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Activation of tyrosine kinase receptors in epithelial cells results in the rapid disassembly of E-cadherin-mediated cell–cell adhesions. New research has identified Hakai, an E3-ubiquitin-ligase related to Cbl that binds E-cadherin in a tyrosine phosphorylation-dependent manner. By promoting the endocytosis and dynamic recycling or destruction of E-cadherin complexes, Hakai may control epithelial–mesenchymal transitions under physiological and pathological conditions.
Human herpesvirus 8 (HHV-8), an agent associated with Kaposi's sarcoma, binds to integrin α3β1 through its gB envelope glycoprotein, and thus gains entry into human fibroblasts. Based on an analogy with other microbial pathogens, integrin interaction with HHV-8 may induce signalling events that promote cell entry and perhaps facilitate disease progression.
Integrins are receptors for extracellular matrix proteins that engage in reciprocal crosstalk with growth factor receptors. Recent work identifies a unique mechanism for the regulation of growth factor receptor phosphorylation by integrins, indicating multiple ways of achieving cooperation between these major signalling systems.
Small GTPases of the Rab family are essential for the control of membrane transport between intracellular compartments. Recent work has shown that on melanosome membranes, Rab27a initiates the formation of a receptor complex that allows the recruitment of the actin-based motor myosin Va. This study provides a molecular basis for several pathologies that result in pigmentation defects in both mouse and humans.
Mutations in either of two polycystin genes can cause kidney failure, but controversy remains regarding the cellular localization and function of the protein products. Polycystin-2 may be a calcium release channel located within the endoplasmic reticulum (ER), and yet may be physically linked to polycystin-1 in the surface membrane.
The assembly of the DNA helicase at replication origins is crucial in initiating DNA synthesis. This process requires the conserved protein Cdt1. Here, a new study identifies a functional homologue of Cdt1 in Saccharomyces cerevisiae. The regulation of its activity reveals an alternative way to assemble prereplicative complexes (pre-RCs) and regulate origin function.
The positioning of a gene within the nucleus is thought to help regulate its transcriptional state. An example is yeast telomeres, which have a propensity to cluster at the nuclear periphery and suppress subtelomeric genes. With a membrane anchoring technique, new data indicate that there may be a second class of perinuclear silencing sites, which require pore-associated myosin-like proteins to establish repression.
The Arp2/3 complex has emerged as a key regulator of signal-dependent changes in the actin cytoskeleton. Recent biochemical and structural studies provide a wealth of information about how the assembly is regulated by WASP family members and actin filaments, and raise important new questions about this cellular machine.
The formin family of proteins have been implicated in regulation of cell polarity and cytoskeletal function in fungal and animal cells, but the manner in which they affect these processes has been mysterious. Two new studies report that formins in budding yeast are specifically required for the assembly of bundles of parallel actin filaments known as cables.
Early endosomes are the first sorting station from which endocytosed materials are targeted to various intracellular destinations. Recent work has identified the FYVE-domain protein rabenosyn-5 as a bifunctional effector of the GTPases rab5 and rab4, physically connecting entry and recycling sites on early endosomes.
Integrins are cell surface receptors for extracellular matrix proteins and, as such, regulate adhesive cell–matrix interactions. New findings indicate that the β4 integrin subunit can also function as a signalling effector for the activated receptor tyrosine kinase Met and enhance invasive growth independent of its adhesive role.
At least one member of the Frizzled (Fz) family of receptors, together with the downstream signalling protein Dishevelled, participates in several distinct, yet closely related signal transduction pathways. Activation of the correct signal transduction pathway is critical. Here, a new study suggests that the transmembrane protein Strabismus participates in this decision.
