A hierarchy of nuclear localization signals governs the import of the regulatory factor X complex subunits and MHC class II expression.
Comprised of RFX5, RFXAP, and RFX-B/ANK, the regulatory factor X (RFX) complex is an obligate transcription factor required for the expression of MHC class II genes. RFX functions by binding to the conserved X1 box sequence located upstream of all MHC class II genes. Using a mutagenesis scheme and a yeast heterologous reporter system, the mechanism by which the RFX complex is transported into the nucleus was examined. The results have identified specific nuclear localization signals (NLS) in both RFX5 and RFXAP that direct the nuclear translocation and expression of MHC class II genes. Additionally, a nuclear export signal was identified in the N terminus of RFXAP. RFX-B was poorly localized to the nucleus, and no specific NLS was identified. Whereas RFX5 could import an RFXAP NLS mutant into the nucleus, it had no effect on the import of RFX-B. The results suggest that although RFX5 and RFXAP could assemble before nuclear import, RFX-B association with the complex does not take place until after the subunits enter the nucleus. The identification of nuclear import and export sites on RFX molecules provides potential targets to modulate MHC class II expression.
Nagarajan
Uma M
UM
Department of Microbiology and Immunology, Emory University School of Medicine, Atlanta, GA 30322, USA.
Long
Alyssa B
AB
Harreman
Michelle T
MT
Corbett
Anita H
AH
Boss
Jeremy M
JM
D021581:Active Transport, Cell Nucleus; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D019556:COS Cells; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D000949:Histocompatibility Antigens Class II / Q000032:analysis* / Q000502:physiology; D006801:Humans; D008969:Molecular Sequence Data; D019913:Nuclear Localization Signals; D021381:Protein Transport; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors / Q000737:chemistry / Q000378:metabolism*
D004268:DNA-Binding Proteins; D000949:Histocompatibility Antigens Class II; D019913:Nuclear Localization Signals; C093524:RFX5 protein, human; C413004:RFXAP protein, human; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors