Association of Lbc Rho guanine nucleotide exchange factor with α-catenin-related protein, α-catulin/CTNNAL1, supports serum response factor activation

The Rho GTPase signaling pathway is required for actin cytoskeletal organization and serum response factor-dependent gene transcription. Lbc is a Rho-specific guanine nucleotide exchange factor that contains a modulatory C-terminal region. To elucidate Lbc regulatory mechanism(s), a yeast two-hybrid screen for proteins that interact with the Lbc C-terminal region was carried out, resulting in multiple isolation of cDNAs encoding the same 734-amino acid Lbc interacting protein. The Lbc interacting protein has homology with the α-catenin cell adhesion component and is identical to the α-catenin-like α-catulin protein of unknown function. The human α-catulin gene (CTNNAL1) maps to 9q31-32. Here we identify the predicted endogenous α-catulin product, document α-catulin and Lbc co-expression in multiple human cell lines, and show α-catulin and Lbc subcellular co-fractionation and intracellular localization. The required regions for Lbc and α-catulin interaction were mapped, and complex formation between Lbc and α-catulin in mammalian cells was detected. Functionally, α-catulin co-expression leads to increased Lbc-induced serum response factor activation in vivo as measured by a transcriptional reporter assay. Furthermore, α-catulin co-expression enhances Lbc-induced GTP-Rho formation in vivo. These results support the concept that the recently identified α-catulin protein may modulate Rho pathway signaling in vivo by providing a scaffold for the Lbc Rho guanine nucleotide exchange factor.