The human oct-4 isoforms differ in their ability to confer self-renewal.

B139

Oct-4 transcription factors play an important role in maintaining the pluripotent state of embryonic stem cells and may prevent expression of genes activated during differentiation. Human Oct-4 isoform mRNAs encode proteins that have identical POU DNA binding domains and C-terminal domains, but differ in their amino-terminal domains. We report here the cloning and characterization of the human Oct-4B isoform. Human Oct-4B cDNA encodes a 265-amino acid protein with a predicted molecular mass of 30 kDa. ES cell-based complementation assays using ZHBTc4 ES cells showed that unlike human Oct-4A, Oct-4B cannot sustain ES cell self-renewal. In addition, Oct-4B does not bind to a probe carrying the Oct-4 consensus binding sequence and we demonstrate that two separate regions of its amino terminal domain are responsible for inhibiting DNA binding. We also demonstrate that Oct-4B is mainly localized to the cytoplasm. Overexpression of Oct-4B did not activate transcription from Oct-4-dependent promoters, whereas Oct-4A did as previously reported. Furthermore, transcriptional activation by human Oct-4A was not inhibited by co-expression of Oct-4B. Taken together, these data suggest that the DNA binding, transactivation, and abilities to confer self-renewal of the human Oct-4 isoforms differ.