UC San Diego
Defining the early steps in nuclear pore assembly : chromatin-associated ELYS initiates pore assembly
- Author(s): Rasala, Beth A.
- et al.
My thesis is focused upon molecularly defining the mechanism of early steps in metazoan nuclear pore assembly. Nuclear pore complexes (NPCs) are large proteinaceous structures that span the nuclear envelope and act as gated aqueous channels to regulate the transport of macromolecules between the nucleus and cytoplasm. In metazoans, the massive NPCs disassemble into soluble subunits at the beginning of mitosis and then somehow reassemble following chromosome segregation; a process that is coordinated with membrane recruitment and fusion. The mechanism and order of NPC assembly is poorly understood. In Chapter 1, I show that ELYS co-purifies with the Nup107-160 complex, the largest subunit of the NPC, in Xenopus egg and human cell extracts. Indeed, I demonstrate that ELYS is a dual nucleoporin/kinetochore protein required for nuclear pore assembly and proper cell division. In Chapter 2, I focus on defining the early steps in nuclear pore assembly, including the mechanism for ELYS as the pore 'targeting' protein. In Chapter 3, I collaborated with Dr. Corinne Ramos on a study to order pore assembly with respect to inner and outer nuclear membrane fusion. Taken the data from chapters 1-3 together, I suggest a model in which NPC assembly is initiated on AT -rich chromatin through an interaction with the C-terminus of ELYS. The data also show the chromatin binding of ELYS precedes and is required for the binding of the Nup107-160 complex. Chromatin-bound ELYS and the Nup107-160 complex then recruit integral pore membrane proteins POM121- and NDC1-containing membrane vesicles. Membrane vesicle fusion takes place to form patches of continuous double nuclear membranes. Oligomerization of ELYS/Nup107-160/POM121 then acts to promote fusion between the inner and outer nuclear membranes to form a diffusion channel. Finally, the remaining soluble pore subunits are recruited to assemble the mature, functional nuclear pore. Finally, Appendix B presents data on the Xenopus binding partners of the C- terminus of Nup160, a member of the Nup107-160 complex, which was derived from mass spectrometry analyses. This data led to the discovery that vertebrate centrin 2 localizes to NPCs and functions in mRNA and protein export, as described in Chapter 4