p0071, a member of the armadillo protein family, is most closely related to p120ctn and the plakophilins 1-3. Whereas plakophilins are desmosomal plaque proteins, p120ctn localizes to adherens junctions and interacts with classical cadherins. In contrast, p0071 has been described as a protein with dual localization in adherens junctions and desmosomes depending on the cell type examined. Here we have analyzed the localization of p0071 and its domains in detail. Although by sequence analysis, p0071 is more closely related to the adherens junction proteins p120ctn, ARVCF and δ-catenin, endogenous p0071 associated preferentially with desmosomes in MCF-7 epithelial cells. Overexpressed p0071 localized along cell borders and overlapped only partially with desmosomal markers but colocalized with non-desmosomal cadherins and recruited cadherins to the membrane. The head domain of p0071 was sufficient for desmosomal targeting, whereas the arm repeat domain associated with adherens junctions and enhanced membrane association of classical cadherins. The tail domain localized preferentially to the nucleus and associated with desmosomes. To examine the mechanism underlying this dual localization more closely we determined binding partners of p0071 by using yeast-two-hybrid and mom-targeting assays. These approaches show that the head domain interacted with desmosomal proteins desmocollin 3a and desmoplakin, whereas the armadillo repeat domain binds to non-desmosomal cadherins. Head and armadillo repeat domains both interacted with plakoglobin by binding to different sites. Our data suggest that, in addition to plakoglobin, p0071 is the second armadillo protein present in both types of adhesive junctions and may play a role in regulating crosstalk between adherens junctions and desmosomes.

The isolation of multispecific B cell hybridomas with a variety of anti-idiotype (anti-Id) activities from the lymphoid organs of fetal and neonatal BALB/c mice suggested that the development of the immune system may depend on Id interactions among autologous B cells. In vitro analysis of antibodies secreted by these hybridomas showed extensive sharing of an idiotope defined by the monoclonal antibody FD5-1. Early and timed administration of this antibody during the perinatal period results in a distortion of the phosphorylcholine (PC) and alpha (1----3)dextran (Dex)-specific B cell precursor compartment of the developing repertoire and is reflected by a drastic reduction of antibody responses to these antigens when challenged as adults. These observations provide strong evidence for the involvement of the early appearing multispecific B cells in Id interactions that bring about the uniform development of the normal adult B cell repertoire. Interference with these interactions at critical stages of developmental results in permanent deficiencies in the adult B cell repertoire.

The pre-B-cell cloning assay is an in vitro differentiation system in which B-lymphocyte precursors expand and generate colonies containing immunoglobulin-secreting cells. Analysis of surface characteristics, growth requirements, and kinetics suggested that these cells represent early stages of the B-cell differentiation pathway. Here we describe a modification of the assay, which allowed us to determine the differentiative potential of these clonable pre-B cells. Using a nitrocellulose protein-transfer technique, we studied immunoglobulin light chain expression in colonies derived from fetal mouse liver B-cell precursors; in particular, we explored whether the B-cell precursors are already committed to the expression of a particular light chain gene at the initiation of culture. Our results show that fetal liver-derived B-cell progenitors generate colonies in vitro that secrete K and A light chains at a ratio similar to that found in colonies derived from adult splenic B cells. Further, we document the existence of colonies that are derived from single cells and that simultaneously secrete both types of light chains. This indicates that the progenitors of (Kc + X)-producing colonies are light chain-uncommitted at the initiation ofculture. These cells are able to rearrange their light chain genes in vitro and differentiate along the B-cell pathway to form colonies secreting both K and X chains.

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Almost all somatic cells in adult murine tissues express all three nuclear lamins (A, B, C). Here we demonstrate that cells of the hemopoietic system of the adult mouse are an exception in that they express only lamin B. Thus T and B lymphocytes as well as granulocytes and monocytic cells directly isolated from spleen, thymus, blood or bone marrow do not express lamin A/C but only lamin B. In agreement with this observation the murine hemopoietic cell lines EL4, BW5147, HK22, 70Z/3, SP2/0 and PAI express only lamin B. In immunoblotting experiments used to confirm the immunofluorescence data no lamin A/C expression was detected. However, we noticed that murine lamin B occurs in two isoforms, which can be distinguished immunologically. These results reinforce the idea that a functional nuclear lamina can be formed from lamin B alone. They also pose the question of whether cells lacking lamin A/C are more plastic in their developmental programs than those that express all three lamins.