Mistargeting of B-Type Lamins at the End of Mitosis

We previously showed that targeting of protein phosphatase 1 (PP1) to the nuclear envelope (NE) by the A-kinase anchoring protein, AKAP149, correlates with nuclear assembly of B-type lamins in vitro. We demonstrate here that failure of AKAP149-mediated assembly of B-type lamins into the nuclear lamina at the end of mitosis is followed by apoptosis, and induces expression of the gene encoding A-type lamins in cells that normally do not express lamins A/C. In HeLa cells, inhibition of PP1 association with the NE mediated by a peptide containing the PP1-binding domain of AKAP149 results in failure of B-type lamins to assemble, and in their rapid caspase-dependent proteolysis. However, assembly of lamins A/C is not affected. Nonetheless, apoptosis follows within hours of nuclear reformation after mitosis. In lymphoid KE37 cells, which do not express lamins A/C, inhibition of B-type lamin assembly triggers rapid synthesis and nuclear assembly of both lamins A and C before apoptosis takes place. The results indicate that nuclear assembly of B-type lamins is essential for cell survival. They also suggest that mistargeting of B-type lamins at the end of mitosis elicits a tentative rescue process to assemble a nuclear lamina in lymphoid cells that normally do not express lamins A/C.