Fubito Nakatsu

μ1B, a novel adaptor medium chain expressed in polarized epithelial cells1

The apical and basolateral plasma membrane domains of polarized epithelial cells contain distinct sets of integral membrane proteins. Biosynthetic targeting of proteins to the basolateral plasma membrane is mediated by cytosolic tail determinants, many of which resemble signals involved in the rapid endocytosis or lysosomal targeting. Since these signals are recognized by adaptor proteins, we hypothesized that there could be epithelial‐specific adaptors involved in polarized sorting. Here, we report the identification of a novel member of the adaptor medium chain family, named μ1B, which is closely related to the previously described μ1A (79% amino acid sequence identity). Northern blotting and in situ hybridization analyses reveal the specific expression of μ1B mRNA in a subset of polarized epithelial and exocrine cells. Yeast two‐hybrid analyses show that μ1B is capable of interacting with generic tyrosine‐based sorting signals. These observations suggest that μ1B may be involved in protein sorting events specific to polarized cells.

Essential role of S‐adenosylmethionine decarboxylase in mouse embryonic development

Background: S‐Adenosylmethionine decarboxylase (AdoMetDC) is one of the key enzymes involved in the biosynthesis of spermidine and spermine, which are essential for normal cell growth. To examine the role of polyamines in embryogenesis, we carried out targeted disruption of the mouse Amd1 gene, encoding AdoMetDC, to generate mice that can not synthesize spermidine and spermine.

E2A and HEB Activate the Pre-TCRα Promoter During Immature T Cell Development

The pre-TCRα (pTα) is exclusively expressed in immature thymocytes and constitutes the pre-TCR complex with TCRβ, which regulates early T cell differentiation. Despite the recent identification of the pTα enhancer, the contribution of the promoter region, the direct DNA-protein interaction, and the regulation of such interaction along with T cell development have not been investigated. We analyzed the pTα promoter region and identified the critical elements for transcription of the pTα gene. The pTα promoter was found to contain two consecutive E-box elements that are critical for pTα transcription. The E-box elements in the promoter region formed the specific DNA-protein complex that was exclusively observed in immature thymocytes, not in mature thymocytes and T cells. The E proteins in this complex were identified as E2A and HeLa E-box binding protein (HEB), and overexpression of E2A and HEB resulted in activation of the pTα promoter. The binding complex in the consecutive E-boxes in the pTα promoter changed along with T cell development, as a distinct DNA-binding complex was observed in mature T cells. Comparing the E-box regions in the enhancer and the promoter, those in the promoter appear to make a greater contribution to pTα gene transcription.

Cytokine-independent Jak3 Activation upon T Cell Receptor (TCR) Stimulation through Direct Association of Jak3 and the TCR Complex

Jak3 is responsible for growth signals by various cytokines such as interleukin (IL)-2, IL-4, and IL-7 through association with the common γ chain (γc) in lymphocytes. We found that T cells from Jak3-deficient mice exhibit impairment of not only cytokine signaling but also early activation signals and that Jak3 is phosphorylated upon T cell receptor (TCR) stimulation. TCR-mediated phosphorylation of Jak3 is independent of IL-2 receptor/γc but is dependent on Lck and ZAP-70. Jak3 was found to be assembled with the TCR complex, particularly through direct association with CD3ζ via its JH4 region, which is a different region from that for γc association. These results suggest that Jak3 plays a role not only in cell growth but also in T cell activation and represents cross-talk of a signaling molecule between TCR and growth signals.

Genomic structure and chromosome mapping of the genes encoding clathrin-associated adaptor medium chains μ1A (Ap1m1) and μ1B (Ap1m2)

The protein μ1B is a member of the medium chain family of the clathrin-associated adaptor complex and is expressed exclusively in epithelial cells. We determined the genomic structure of previously cloned murine genes for μ1B (Ap1m2) and its closely related homolog, μ1A (Ap1m1). Comparison of their genomic structures revealed that the positions of introns are identical between these two genes, except for the insertion of an additional intron in Ap1m1 (intron 4). By contrast, these structures are different from that of the more distantly related Ap2m1 gene encoding μ2. Taken together with the similarity of amino acid sequences among these genes, the data presented in this study suggest that Ap1m1/2 and Ap2m1 diverged long before the separation of Ap1m1 and Ap1m2, which most likely resulted from a relatively recent gene duplication. We also mapped AP1M2 to human chromosome 19p13.2 and Ap1m2 to the proximal region of mouse chromosome 9. The results are consistent with the fact that these regions are syntenic.