Isolation of a cDNA clone encoding a novel membrane protein expressed in lymphocytes

CD45-associated protein is not essential for the regulation of antigen receptor-mediated signal transduction

CD45 is a transmembrane protein tyrosine phosphatase required for signaling through the T-and B-cell antigen receptors. In lymphocytes, CD45 interacts with CD45-associated protein (CD45AP), a 32 000 Mr phosphoprotein, through their respective transmembrane domains. To determine whether CD45AP affects the ability of CD45 to regulate antigen receptor signaling, CD45AP-deficient mice were generated. Thymocyte development was grossly normal. Moreover, the cellularity of the thymus and spleens were normal. CD45 expression on thymocytes and splenocytes, ascertained by flow cytometry, was comparable between CD45AP-deficient mice and littermate controls. In contrast to a previous report (Matsuda et al., J. Exp. Med. 1998 187: 1863 - 1870). CD45AP-deficient and normal thymocytes and splenocytes proliferated similarly in response to various mitogens or antigen receptor cross-linking. Furthermore, thymocyte CD45-associated p56(lck) kinase activity was similar between CD45AP-deficient and normal cells. We conclude that CD45AP is not essential for the regulation of Src-family kinase activity by CD45.

Disruption of lymphocyte function and signaling in CD45-associated protein-null mice

CD45-AP specifically associates with CD45, a protein tyrosine phosphatase essential for lymphocyte differentiation and antigen receptor-mediated signal transduction. CD45 is thought to mediate antigen receptor signaling by dephosphorylating regulatory tyrosine residues on Src family protein tyrosine kinases such as Lck. However, the mechanism for regulating CD45 protein tyrosine phosphatase activity remains unclear. CD45-AP-null mice were created to examine the role of CD45-AP in CD45-mediated signal transduction. T and B lymphocytes showed reduced proliferation in response to antigen receptor stimulation. Both mixed leukocyte reaction and cytotoxic T lymphocyte functions of T cells were also markedly decreased in CD45-AP-null mice. Interestingly, the interaction between CD45 and Lck was significantly reduced in CD45-AP-null T cells, indicating that CD45-AP directly or indirectly mediates the interaction of CD45 with Lck. Our data indicate that CD45-AP is required for normal antigen receptor signaling and function in lymphocytes.

Regulation of cell signaling by the protein tyrosine phosphatases, CD45 and SHP-1

An equilibrium between positive and negative regulation of immunoreceptor signaling leads to the proper execution of lymphocyte activation. Tyrosine phosphorylation is the initial event in antigen receptor-induced lymphocyte activation. It is generally accepted that protein tyrosine kinases are involved in positive regulation, whereas protein tyrosine phosphatases are important for the negative regulation of tyrosine phosphorylation-dependent processes. However, the interaction between protein tyrosine kinases and protein tyrosine phosphatases is complex. This article discusses the role of two protein tyrosine phosphatases. CD45 and SHP-1, in the regulation of immunoreceptor signaling. SHP-1 acts as a negative regulator for several immunoreceptors, including those for T- and B-cell antigen receptors. The major role of CD45 is in the positive regulation of T- and B-cell antigen receptor signaling.

CD45-associated protein is a lymphocyte-specific membrane protein expressed in two distinct forms

CD45-AP is a recently identified CD45-associated protein. The two proteins interact specifically through their respective transmembrane segments. Northern hybridization analysis of CD45+ T lymphocytes and their CD45- variants demonstrated that the production of CD45-AP and CD45 mRNA is regulated independently. On the other hand, Western blotting analysis indicated that the CD45-AP protein has a shorter half-life in the absence of CD45 in three out of four variants. Similar analysis of various types of leukocytes demonstrated that CD45-AP is expressed in T, B, and pre-B cells, but not in plasma cells or cells of the monocyte/macrophage lineage. Two forms of CD45-AP mRNA exist in all types of CD45-AP-expressing lymphocytes analyzed. One form corresponds to the previously reported CD45-AP cDNA and the other form encodes an additional 12 amino acids at the N terminus. The two CD45-AP proteins are identical in their capacity for specific binding to CD45, but employ different mechanisms for endoplasmic reticulum membrane translocation.

Lineage- and differentiation stage-specific expression of LSM-1 (LPAP), a possible substrate for CD45, in human hematopoietic cells

CD45, a transmembrane tyrosine phosphatase, is found on almost all nucleated hematopoietic cells and plays a crucial role in lymphocyte activation and differentiation. We recently achieved isolation of the human LSM-1 (hLSM-1) gene, whose product is a possible substrate for CD45, and we raised antibodies against the hLSM-1 protein. hLSM-1 expression in hematopoietic cells was examined with Northern and Western blot, fluorescence-activated cell sorter, and immunocytochemical analyses. It was found that in the lymphoid lineage, T and B lymphocytes as well as NK cells expressed LSM-1, whereas terminally differentiated plasma cells did not. As for the myeloid lineage, immature myeloid cells expressed LSM-1, whereas terminally differentiated granulocytes and monocytes did not. In the erythroid lineage, normal erythroblasts expressed very low levels of LSM-1, while erythroid cell lines (K562 and HEL) did not. Megakaryocytes did not express LSM-1. Both CD34+/CD33- and CD34+/CD33+ hematopoietic progenitor cells weakly expressed LSM-1. These results showed that LSM-1 is expressed in a lineage- and differentiation stage-specific fashion.

CD45 protein-tyrosine phosphatase associates with the WW domain-containing protein, CD45AP, through the transmembrane region

CD45 is a transmembrane protein-tyrosine phosphatase required for antigen receptor signaling in lymphocytes. CD45 activates the Src family protein-tyrosine kinases, p56lck and p59fyn, by dephosphorylating a negative regulatory tyrosine in the carboxyl terminus. Immunoprecipitation of CD45 precipitates p56lck and CD45AP. Although the function of CD45AP is unknown, it has been proposed to be an adapter between p56lck and CD45. To assess the ability of CD45AP to function as an adapter, we determined the regions required for the interaction with CD45 by expressing chimeric proteins in HeLa cells. CD45AP has a region similar to a potential protein-protein interaction domain, the WW domain. Surprisingly, this domain was not necessary for the association with CD45. Rather, a 40-amino acid sequence encompassing the putative transmembrane domain of CD45AP was sufficient to mediate binding to CD45. Similarly, a 39-amino acid sequence encompassing the CD45 transmembrane region was sufficient to direct the interaction with CD45AP. Expression of p56lck with CD45AP resulted in an interaction that could only be detected by in vitro kinase reaction, suggesting that the association of p56lck and CD45AP is weak. These data support a model in which CD45AP links CD45 with other proteins but not necessarily p56lck.