Opponent: Professor Katherine Wilson, Department of Cell Biology, Johns Hopkins University School of Medicine, 725 N. Wolfe Street, G10 WBSB, Baltimore, MD 21205​, USA

Abstract

The nuclear envelope forms the interface between the nucleus and the cytoplasm. The nuclear envelope consists of the two concentric lipid membranes, the nuclear pores and the nuclear lamina. The inner nuclear membrane contains hundreds of unique transmembrane proteins showing high tissue diversity. Mutations of some proteins in the nuclear envelope give rise to a broad spectrum of diseases called envelopathies or laminopathies. In this thesis, I aimed to study the functional organization of the nuclear envelope by identifying and characterizing interactions between the nuclear envelope proteins. For this, we developed a novel method called the Membrane Protein Crosslink Immuno-Precipitation, which enable identification of protein-protein interactions in the nuclear envelope in live cells. We identified several novel interactions of the inner nuclear membrane protein, Samp1, and studied the interaction between the Samp1 and the nuclear GTPase, Ran in detail. Samp1 can bind to Ran and is thus the first known transmembrane Ran binding protein and Samp1 might provide a local binding site for Ran in the inner nuclear membrane. We found that Samp1 also binds to the inner nuclear membrane protein, Emerin and Ran can regulate the Samp1-Emerin interaction in the nuclear envelope. During mitosis, Samp1 distributes in the mitotic spindle. Therefore, we investigated a possible functional role of Samp1 in the mitotic machinery. Samp1 depletion resulted in aneuploid phenotypes, metaphase prolongation and decreased distribution of γ-tubulin and β-tubulin in the mitotic spindle. We found that Samp1 can bind to γ-tubulin, which is essential for the microtubule nucleation and hence for the spindle stability. The new interesting features of Samp1 provide insights on the unforeseen functions of the nuclear envelope proteins.