Recent biochemical and genetic studies indicate that in addition to the octamer-binding proteins Oct-1 and Oct-2, other B cell components are required for lymphoid-restricted, octamer site-mediated immunoglobulin gene promoter activity. Using a genetic screen in yeast, we have isolated B cell-derived cDNAs encoding Oct-binding factor 1 (OBF-1), a novel protein that specifically associates with Oct-1 and Oct-2. Biochemical studies demonstrate that OBF-1 has no intrinsic DNA-binding activity and recognizes the POU domains of Oct-1 and Oct-2, but not those of Oct-4 and Oct-6. The OBF-1 mRNA is expressed in a highly cell-specific manner, being most abundant in B cells and essentially absent in most of the other cells or tissues tested. Furthermore, expression of OBF-1 in HeLa cells selectively stimulates the activity of a natural immunoglobulin promoter in an octamer site-dependent manner. Thus, OBF-1 has all the properties expected for a B cell-specific transcriptional coactivator protein.

The congenital disorders of glycosylation (CDG) are characterized by defects in N-linked glycan biosynthesis that result from mutations in genes encoding proteins directly involved in the glycosylation pathway. Here we describe two siblings with a fatal form of CDG caused by a mutation in the gene encoding COG-7, a subunit of the conserved oligomeric Golgi (COG) complex. The mutation impairs integrity of the COG complex and alters Golgi trafficking, resulting in disruption of multiple glycosylation pathways. These cases represent a new type of CDG in which the molecular defect lies in a protein that affects the trafficking and function of the glycosylation machinery.

Summary Streptococcus thermophilus

The transcriptional coactivator OBF‐1, which interacts with Oct‐1 and Oct‐2 and the octamer site DNA, has been shown to be critical for development of a normal immune response and the formation of germinal centers in secondary lymphoid organs. Here we have identified the RING finger protein Siah‐1 as a protein interacting specifically with OBF‐1. This interaction is mediated by the C‐terminal part of Siah‐1 and by residues in the N‐terminus of OBF‐1, partly distinct from the residues required for formation of a complex with the Oct POU domains and the DNA. Interaction between Siah‐1 and OBF‐1 leads to downregulation of OBF‐1 protein level but not mRNA, and to a corresponding reduction in octamer site‐dependent transcription activation. Inhibition of the ubiquitin‐proteasome pathway in B cells leads to elevated levels of OBF‐1 protein. Furthermore, in immunized mice, OBF‐1 protein amounts are dramatically increased in primary activated B cells, without concomitant increase in OBF‐1 mRNA. These data suggest that Siah‐1 is part of a novel regulatory loop controlling the level of OBF‐1 protein in B cells.