Immunoaffinity purification and functional characterization of interphase and meiotic Drosophila nuclear lamin isoforms

Three isoforms of a single nuclear lamin have been identified in Drosophila. Two, lamins Dm₁ and Dm₂, are present during interphase and are apparently in equilibrium with each other in vivo. The third, lamin Dm_mit, is found in cells that have undergone nuclear envelope breakdown, either during meiosis or mitosis. All three isoforms were purified under nondenaturing conditions using a novel technique of immunoaffinity chromatography and their in vitro activities were examined. Interphase lamins Dm₁ and Dm₂ can assemble into filaments at physiologic ionic strength; assembly is reversible upon addition of concentrated NaCl. Negative staining of filaments formed in vitro shows long, unbranched bundles approximately 20 nm in diameter. Addition of specific antilamin antibodies blocks in vitro assembly completely. In contrast with lamins Dm₁ and Dm₂, lamin Dm_mit remains soluble at physiologic ionic strength. These observations are consistent with the notion that lamina disassembly in vivo is due, at least in part, to changes in properties of the lamins themselves.