Poster Presentation 2014 International Biophysics Congress

Lamin-A/C/LAP2a/BAF1 protein complex regulates mitotic spindle assembly and positioning (#615)

Ran Qi 1 2 , Nan Xu 1 2 , Gang Wang 1 2 , He Ren 1 2 , Jun Lei 1 2 , Qiaoyu Lin 1 2 , Lihao Wang 1 2 , Xin Gu 1 2 , Qing Jiang 1 2 , Chuanmao Zhang 1 2
  1. The Key Laboratory of Cell Proliferation and Differentiation of Ministry of Education, Beijing, China
  2. The State Key Laboratory of Biomembrane and Membrane Biotechnology, College of Life Sciences, Peking University, Beijing, China

Mitotic spindle assembly and positioning are dynamic processes in mammalian cells that require precise coordination of nuclear disassembly, chromatin condensation, microtubule cytoskeleton reorganization and microtubule-kinetochore connection during the mitotic entry. In these processes some nuclear proteins relocate to the spindle to perform their distinct roles, although the mechanisms are largely unclear. In interphase, the nuclear proteins lamin-A/C, LAP2a and barrier-to-autointegration factor 1 (BAF1) perform their roles for the nuclear structure maintenance and functions. LAP2a forms a stable complex with lamin-A/C in the nucleoplasm. BAF1 is an evolutionarily conserved small protein that binds chromatin DNA and histones as well as LAP2, Emerin, and MAN1  on the inner nuclear membrane and plays important roles for chromosome segregation and NE assembly. Mutations of lamin-A/C and BAF1 cause a variety of diseases. However, the molecular nature of the nuclear periphery proteins in taking their distinct roles in mitosis remains elusive. In this work, we found that lamin-A/C, LAP2a and BAF1 relocate to spindle and cell cortex and regulate the spindle assembly and positioning in mitosis. RNAi knockdown of these proteins induced short and fluffy spindle formation and disconnection of the spindle with the cell cortex. Disrupting the microtubule disassembly by special treatments led to accumulation of these proteins in the cell cortex, whereas depolymerizing the actin microfilaments resulted in short spindle formation. We further demonstrated that these proteins form a stable complex that links the spindle to cell cortex and spindle matrix by binding with the spindle-associated dynein and the cell cortex and spindle matrix -localized actin filaments. Through balance of pulling and pushing forces between the spindle and the cell cortex generated by dynein, the spindle is properly positioned and orientated. Together, our findings unveil a unique mechanism that lamin-A/C/LAP2a/BAF1 protein complex regulates the mitotic spindle assembly and positioning in mitosis.