Rab proteins expressed in intestinal epithelial cells : specific links to the cytoskeleton

ABSTRACT: The large variety of vesicular transport pathwaysthat can be distinguished in cells are essential for the correct delivery of cellular components to the appropriate sites where they exhibit their biological function. This involves the targeting of newly synthesized proteins, like hormones and enzymes, along the exocytotic/secretory pathway, and internalization or endocytosis of molecules, like receptors and signal peptides, towards intracellular organelles. Rab proteins, a subfamily of the Ras superfamily of small GTPases, are key players in the regulation of these cellular processes. Each individual member is involved in the orchestration of one specific intracellular transport route. In this thesis, we investigated which Rab proteins may act along the exocytic transport pathway that is followed by digestive enzymes in polarized intestinal epithelial Caco-2 cells. In particular, we were interested how transport is achieved in respect to the molecular environment of involved Rab proteins. As such the starting point was to determine Rab proteins that are specifically expressed in Caco-2 cells, and to establish if and how they direct the transport of cargo along steps of the exocytic pathway. In this thesis we describe seven sequences encoding novel human Rab genes. We decided to further analyze four of them, i.e. Rab21, Rab6A', Rab6B, and Rab22B. We propose a role for Rab21 in an apically oriented transport route specific for polarized intestinal epithelial cells. We further describe that cells organize and/or fine-tune transport pathways by the expression of Rab6 isoforms via alternative splicing of exons, thereby introducing forms with different functions and that interact with distinct sets of effectors. Finally, we found a direct interaction between Rab22B and a core component of the cytoskeleton, filamin B, which indicates a role for the actin cytoskeleton in the regulation of membrane traffic in cooperation with Rab proteins