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Transactivation activity of LBP‐1 proteins and their dimerization in living cells

LBP‐1 proteins form dimers and act as transcription factors that activate a number of genes related to cell growth and differentiation. LBP‐1a and LBP‐1c are localized in the cytoplasm when transiently expressed in cultured cells, but translocated into the nucleus after forming heterodimers with LBP‐1b, which is a splicing variant of LBP‐1a with an intrinsic nuclear localization signal (NLS). Here, we report that LBP‐1b showed potent transactivation activity, and that forcibly expressed LBP‐1a and LBP‐1c in the nucleus essentially exhibited very little or no transactivation activity. Mutations in the NLS that abolished the NLS activity of LBP‐1b also abrogated the transactivation activity. We have found that LBP‐1 proteins contain a putative sterile alpha motif domain indispensable for their dimerization capability in the C‐terminal region. To demonstrate whether homo‐ and heterodimers composed of LBP‐1a and/or LBP‐1c are generated in the nucleus, we applied the FLIM‐based fluorescence resonance energy transfer imaging technique to living cells. It revealed that dimers composed of LBP‐1a and LBP‐1c were re‐formed probably by a partner‐exchange of LBP‐1b‐containing heterodimers.