Talin for presynaptic endocytosis

plicing factors, usually associated with RNA polymerase II transcripts, have a temporary home near sites of RNA polymerase I transcription, as shown on page 51 by Bubulya et al. During interphase, premRNA splicing factors such as the serine arginine–rich (SR) proteins reside in nuclear speckles, along with other pre-mRNA processing proteins. The speckles disassemble during mitosis, and then reform in G1. By viewing speckle reformation, the group finds that SR proteins first make a side trip and gather in nucleolar organizing region–associated patches (NAPs), around areas where rDNA is transcribed. Inhibiting mRNA transcription prolonged the life of NAPs and also caused SR proteins to accumulate near nucleoli in interphase cells. Without their pre-mRNA transcript targets available , SR protein traffic was backed up at a location it would otherwise move through rapidly. NAPs contained hypophosphorylated SR proteins, which may self-associate and thus accumulate in patches. NAPs also contained an SR protein kinase, whose activity may release the splicing factors to transcription sites, although it is not clear how active RNA polymerase II might signal to effect such a change. At NAPs, the SR proteins are se-questered from other splicing factors. Their isolation during telophase may be needed for modification or SR complex assembly in reforming daughter nuclei. Why SR proteins choose NAPs for their meeting point remains to be determined. S Splicing factors (green) are found around rDNA transcription sites (red). Talin for presynaptic endocytosis alin links integrins to the actin cytoskeleton at focal adhesions. On page 43, Morgan et al. show that talin also links synaptic activity to actin rearrangements. Synapses are specialized sites of cell adhesion that were recently shown to hold talin. Talin interacts with PIPKI ␥ , a PI(4,5)P 2-producing enzyme, which in turn regulates talin and other actin regulatory proteins. The importance of this interaction is now shown at synapses, where PI(4,5)P 2 also controls clathrin coat dynamics. The authors interfered with talin–PIPKI ␥ interactions in the ax-ons of giant lamprey neurons. The interference inhibited the recycling of synaptic vesicles on the pre-synaptic side. Clathrin-coated pits formed but did not bud off the plasma membrane. These changes were associated with reduced synaptic actin networks. The defects are probably due to low levels of PI(4,5)P 2 resulting from a block in PIPKI ␥ recruitment to the membrane. The same lab has also recently shown genetically that PI(4,5)P 2 is necessary for synaptic vesicle trafficking (Di Pa-olo …