The apparent absence of lamin B1 and emerin in many tissue nuclei is due to epitope masking.

Immunolocalization studies have concluded that the nuclear membrane protein, emerin, is absent from many cell types and that lamin B1 is absent from adult heart and skeletal muscle. We now show that epitope masking in the nucleus is often responsible for failure to detect emerin and lamins in human, rat and pig tissues. Human heart cardiomyocyte nuclei were negative for lamin B1 using a commercial mAb, but were positive using two other lamin B1 antibodies, mAb8D1 and pAbB1-cbs. Rat hippocampal neuronal nuclei were immunostained by mAb8D1, but not pAbB1-cbs, while the commercial antibody stained only a subset. These data suggest that different regions of the lamin B1 molecule are masked in different tissues. Similarly, pig spleen had fewer emerin-positive nuclei than lung (5% vs. 32%), although their emerin content was similar by Western blotting. As mAbs against six epitopes gave the same result, the whole emerin molecule is either masked or redistributed in a subset of cells. Our findings argue that immunostaining evidence can be misleading for expression of nuclear envelope proteins. Problems with lamin B1 immunostaining can be avoided by using mAb8D1, but use of antibodies recognizing different epitopes may reveal cell-specific protein interactions in the nucleus.