Nuclear transport

Binding of the Mex67p/Mtr2p heterodimer to FXFG, GLFG, and FG repeat nucleoporins is essential for nuclear mRNA export. Strässer, K., Bassler, J. & Hurt, E. J. Cell Biol. 150 , 695?706 (2000). [ PubMed]

Messenger RNA export through the nuclear pore involves a number of nucleoporins and the Mex67p/Mtr2p complex, but its mechanism is not well understood. Hurt and colleagues show that Mex67p and Mtr2p bind as a heterodimer to RNA and also to several conserved nucleoporin motifs. They propose that transport through the pore is directed by sequential interactions with first the Nup82 and then the Nup116 complex. This mechanism resembles that used by importin family members to transport proteins through the nuclear pore.

Cell polarity

CHE-14, a protein with a sterol-sensing domain, is required for apical sorting in C. elegans ectodermal epithelial cells.  Michaux, G. et al. Curr. Biol. 10 , 1098?1107 (2000). [ Contents page]

It's a mystery how trafficking of proteins to the apical or basolateral membranes of polarized cells is controlled, but evidence implicates proteins containing sterol-sensing domains. Here, Michaux and colleagues characterize CHE-14, a new member of the Patched family of sterol-sensing proteins, in Caenorhabditis elegans. CHE-14 is most closely related to Dispatched, a Drosophila protein with 12 putative transmembrane domains that is involved in releasing the secreted signalling molecule Hedgehog from cells. CHE-14 mutants accumulate vesicles at the apical membranes of epithelial cells, and CHE-14 tagged with green fluorescent protein rescues the mutant phenotype and localizes to apical membranes. Deletion of the predicted extracellular loops and transmembrane domains, including the sterol-sensing domain, abolishes its ability to rescue the secretion phenotype, whereas the predicted cytoplasmic loops seem dispensable. The authors propose a model in which CHE-14 and its close relative Dispatched are required for exocytosis, whereas Patched and the Niemann?Pick C protein ? another sterol-sensing protein ? are required for endocytosis.

Angiogenesis

Genes expressed in human tumor endothelium. St. Croix, B. et al. Science 289 , 1197?1202 (2000). [ PubMed]

To survive and grow, tumours need their own supply of blood. They produce factors to stimulate the formation of new blood vessels, but does the endothelial lining of these vessels differ from that in vessels from healthy tissues? This paper indicates that they do ? and dramatically so. The authors compare gene-expression profiles in endothelium derived from normal and tumour tissue, and find that of the 170 transcripts predominantly expressed in the endothelium, 79 are differentially expressed.