Mechanisms of nuclear import and export that control the subcellular localization of class II transactivator

The presence of the class II transactivator (CIITA) activates the transcrip tion of all MHC class II genes. Previously, we reported that deletion of a carboxyl-terminal nuclear localization signal (NLS) results in the cytoplas mic localization of CIITA and one form of the type II bare lymphocyte syndr ome. However, further sequential carboxyl-terminal deletions of CIITA resul ted in mutant forms of the protein that localized predominantly to the nucl eus, suggesting the presence of one or more additional NLS in the remaining sequence. We identified a 10-aa motif at residues 405-414 of CIITA that co ntains strong residue similarity to the classical SV40 NLS. Deletion of thi s region results in cytoplasmic localization of CIITA and loss of transacti vation activity, both of which can be rescued by replacement with the SV40 NLS. Fusion of this sequence to a heterologous protein results in its nucle ar translocation, confirming the identification of a NLS. In addition to nu clear localization sequences, CIITA is also controlled by nuclear export. L eptomycin B, an inhibitor of export, blocked the nuclear to cytoplasmic tra nslocation of CIITA; however, leptomycin did not alter the localization of the NLS mutant, indicating that this region mediates only the rate of impor t and does not affect CIITA export. Several candidate nuclear export sequen ces were also found in CIITA and one affected the export of a heterologous protein. In summary, we have demonstrated that CIITA localization is balanc ed between the cytoplasm and nucleus due to the presence of NLS and nuclear export signal sequences in the CIITA protein.