Lamina-associated polypeptide 2β (LAP2β) is contained in a protein complex together with A- and B-type lamins

Abstract

Lamina-associated polypeptide 2β (LAP2β) of vertebrates is an integral membrane protein of the inner nuclear membrane that is generated by alternative splicing from the LAP2 gene. In the majority of Xenopus somatic cells including cultured kidney epithelial cells (A6 cells) there is only one major LAP2 isoform expressed that has the highest similarities with the mammalian LAP2β whereas isoforms corresponding in size to the mammalian LAP2γ and α are not detectable. We selected A6 cells and A6 cells stably expressing GFP fusion proteins of Xenopus LAP2β (XLAP2β) as a model system to study interactions between LAP2β and lamins. In vitro binding experiments with GST-XLAP2β fusion proteins and immunoprecipitations with antibodies to GFP revealed that XLAP2β is part of a complex that contains A- and B-type lamins. For the targeting to the nuclear envelope and the in vivo formation of this complex, GFP fusion proteins were sufficient comprising only the carboxyterminal 135 amino acids of XLAP2β or the comparable region of zebrafish LAP2β. A highly conserved 36 amino acids long sequence is located in this region of LAP2β that is part of the lamina-binding domain previously identified in rat LAP2β. GFP-LAP2β fusion proteins of Xenopus, zebrafish, and rat that contained this sequence do compete with endogenous LAP2 in transfected cells for the same binding sites in the lamina. Our data indicate that the lamina-binding site of LAP2β has been highly conserved during vertebrate evolution and suggests that this region of LAP2β mediates the interactions between polymers of A- and B-type lamins.