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.