Nuclear staining for the small heat shock protein αB-crystallin colocalizes with splicing factor SC35

αβ-Crystallin has for a long time been considered a specific eye lens protein. Later on it appeared that this protein belongs to the family of the small heat shock proteins and that it occurs also extra-lenticularly in many different cell types. αB-Crystallin is mainly present in the cytoplasm, but there are some indications that it might have a function in the nucleus too. However, till now its presence in the nucleus is uncertain. We therefore compared the localization of αB-crystallin in nine cell lines cultured under normal conditions using four different antisera. All four antisera gave a diffuse staining for αB-crystallin in the cytoplasm, but one of the antibodies consistently showed nuclear staining in eight of the cell types, in the form of distinct speckles. These speckles are equally pronounced in the different cell types, whether or not cytoplasmic αB-crystallin is present. Preabsorption of the antiserum with αB-crystallin abolished the staining. Furthermore we demonstrate that if only minor amounts of αB-crystallin are present, the protein seems to be located exclusively in the nucleus. However, in case of higher amounts of protein, αB-crystallin is distributed between cytoplasm and nucleus. The nuclear αB-crystallin exists, like the cytoplasmic αB-crystallin, in non-phosphorylated and phosphorylated forms, is Triton-insoluble but can be extracted by 2 M NaCl. These data suggest that αB-crystallin might be bound to the nuclear matrix per se or to nuclear matrix proteins via other proteins. In agreement with other nuclear matrix proteins, nuclear αB-crystallin staining turns diffuse upon mitosis and leaves the chromosomes unstained. Double staining experiments revealed colocalization of αB-crystallin with the splicing factor SC35 in nuclear speckles, suggesting a role for αB-crystallin in splicing or protection of the splicing machinery.