Palmitoylation of the oncogenic RhoGEF TGAT is dispensable for membrane affinity and consequent activation of RhoA
Rho guanine exchange factors (RhoGEFs) control many aspects of the cellular cytoskeleton, and thereby regulate and control processes such as cell migration, cell adhesion and proliferation. TGAT is a splice variant of the RhoGEF Trio, with oncogenic potential. Whether the subcellular location of TGAT is critical for its activity is unknown. Confocal microscopy of fluorescent protein tagged TGAT revealed co-localization with a Golgi marker. Because plasma membrane localized RhoGEFs are particularly effective at activating RhoA, plasma membrane localization of TGAT was studied. In order to quantitatively measure plasma membrane association we developed a novel, highly sensitive image analysis method. The method requires a cytoplasmic marker and a plasma membrane marker, which are co-imaged with the tagged protein of interest. Linear unmixing is performed to determine the plasma membrane and cytoplasmic component in the fluorescence signal of protein of interest. The analysis revealed that wild-type TGAT is partially co-localized with the plasma membrane. Strikingly, cysteine TGAT-mutants lacking one or more palmitoylation sites in the C-tail, still showed membrane association. In contrast, a truncated variant, lacking the last 15 amino acids, TGAT{Delta}15, lost membrane association. The functional role of membrane localization was determined by measuring TGAT activity in single cells with a RhoA FRET-sensor and F-actin levels. Mutants of TGAT that still maintained membrane association showed similar activity as wild-type TGAT. In contrast, the activity was abrogated for the cytoplasmic TGAT{Delta}15 variant. Synthetic recruitment of TGAT{Delta}15 to membranes confirmed that TGAT effectively activates RhoA at the plasma membrane. Together, these results show that membrane association of TGAT is critical for its activity, but that palmitoylation is dispensable.