Mammalian phosphoinositide-specific phospholipase C enzymes (PI-PLC) act as signal transducers that generate two second messengers, inositol-1,4,5-trisphosphate and diacylglycerol. The 2.4-A structure of phospholipase C delta 1 reveals a multidomain protein incorporating modules shared by many signalling proteins. The structure suggests a mechanism for membrane attachment and Ca2+-dependent hydrolysis of second-messenger precursors. The regulation and reversible membrane association of PI-PLC may serve as a model for understanding other multidomain enzymes involved in phospholipid signalling.

Studies of inositol lipid-specific phospholipase C (PLC) have elucidated the main regulatory pathways for PLCbeta and PLCgamma but the regulation of PLCdelta isoenzymes still remains obscure. Here we demonstrate that an increase in Ca2+ ion concentration within the physiological range (0.1-10 microM) is sufficient to stimulate PLCdelta1, but not PLCgamma1 and PLCbeta1, to hydrolyse cellular inositol lipids present in permeabilized cells. The activity of PLCdelta1 is further enhanced in the presence of phosphatidylinositol transfer protein (PI-TP). Both full activation by Ca2+ ions and stimulation in the presence of PI-TP require an intact PH domain involved in the membrane attachment of PLCdelta1. The physiological implication of this study is that PLCdelta1 could correspond to a previously uncharacterized PLC responsible for Ca2+ ion-stimulated inositol lipid hydrolysis observed in many cellular systems.

The pleckstrin homology (PH) domain of phosphatidylinositol-specific phospholipase C-␦1 (PLC-␦1) binds to both D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3 ) and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P 2 ) with high affinities. We have previously identified a region rich in basic amino acids within the PH domain critical for ligand binding (Yagisawa, H., Hirata, M., Kanematsu, T., Watanabe, Y., Ozaki, S., Sakuma, K., Tanaka, The pleckstrin homology (PH) 1 domain has been initially identified as a region of sequence similarity of about 120 amino acid residues (3, 4). At the last count, more than 100 proteins have been reported to have this sequence motif; many of these proteins are involved in cellular signaling and cytoskeletal functions (5-8). Studies of several PH domains using x-ray crystallography or NMR (9 -12) revealed a conserved structural module containing a seven-stranded ␤-sandwich formed by two orthogonal antiparallel ␤-sheets and a C-terminal amphiphilic ␣-helix. The loops between the ␤-strands, particularly the ␤1/ ␤2, ␤3/␤4, and ␤6/␤7, differ greatly in length and sequence. Each PH domain is electrostatically polarized, and the most variable loops coincide with the positively charged face.By analogy with other conserved structural modules (e.g. SH2 and SH3 domains), it has been proposed that the PH domain could be involved in signaling by mediating intermolecular interactions. Consequently, a great effort has been made to identify ligand(s) for this domain. Although there are examples of PH domains involved in protein-protein interactions (e.g. binding of G␤␥ by ␤-adrenergic receptor kinase PH domain (13) or recognition of phosphotyrosine by Sch PH/PTB domain (14)) there is an increasing evidence that many PH domains interact with different inositol lipids and inositol phosphates (15,16). In this respect, the PH domain of phospholipase C-␦1 (PLC-␦1) has been studied most extensively. Determination of association constants for different inositol lipids and their head groups (1, 2, 17), and relative abundance of these phospholipids in the cell identified PtdIns(4,5)P 2 as a potentially important physiological ligand (18,19). Ins(1,4,5)P 3 can bind to the same binding pocket as the head group of

The structural requirements of phospholipase C delta 1 for interaction with the plasma membrane were analysed by immunofluorescence after microinjection into living cells. Microinjection of deletion mutants revealed that the region required for membrane attachment and binding of inositol 1,4,5-trisphosphate in vitro corresponded to the pleckstrin homology domain, a structural module described in more than 90 proteins.

Homozygous mice overexpressing Claudin-6 (Cldn6) exhibit a perturbation in the epidermal differentiation program leading to a defective epidermal permeability barrier (EPB) and dehydration induced death ensuing within 48 h of birth [Turksen, K., Troy, T.C., 2002. Permeability barrier dysfunction in transgenic mice overexpressing claudin 6. Development 129, 1775-1784]. Their heterozygous counterparts are also born with an incomplete EPB; however, barrier formation continues after birth and normal hydration levels are achieved by postnatal day 12 allowing survival into adulthood. Heterozygous Inv-Cldn6 mice exhibit a distinct coat phenotype and histological analysis shows mild epidermal hyperkeratosis. Expression of K5 and K14 is aberrant, extending beyond the basal layer into the suprabasal layer where they are not co-localized suggesting that their expression is uncoupled. There is also atypical K17 and patchy K15 expression in the basal layer with no K6 expression in the interfollicular epidermis; together with marked changes in late differentiation markers (e.g. profilaggrin/filaggrin, loricrin, transglutaminase 3) indicating that the normal epidermal differentiation program is modified. The expression compartment of various Cldns is also perturbed although overall protein levels remained comparable. Most notably induction of Cldn5 and Cldn8 was observed in the Inv-Cldn6 epidermis. Heterozygous Inv-Cldn6 animals also exhibit subtle alterations in the differentiation program of the hair follicle including a shorter anagen phase, and altered hair type distribution and length compared to the wild type; the approximately 20% increase in zig-zag hair fibers at the expense of guard hairs and the approximately 30% shorter guard hairs contribute to coat abnormalities in the heterozygous mice. In addition, the transgenic hair follicles exhibit a decreased expression of K15 as well as some hair-specific keratins and express Cldn5 and Cldn18, which are not detectable in the wild type. These data indicate that Cldn6 plays a role in the differentiation processes of the epidermis and hair follicle and supports the notion of a link between Cldn regulation and EPB assembly/maintenance as well as the hair cycle.