THE ALPHA-HELICAL ROD DOMAIN OF HUMAN LAMIN-A AND LAMIN-C CONTAINS A CHROMATIN BINDING-SITE

We examined regions Of human lamins A and C involved in binding to sur faces of mitotic chromosomes. An Escherichia coli expression system wa s used to produce full-length lamin A and lamin C, and truncated lamin s retaining the central alpha-helical rod domain (residues 34-388) but lacking various amounts of the amino-terminal 'head' and carboxy-term inal 'tail' domains. We found that lamin A, lamin C and lamin fragment s lacking the head domain and tail sequences distal to residue 431 eff iciently assembled into paracrystals and strongly associated with mito tic chromosomes. Furthermore, the lamin rod domain also associated wit h chromosomes, although efficient chromosome coating required the pH 5 -6 conditions needed to assemble the rod into higher order structures. Biochemical assays showed that chromosomes substantially reduced the critical concentration for assembly of lamin polypeptides into pelleta ble structures. Association of the lamin rod with chromosomes was abol ished by pretrypsinization of chromosomes, and was not seen for viment in (which possesses a similar rod domain). These data demonstrate that the alpha-helical rod of lamins A and C contains a specific chromosom e binding site. Hence, the central rod domain of intermediate filament proteins can be involved in interactions with other cellular structur es as well as in filament assembly.