Intermediate filament protein polymerization: Molecular analysis of Drosophila nuclear lamin head-to-tail binding

Polymerization of intermediate filament proteins results from interactions among several distinct binding sites on the constituent proteins. Nuclear lamin head-to-tail polymers arise from one such interaction. We studied this binding using Drosophila lamin Dm₀-derived fragments containing either the NH₂-terminal or COOH-terminal binding site with a combination of co-immunoprecipitation, yeast two-hybrid, analytical ultracentrifugation, and electron microscopic assays. Fragment binding and fall-length lamin head-to-tail polymerization were similar to each other in morphology, buffer requirements, and inhibition after phosphorylation with cdc2 kinase. Deletion analysis localized the binding sites to the ends of the rod domain that are highly conserved among all intermediate filament proteins. Point mutants, defective in binding, were isolated. Two were identical to point mutations in specific human keratin genes known to affect keratin assembly and to cause genetic skin diseases. Results further indicate that the binding sites only function in specific sequence contexts and that binding can be modulated by elements outside the binding sites (like the cdc2 kinase phosphorylation site). Our data indicate that one type of interaction in intermediate filament protein polymerization is the longitudinal binding of dimers via the conserved end segments of the coiled-coil rod domain.