression of JRABΔCC caused cell spreading with membrane ruffles. JRABΔCT induced stress fibers, indicating opposing effects. Filamin coimmunoprecipitated with HA-JRABΔCC, suggesting a direct interaction. ASB2 expression degraded all three filamin isoforms. Degradation of filamin inhibited JRABΔCC-induced cell spreading. Actinin-1/-4 also coimmunoprecipitated with HA-JRABΔCC. However, actinin-1/-4 had no effect on cell spreading. These findings suggest filamin is essential for JRABΔCC's morphological effects.

The data suggest that JRAB contributes to actin cytoskeletal reorganization via filamin. JRABΔCC induces cell spreading, while JRABΔCT causes stress fibers. Filamin isoforms coimmunoprecipitate with JRABΔCC. ASB2-induced filamin degradation inhibits JRABΔCC effects. Actinin-1/-4 also interact with JRABΔCC but do not affect spreading. These findings may propose a new regulatory pathway for cell morphology. The study highlights the role of filamin in JRAB signaling. The authors suggest that filamin is a key mediator of JRAB's effects.