THE MURINE VPREB1 AND VPREB2 GENES BOTH ENCODE A PROTEIN OF THE SURROGATE LIGHT-CHAIN AND ARE COEXPRESSED DURING B-CELL DEVELOPMENT

The surrogate light chain is composed of two polypeptides, VpreB and l ambda 5. In the mouse there are two VpreB genes which are 99% identica l within the coding regions. Extensive restriction enzyme mapping and sequencing of these two genes showed that only the coding region and i mmediate 5' and 3' flanking sequences exhibited such high homology. Mo re distal sequences have diverged considerably. The region 5' of the r espective gene directed transcription of a reporter gene in a pre-B ce ll line, indicating that it contained promoter, and perhaps enhancer f unction. The VpreB2 gene is functional, as it directed the production in COS cells of a 16-kDa protein that assembled with lambda 5 and was recognized by a VpreB-specific monoclonal antibody. Using transfected COS cells expressing either VpreB1 or VpreB2, a PCR assay was develope d to examine the steady state level of transcripts from each gene. Whe n this assay was applied to a number of cell lines representing early stages of B cell differentiation, co-expression of the two genes was o bserved in every case. VpreB1 and VpreB2 were co-expressed in the feta l liver of CB17 mice, where peak expression of each gene occurred at d ays 16-17 of gestation. Similarly, adult bone marrow from several stra ins of mice expressed both genes. In sorted bone marrow cells expressi on of both VpreB genes was detected in pro-B/pre-BI and large pre-BII cells, while the RNA steady state levels were at least 100-fold lower in small pre-BII and immature/mature B cells. Finally, single-cell rev erse transcriptase-polymerase chain reaction on such sorted bone marro w cells detected VpreB1 and VpreB2 expression in at least 30% of all p ro-B/pre-BI cells and large Ig heavy chain, surrogate light chain (pre -B receptor) expressing pre-BII cells. These results demonstrate that the control of expression of the two VpreB genes overlaps during devel opment. They suggest that both VpreB1 and VpreB2 polypeptides can asse mble with lambda 5 and mu to form pre-B cell receptor complexes.