DETERMINANTS FOR INTRACELLULAR SORTING OF CYTOPLASMIC AND NUCLEAR INTERMEDIATE FILAMENTS

The mechanism by which nuclear and cytoplasmic filaments are sorted in vivo was studied by examining which lamin sequences are required to t arget an otherwise cytoplasmic IF protein, the small neurofilament sub unit (NF-L), to the nuclear lamina. By swapping corresponding domains between NF-L and lamin A, nuclear envelope targeting of NF-L was shown to require the presence of the ''head'' domain, a 42-amino acid seque nce unique to lamin rod domains, a nuclear localization signal and the CAAX motif. Replacement of the entire COOH-terminal tail of lamin A w ith that of NF-L had no discernible effect on nuclear localization of lamin A, provided the substituted NF-L tail contained a NLS and a CAAX motif. This chimeric protein exhibited characteristics more typical o f lamin B than that of the parental lamin A. With regard to cytoplasmi c assembly properties, substitution of the head domain of lamin A for that of NF-L did not substantially affect the ability of NF-L to coass emble with vimentin in the cytoplasm. In contrast, insertion of a 42-a mino acid sequence unique to lamin rod domains into NF-L profoundly af fected NF-L coassembly with vimentin indicating that the 42-amino acid insertion in lamins may be important for sorting lamins from cytoplas mic IF proteins.