Mechanisms of Nuclear Import and Export That Control the Subcellular Localization of Class II Transactivator

Abstract

The presence of the class II transactivator (CIITA) activates the transcription of all MHC class II genes. Previously, we reported that deletion of a carboxyl-terminal nuclear localization signal (NLS) results in the cytoplasmic localization of CIITA and one form of the type II bare lymphocyte syndrome. However, further sequential carboxyl-terminal deletions of CIITA resulted in mutant forms of the protein that localized predominantly to the nucleus, 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 contains strong residue similarity to the classical SV40 NLS. Deletion of this region results in cytoplasmic localization of CIITA and loss of transactivation activity, both of which can be rescued by replacement with the SV40 NLS. Fusion of this sequence to a heterologous protein results in its nuclear translocation, confirming the identification of a NLS. In addition to nuclear localization sequences, CIITA is also controlled by nuclear export. Leptomycin B, an inhibitor of export, blocked the nuclear to cytoplasmic translocation of CIITA; however, leptomycin did not alter the localization of the NLS mutant, indicating that this region mediates only the rate of import and does not affect CIITA export. Several candidate nuclear export sequences were also found in CIITA and one affected the export of a heterologous protein. In summary, we have demonstrated that CIITA localization is balanced between the cytoplasm and nucleus due to the presence of NLS and nuclear export signal sequences in the CIITA protein.

Identification and characterization of interferon regulatory factor-1 from orange-spotted grouper (Epinephelus coioides)

“…Protein translocation from cytoplasm into nucleus is dependent on the presence of a nuclear localization signal (NLS) within the protein [45]. Constitutive nuclear localization of mammalian IRF-1 has been confirmed, and its NLS has been localized to a region C-terminal beside the DBD, spanning amino acids 117-141 [46].…”

Inhibition of human T cell leukemia virus type 2 replication by the suppressive action of class II transactivator and nuclear factor Y

“…This dual subcellular distribution is the net result of multiple nuclear import and nuclear export signals (53,(60)(61)(62)(63)(64)(65). Because Tax-2 activates the viral promoter in the nucleus, it is likely that CIITA inhibits Tax-2 at the nuclear level.…”

A novel CARD containing splice-isoform of CIITA regulates nitric oxide synthesis in dendritic cells

“…Interestingly, our study shows that like other CIITA, DC-CIITA localizes to cytoplasm and nucleus but not mitochondria (unpublished data). It has been reported that the C-terminal two-third of the CIITA protein contains at least two nuclear-localization signals (NLS2 and NLS3) (Cressman et al, 2001). Thus, it is possible that these signals in the full length DC-CIITA might cover mitochondria targeting signal in the CARD-like domain.…”

“…Consistently, our study shows that a novel DC-CIITA splicing isoform, DC-CASPIC, regulates the expression of NOS2 and NO synthesis in the maturing DC. This is an efficient mechanism where transcription via the type-1 CIITA promoter produces two splice-isoforms of mRNA encoding for two proteins (DC-CIITA and DC-CASPIC) with distinct functions in the maturing DC, where DC-CIITA preferentially localizes to nucleus (Cressman et al, 2001) and regulates transcription of MHC genes while DC-CASPIC localizes to mitochondrial organelle (as well as cytoplasm) and regulates NO synthesis. -202), and Jurkat T cell lymphocyte cell line (TIB-152) were obtained from the American Type Culture Collection (ATCC, Manassas, VA, USA).…”