Unique Function for Carboxyl-Terminal Domain of Oct-2 in Ig-Secreting Cells

Abstract

The activity of Ig gene promoters and enhancers is regulated by two related transcription factors, Oct-1 (ubiquitous) and Oct-2 (B lineage specific), which bind the octamer motif (ATTTGCAT) present in these elements. As Ig promoter-binding factors, Oct-1 and Oct-2 each work together with a B lymphocyte-specific cofactor OCA-B/OBF-1/Bob-1 that interacts with them through their POU (DNA-binding) domains. Because both can mediate Ig promoter activity in B cells, there has been some question as to whether these two octamer-binding factors serve distinct functions in lymphocytes. We have shown previously that the silencing of B lymphocyte-specific genes in plasmacytoma × T lymphoma hybrids can be prevented by preserving Oct-2 expression. The pronounced effect of this transcription factor on the phenotype of plasmacytoma × T lymphoma hybrids established a critical role for Oct-2 not only in maintaining Ig gene expression, but in maintaining the overall genetic program of Ig-secreting cells. In the present study, we have explored the functional differences between Oct-1 and Oct-2 using chimeric Oct-1/Oct-2 proteins in cell fusion assays. Our results provide further evidence for an essential role for Oct-2 in Ig-secreting cells and identify the C-terminal domain of Oct-2 as responsible for its unique function in these cells.

NF-κB and Oct-2 Synergize to Activate the Human 3′ Igh hs4 Enhancer in B Cells

“…ChIP data for BL-2 cells have confirmed the endogenous binding of NF-B family members, p50 and Rel-B, and of Oct-2 to human hs4. These data are consistent with a previously reported requirement for Oct-2 for the octamer-dependent portion of mouse 3Ј enhancer activity in the M12 B cell line (48) and may reflect intrinsic differences between the trans-activation domains of Oct-2 and Oct-1 (49).…”

Critical Role for the Oct-2/OCA-B Partnership in Ig-Secreting Cells

“…(e.g., hybrids 38A and 13A), whereas others expressed Oct-2 at much lower levels (e.g., hybrids 55A and 51B). The variation in expression levels for rescued genes has been observed previously in experiments involving hOct-2 as the rescuing transcription factor (17,18). As was seen in the latter studies, rescuing factor levels and rescued gene expression levels did not covary (e.g., compare hOCA-B and Oct-2 levels in hybrids 55A and 38A; Fig.…”

“…One possibility was that overexpression of any of the plasmacytoma-specific genes would lead to dominance of the Ig-secreting cell profile. We have previously shown that overexpression of the ubiquitous, octamer-binding factor Oct-1 does not affect the plasmacytoma ϫ T lymphoma hybrid phenotype, nor does expression of a protein that differs from Oct-2 by only its C-terminal domain (Oct2.2.1) (18). In the present study we have demonstrated that PU.1, a factor critical to early events in B cell development, is also unable to affect the hybrid cell phenotype.…”

“…This led us to conclude that Oct-2 was interacting with another plasmacytomarestricted factor to achieve its effect on the hybrid cell phenotype (17,18). We have previously hypothesized that such a factor would interact with the C-terminal domain of Oct-2.…”

“…In more recent experiments we have shown that the related octamer-binding transcription factor Oct-1 cannot supplant Oct-2 in this function and that the C-terminal domain of Oct-2 distinguishes it from Oct-1 in this regard (18). One question arising from these findings is whether any plasmacytoma-specific gene encoding a regulatory factor can shift the balance in favor of the plasmacyte's genetic program if this gene is shielded from silencing at the time of cell fusion.…”