Phage Display Screening Reveals an Association Between Germline-specific Transcription Factor Oct-4 and Multiple Cellular Proteins

Oct-4 is a transcription factor that is specifically expressed in mouse embryonic stem cells and in cell lines derived thereof. In these cells, Oct-4 activates transcription from remote binding sites due to as of yet unknown co-activators. Expression of Oct-4 in differentiated cells is not sufficient to activate transcription from a distance, rather it requires the co-expression of co-activators such as the adenoviral oncoprotein E1A. In this paper, we used phage display to identify Oct-4-interacting proteins. We first analyzed the interaction between Oct-4 and E1A in order to optimize the biochemical conditions that enable Oct-4-specific interactions with other interacting proteins. A panning approach was used to enrich Oct-4 interacting phages from a pool of excess unspecific phages. The biochemical conditions established in our interaction assays were then used to screen a P19 EC cell cDNA expression library in M13 filamentous phage. A number of phage clones displaying portions of unknown and known transcription factors were obtained, from which the HMG-1 transcription factor was identified. HMG-1, and the closely related factor HMG-2, interact with Oct-4 when co-expressed in mammalian cells. In addition, HMG-1 was found to cooperate with Oct-4 in P19 EC cells. These results provide the first evidence of a non-viral factor that enhances Oct-4 distance-dependent transactivation in stem cells.