Mammalian heterochromatin proteins 1 (HP1alpha, HP1beta, and HP1gamma) are nonhistone proteins that interact in vitro with a set of proteins that play a role in chromatin silencing, transcription, and chromatin remodeling. Using antibodies specific for each HP1 isoform, we showed that they segregate in distinct nuclear domains of human HeLa cells. By contrast, in mouse 3T3 interphase cells, HP1alpha and HP1beta are strictly colocalized. In mitotic HeLa cells, all of HP1alpha and a fraction of HP1beta and HP1gamma remain associated with chromosomes. Immunostaining of spread HeLa chromosomes showed that HP1alpha is mainly localized on centromeres as shown previously for HP1beta, while HP1gamma is distributed on discrete sites on the arms of chromosomes. Biochemical analysis showed that HP1alpha and HP1gamma are phosphorylated throughout the cell cycle, although more extensively in mitosis than in interphase, while HP1beta apparently remains unphosphorylated. Therefore, despite their extensive sequence conservation, mammalian HP1 isoforms differ widely in their nuclear localization, mitotic distribution and cell cycle-related phosphorylation. Thus, subtle differences in primary sequence and in posttranslational modifications may promote their targeting at different chromatin sites, generating pleiotropic effects.

Lamins are nuclear intermediate filaments that, together with lamin-associated proteins, maintain nuclear shape and provide a structural support for chromosomes and replicating DNA. We have determined the solution structure of the human lamin A/C C-terminal globular domain which contains specific mutations causing four different heritable diseases. This domain encompasses residues 430-545 and adopts an Ig-like fold of type s. We have also characterized by NMR and circular dichroism the structure and thermostability of three mutants, R453W and R482W/Q, corresponding to "hot spots" causing Emery-Dreifuss muscular dystrophy and Dunnigan-type lipodystrophy, respectively. Our structure determination and mutant analyses clearly show that the consequences of the mutations causing muscle-specific diseases or lipodystrophy are different at the molecular level.