Requirment Of DDB1-DCAF2 Axis In Early B Cell Development

B lymphocytes developed from hematopoietic stem cells（HSC）in the liver during mid-to-late fetal development and in the bone marrow after birth.Mature B cells migrate to the peripheral lymphoid organs and exert humoral immunity by secreting different classes of immunoglobulins to maintain the homeostasis.Prepro-B cells are considered to be the earlist B-cell precursor,highly expressing B220 and other B-cell-related molecules.Prepro-B cells then develop into pro-B cells,in which rearrangement of the heavy chain of immunoglobulin occurred.Rearrangedμchain forms a pre-BCR on the surface of pre-B cells together with the surrogate light chain,λchain and Vpre B.The pre-BCR signaling is important for the proliferation and rearrangement of light chain of pre-B cells.DDB1,which is short for DNA damage binding protein 1,forms a complex with DDB2and participates in the repair of DNA damage caused by UV.Recent studies find out that DDB1 is involved in the formation of CRL4 E3 ubiquitin ligase as a bridge,β-propella B binding the N-terminal of CUL4 whileβ-propella A and C binding DCAFs（DDB1-CUL4-associate factor）.DCAFs are responsible for identifying specific substrates in proper place and at proper time.DDB1 is highly expressed in a variety of precursor cells with vital proliferation,including pro-B cells.Knock-out of DDB1 in the hematopoietic stem cells causes great loss of blood cells in mice and these mice die rapidly after birth.In order to study the function of DDB1 in B cells,we crossed Ddb1^fl/fl mice with Mb1^Cremice to induce DDB1-deletion from the late stage of prepro-B cells.Flocytometry analysis of B cells in bone marrow revealed that DDB1 deficiency led to a large reduction of pro-B cells.And the loss of pro-B cells was not due to increased apoptosis rate,nor the failure of immunoglobulin reccombination,but because the cell cycle of pro-B cells was blocked in the G2/M phase.Further mass spectrometry analysis found that DDB1 recruited DCAF2 to regulate the development of pro-B cells.The phenotype of Dcaf2^fl/flMb1^Cre was similar to that of Ddb1^fl/flMb1^Cre mice.CHK1 and P21,proteins involved in the G2/M checkpoint,are two known substrates of DCAF2,accumulated in DDB1-or DCAF2-deficient pro-B cells.In summary,these results suggest that in the B cell development,DDB1 interacts with DCAF2 to identify CHK1 and P21 for ubiquitination,which ensures the proper cell cycle of pro-B cells and guarantee the B cell development.