Regulation of V(D)J recombination activator protein RAG-2 by phosphorylation

Definition of a core region of RAG-2 that is functional in V(D)J recombination

The products of the RAG-1 and RAG-2 genes cooperate to allow V(D)J recombination in lymphoid and non-lymphoid cells. As one step toward understanding the role of RAG-2, we have constructed mutated RAG-2 genes and examined their ability to support recombination of plasmid substrates in a fibroblast cell line. The mutations define essential and dispensable parts of the RAG-2 gene. Mutations in the N-terminal part eliminate almost all activity. In the central region of the protein, some but not all local alterations still allow recombination. On the other hand, proteins with large deletions from the C-terminal end, including one truncated by 25%, still retain activity, even though this part of the protein is highly conserved between species. Similar results were obtained with substrates that retain either a signal joint or a coding joint, or perform an inversion. Thus all basic features of V(D)J joining are retained in a RAG-2 protein with only the first 75% of the sequence.

T cell receptor beta chain gene rearrangement and selection during thymocyte development in adult mice

The identification is made in normal mice of the stages in T cell development at which the rearranged beta chain of the T cell receptor (TCR) is utilized to promote T cell maturation, independent of the TCR alpha chain. In addition, evidence is provided that utilization of beta chains in T cell progenitors does not preclude differentiation to TCR gamma delta + T cells. This is consistent with the view that an initial consequence of beta chain expression by early thymocytes is clonal expansion, increasing the size of the pool of useful precursors. This allows the proposal to be made that allelic exclusion may be a byproduct of cell cycle regulation during early thymocyte differentiation, which may in turn explain why the efficiency of allelic exclusion varies at different TCR or immunoglobulin loci.

A regulatory role for recombinase activating genes, RAG-1 and RAG-2, in T cell development

RAG-1 and RAG-2 are developmentally regulated genes that are essential for the assembly of antigen receptors in lymphoid cells. Here we describe transgenic mice that carry RAG-1 and RAG-2 under the control of the proximal lck promoter. Persistent expression of RAG-1 and RAG-2 was associated with incomplete thymopoiesis and profoundly compromised cellular immunity. In addition, RAG transgenic mice rapidly developed lymphadenopathy, splenomegaly, and lymphocytic perivascular infiltrates. These effects required both RAG-1 and RAG-2, since mice that carried either gene exclusively were indistinguishable from wild-type controls. We propose that in addition to a previously documented role in V(D)J recombination, RAG-1 and RAG-2 expression must be properly regulated for completion of normal T cell development

The B cell antigen receptor in B-cell development

During B-cell development, immature and mature forms of the B cell antigen receptor complex are deployed in a regulated fashion; thus, B cell antigen receptor complexes play essential roles in the transit of cells through ontogeny. The past year has seen progress in our understanding of how antigen receptor gene assembly is controlled and in defining the requirements for antigen receptor mediated signaling at specific developmental stages. The discovery that a defective protein tyrosine kinase is responsible for X-linked agammaglobulinemia in man and X-linked immunodeficiency in the mouse is particularly interesting, as it may provide the means to link a specific intracellular signaling pathway with a particular step in B-cell development.

Cell cycle regulation of V(D)J recombination-activating protein RAG-2

The antigen receptors of B and T lymphocytes are encoded in multiple germ-line DNA segments that are joined during lymphocyte development. The recombination-activating proteins RAG-1 and RAG-2 are both essential for this process, termed V(D)J rearrangement. Phosphorylation of the RAG-2 protein at Thr-490 by one or more cyclin-dependent kinases is associated with its rapid degradation. In an immature B-cell line and in normal thymocytes, RAG-2 protein accumulates preferentially in the G0/G1 phases of the cell cycle and declines by at least 20-fold before cells enter S phase. The amount of RAG-2 protein remains low throughout the S, G2, and M phases. The amount of RAG-1 protein shows considerably less fluctuation. The variation in RAG-2 protein is likely to be established, at least in part, by a posttranscriptional mechanism. These observations suggest that V(D)J rearrangement occurs entirely or preferentially within G0/G1.

An active v-abl protein tyrosine kinase blocks immunoglobulin light-chain gene rearrangement

Lymphoid cells transformed by Abelson murine leukemia virus have provided one of the classic models for study of early B-cell development and immunoglobulin rearrangement. Most of these cells have rearranged their heavy-chain locus but not their light chain genes, suggesting that an active v-abl protein interferes with this differentiation step. To test this hypothesis, light-chain gene structure was examined in pre-B cells transformed by temperature-sensitive mutants of the Abelson virus and in derivatives that survive at the nonpermissive temperature because they express a human BCL-2 gene. Our studies reveal that inactivation of the v-abl protein tyrosine kinase triggers high-frequency rearrangement of kappa and lambda light-chain genes. These events are accompanied by marked increases in the expression of RAG-1 and RAG-2 RNAs. These increases occur in the absence of protein synthesis but are dependent on inactivation of the v-abl protein tyrosine kinase. As documented in the accompanying paper (Klug et al., this issue), an active v-abl protein also suppresses the activity of NF-kappa B/rel and expression controlled by the kappa intron enhancer. Together these data demonstrate that the v-abl protein specifically interferes with light-chain gene rearrangement by suppressing at least two pathways essential for this stage of B-cell differentiation and suggest that tyrosine phosphorylation is important in regulating RAG gene expression.

DNA damage and the DNA-activated protein kinase

DNA-activated protein kinase (DNA-PK) is a nuclear serine/threonine protein kinase that is activated in vitro by DNA fragments. The cellular targets of DNA-PK are nuclear, DNA-binding, regulatory proteins including Sp1, Fos, Jun, Myc, the tumor suppressor protein p53, and RNA polymerase II. These characteristics suggest a role for DNA-PK in coordinating nuclear processes and as a modulator of checkpoint mechanisms activated by DNA damage.

A V(D)J recombinase-inducible B-cell line: role of transcriptional enhancer elements in directing V(D)J recombination

Rapid analysis of mechanisms that regulate V(D)J recombination has been hampered by the lack of appropriate cell systems that reproduce aspects of normal prelymphocyte physiology in which the recombinase is activated, accessible antigen receptor loci are rearranged, and rearrangement status is fixed by termination of recombinase expression. To generate such a system, we introduced heat shock-inducible V(D)J recombination-activating genes (RAG) 1 and 2 into a recombinationally inert B-cell line. Heat shock treatment of these cells rapidly induced high levels of RAG transcripts and RAG proteins that were accompanied by a parallel induction of V(D)J recombinase activity, strongly suggesting that RAG proteins have a primary role in V(D)J recombination. Within hours after induction, these cells began to rearrange chromosomally integrated V(D)J recombination substrates but only if the substrates contained an active transcriptional enhancer; substrates lacking an enhancer were not efficiently rearranged. Activities necessary to target integrated substrates for rearrangement were provided by two separate lymphoid-specific transcriptional enhancers, as well as an active nonlymphoid enhancer, unequivocally demonstrating that such elements enhance both transcription and V(D)J recombinational accessibility.

A V(D)J recombinase-inducible B-cell line: role of transcriptional enhancer elements in directing V(D)J recombination

Rapid analysis of mechanisms that regulate V(D)J recombination has been hampered by the lack of appropriate cell systems that reproduce aspects of normal prelymphocyte physiology in which the recombinase is activated, accessible antigen receptor loci are rearranged, and rearrangement status is fixed by termination of recombinase expression. To generate such a system, we introduced heat shock-inducible V(D)J recombination-activating genes (RAG) 1 and 2 into a recombinationally inert B-cell line. Heat shock treatment of these cells rapidly induced high levels of RAG transcripts and RAG proteins that were accompanied by a parallel induction of V(D)J recombinase activity, strongly suggesting that RAG proteins have a primary role in V(D)J recombination. Within hours after induction, these cells began to rearrange chromosomally integrated V(D)J recombination substrates but only if the substrates contained an active transcriptional enhancer; substrates lacking an enhancer were not efficiently rearranged. Activities necessary to target integrated substrates for rearrangement were provided by two separate lymphoid-specific transcriptional enhancers, as well as an active nonlymphoid enhancer, unequivocally demonstrating that such elements enhance both transcription and V(D)J recombinational accessibility.