ASSEMBLY OF A-TYPE AND B-TYPE LAMINS STUDIED IN-VIVO WITH THE BACULOVIRUS SYSTEM

We have expressed an A-type lamin (Xenopus lamin A), a probable A-type lamin (Drosophila lamin C), two B-type lamins (Xenopus lamin LI, Dros ophila lamin Dmo), and two mutants of Xenopus lamin A in Sf9 cells, Al l proteins were synthesized at high levels resulting in formation of p aracrystals with an axial repeat of 18.5-20.0 nm by A-type lamins; in contrast B-type lamins assembled into aggregates with a fibrillar ultr astucture. Of the four wild-type proteins analyzed only lamin Dmo was found in the nuclear compartment of Sf9 cells in association with the lamina whereas the three other lamins assembled into polymers localize d in the cytoplasm as well as the nucleoplasm. The Xenopus lamin A mut ant lacking the complete carboxy-terminal tail assembled in the cytopl asm into long filament bundles consisting of fibrils of less than 6 nm diameter, In vitro the non-helical amino-terminal head domain of lami ns is required for the formation of 'head-to-tail' polymers, A lamin A mutant lacking this domain could be efficiently extracted from Sf9 ce lls with physiological buffers containing Triton X-100, demonstrating the importance of this domain for lamin assembly in vivo.