SUNDAY, DECEMBER 4- Late Abstracts 1 - Molecular Biology of the ...

SUNDAY
2017
Samp1 is functionally associated with the LINC complex and A-type lamina networks.
S. Gudise 1 , R. Figueroa 2 , V. Larsson 3 , E. Hallberg 3 ; 1 Department of Biosciences and Nutrition,
Karolinska Institute, Sweden, 2 Department of Biochemistry and Biophysics, Stockholm
University, 3 Department of Neurochemistry, Stockholm University, Stockholm, Sweden
The transmembrane inner nuclear membrane (INM) protein Samp1 is required for anchoring
centrosomes near the nuclei. Using highresolution
fluorescence microscopy we show that Samp1 is distributed in a distinct and characteristic
pattern in the nuclear envelope (NE), where it partially colocalizes with the LINC complex
protein Sun1. By studying the localization of Samp1 deletion mutants and fusion proteins, we
conclude that the cysteine-rich N-terminal half of Samp1 is nucleoplasmically exposed and is
responsible for targeting to the INM. It contains four conserved CxxC motifs with the potential to
form zinc fingers. The distribution of cysteine-toalanine substitution mutants, designed to
prevent zinc finger formation, showed that NE localization of Samp1 depends on intact CxxC
motifs. Overexpression of Samp1 zinc finger mutants produced an abnormal dominant
phenotype characterized by disrupted organization
of a selective subset NE proteins, including emerin, Sun1, endogenous Samp1 and, in some
cases, lamin A/C, but not lamin B, Sun2
or nucleoporins. Silencing of Samp1 expression showed that emerin depends on Samp1 for its
correct localization in the NE. Our results demonstrate that Samp1 is functionally associated
with the LINC complex protein Sun1 and proteins of the A-type lamina network.
2018
NE localization of the LINC complex requires multiple elements of SUN and KASH
proteins.
Y. G. Turgay 1 , R. Ungricht 2 , A. Rothballer 2 , A. Kiss 2 , G. Csucs 2 , P. Horvath 2 , U. Kutay 2 ; 1 Institute
of Biochemistry, ETH Zurich, Zurich, Switzerland, 2 ETH Zurich, Zurich, Switzerland
Inner nuclear membrane (INM) proteins are co-translationally inserted into the endoplasmic
reticulum membrane (ER) and need to cross the nuclear pore complex (NPC) to reach their final
destination. Originally, targeting of INM proteins to the INM was solely explained by a “diffusionretention”
model. However, recent studies on yeast INM proteins revealed the existence of
classical nuclear localization signals (cNLS) that mediate the active transport of the proteins
across the NPC. Whether active transport mechanisms also play a role in the targeting of INM
proteins in mammalian cells is still unclear. We have used human SUN2 to study INM targeting
and identified three different elements, which collectively mediate NE targeting. The N-terminal
nucleoplasmic domain of SUN2 comprises a cNLS and a Golgi retrieval signal. The cNLS was
shown to constitute a functional binding site for the heterodimeric transport receptor importin
α/β. A nearby arginine cluster was found to serve as binding platform for the coatomer complex,
which mediates retrieval of SUN2 from the Golgi to the ER and ensures efficient INM targeting.
The conserved SUN domain at the C terminus represents the third targeting element we have
identified in our study. Together, our study shows that multiple elements in SUN2 contribute to
NE targeting and that these elements are not limited to cytoplasmic or transmembrane domains.