A lamin-B envelope confers robustness to the steady-state prometaphase spindle by stabilizing it to phenotypic variations in the KLP61F to Ncd ratio. (A) Localization of the Drosophila lamin-B (Dm0) surrounding the prometaphase (phase I and II) spindle in fly embryos stably expressing GFP-Dm0; t = 0 is NEB. Images show a time series from a live embryo, each image being a projection of six z stacks along the x–y plane. Note the indentation of the lamin-B envelope surrounding the spindle by the centrosomes at the opposite ends of the spindle at t = 36 and 72 s during prometaphase phase I. (B) Schematic representation of an elastic lamin-B envelope surrounding the spindle during phase I of prometaphase and the representation of this envelope as a linear Hookean spring between the spindle poles (simple drawings above the spindle cartoons). Green and red arrows depict KLP61F- and Ncd-driven ipMT sliding. The time-dependent disassembly of this elastic envelope is incorporated into the quantitative model as a linearly decreasing spring constant, ζ(t), responding equally to extension and compression as shown in the graph on the right (solid line). Superimposed is an asymmetric spring constant of the lamin-B envelope with limited compressibility (dashed line). (C) Quantitative model results of the role of the elastic lamin-B envelope on the robustness of the prometaphase steady-state to the motors’ molar ratio. The left panel shows the mean prometaphase spindle length over time (averaged over 10 virtual spindles) for the varying mole fraction of Ncd, increasing from 0 to 1 in 31 equally spaced steps. The right panel shows the mean pole–pole separation rate for increasing Ncd mole fraction from 0 to 1; results of the model augmented with an elastic lamin-B envelope (red) are superimposed on the model solutions without lamin-B (black), as in Fig. 4 D, for comparison. A stable spindle length (mean velocity ≤|0.02| μm s⁻¹) is maintained for a wide range of motors’ molar ratio (approximately within [0.35 to 0.8] Ncd mole fraction) in the solution of the augmented model (red). The solution of the model including a lamin-B envelope with asymmetric response to extension and compression as in B (right, dashed line) is shown superimposed in blue. This extends the stability regime further, particularly at high Ncd mole fractions. Error bars show SD. Bar, 5 μm.