Regulation of Cadherin Function by Rho and Rac: Modulation by Junction Maturation and Cellular Context

Cadherins are cell–cell adhesion receptors whose adhesive function requires their association with the actin cytoskeleton via proteins called catenins. The small guanosine triphosphatases (GTPases), Rho and Rac, are intracellular proteins that regulate the formation of distinct actin structures in different cell types. In keratinocytes and in other epithelial cells, Rho and Rac activities are required for E-cadherin function. Here we show that the regulation of cadherin adhesiveness by the small GTPases is influenced by the maturation status of the junction and the cellular context. E-cadherin localization was disrupted in mature keratinocyte junctions after inhibition of Rho and Rac. However, an incubation of 2 h was required after GTPase inhibition, when compared with newly established E-cadherin contacts (30 min). Regarding other cadherin receptors, P-cadherin was effectively removed from mature keratinocytes junctions by blocking Rho or Rac. In contrast, VE-cadherin localization at endothelial junctions was independent of Rho/Rac activity. We demontrate that the insensitivity of VE-cadherin to inhibition of Rho and Rac was not due to the maturation status of endothelial junction, but rather the cellular background: when transfected into CHO cells, the localization of VE-cadherin was perturbed by inhibition of Rho proteins. Our results suggest that the same stimuli may have different activity in regulating the paracellular activity in endothelial and epithelial cells. In addition, we uncovered possible roles for the small GTPases during the establishment of E-cadherin–dependent contacts. In keratinocytes, Rac activation by itself cannot promote accumulation of actin at the cell periphery in the absence of cadherin-dependent contacts. Moreover, neither Rho nor Rac activation was sufficient to redistribute cadherin molecules to cell borders, indicating that redistribution results mostly from the homophilic binding of the receptors. Our results point out the complexity of the regulation of cadherin-mediated adhesion by the small GTPases, Rho and Rac.

Comparison of circumsporozoite proteins from avian and mammalian malarias: biological and phylogenetic implications.

The circumsporozoite (CS) protein of malaria parasites (Plasmodium) covers the surface of sporozoites that invade hepatocytes in mammalian hosts and macrophages in avian hosts. CS genes have been characterized from many Plasmodium that infect mammals; two domains of the corresponding proteins, identified initially by their conservation (region I and region II), have been implicated in binding to hepatocytes. The CS gene from the avian parasite Plasmodium gallinaceum was characterized to compare these functional domains to those of mammalian Plasmodium and for the study of Plasmodium evolution. The P. gallinaceum protein has the characteristics of CS proteins, including a secretory signal sequence, central repeat region, regions of charged amino acids, and an anchor sequence. Comparison with CS signal sequences reveals four distinct groupings, with P. gallinaceum most closely related to the human malaria Plasmodium falciparum. The 5-amino acid sequence designated region I, which is identical in all mammalian CS and implicated in hepatocyte invasion, is different in the avian protein. The P. gallinaceum repeat region consists of 9-amino acid repeats with the consensus sequence QP(A/V)GGNGG(A/V). The conserved motif designated region II-plus, which is associated with targeting the invasion of liver cells, is also conserved in the avian protein. Phylogenetic analysis of the aligned Plasmodium CS sequences yields a tree with a topology similar to the one obtained using sequence data from the small subunit rRNA gene. The phylogeny using the CS gene supports the proposal that the human malaria P. falciparum is significantly more related to avian parasites than to other parasites infecting mammals, although the biology of sporozoite invasion is different between the avian and mammalian species.