Mlp1 and Mlp2 cooperate to build a stoichiometric nuclear pore basket in budding yeast

The nuclear pore complex (NPC) is the only gateway between the nucleus and the cytoplasm in eukaryotic cells. Its nucleoplasmic face is decorated by the nuclear basket, a filamentous structure with important roles in mRNA export and chromatin organization. In contrast to major parts of the nuclear pore scaffold, the architecture of the nuclear basket remains poorly defined. Here, we investigate the interactions required for formation and maintenance of the nuclear basket in vivo using budding yeast. Although previous work has often focused on Mlp1, the largest and most abundant nuclear basket protein, we demonstrate that its paralogue, Mlp2, also plays a central role in nuclear basket architecture. Mlp2 can interact with the NPC scaffold independently of Mlp1, and interactions between the coiled-coil regions of both proteins stabilize their binding. Furthermore, the N-termini of both Mlp1 and Mlp2 are required for recruitment of Pml39. In addition, we show that Pml39 uses its N- and C-terminal helices to recruit additional Mlp1 subunits. We propose a refined model of nuclear basket architecture with a stoichiometry of 4:2:1 per spoke for Mlp1:Mlp2:Pml39.