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Abstract
The fissioning of biological membranes is an essential aspect of intracellular transport. While several fissioning and trafficking steps are mediated by the large GTPase dynamin, others are dynamin-independent and are instead mediated by C-terminal-binding-protein1/brefeldinA-ADP ribosylated substrate (CtBP1/BARS). CtBP1/BARS is a bi-functional protein that shuttles between the cytoplasm and the nucleus, and it is involved in both membrane fission events and regulation of gene transcription. To gain insights into the molecular mechanisms of CtBP1/BARS in membrane fission and to understand how the same protein can perform different functions in very distinctive subcellular locations, I have studied its role in macropinocytosis, a dynamin-independent endocytic pathway that can be synchronously activated by growth factors. Here, I show that upon activation of the EGF receptor, CtBP1/BARS is translocated to the macropinocytic cup and the surrounding membrane, where it is required for fission of the macropinocytic cup. I have also analyzed the mechanisms that lead to CtBP1/BARS activation in this context: CtBP1/BARS is phosphorylated by p21-activated kinase (Pak1) on a specific serine after EGF stimulation, and this phosphorylation is essential for the fission of the macropinocytic cup. An important aspect of my thesis concerns the relationship between the two functions of CtBP1/BARS, one in membrane fissioning events and the other as a transcriptional co-repressor. These data identify the phosphorylation of CtBP1/BARS by Pak1 as a mechanistic link between these two activities of CtBP1/BARS: Pak1 switches CtBP1/BARS on for membrane fission and at the same time it turns it off as a co-repressor.
