To understand how nuclear processes involving DNA are regulated, knowledge of the determinants of chromatin condensation is required. From recent structural studies it has been concluded that the formation of the 30-nm chromatin fiber does not require the linker histone. Here, by comparing the linker histone-dependent compaction of long, reconstituted nucleosome arrays with different nucleosome repeat lengths (NRLs), 167 and 197 bp, we establish that the compaction behavior is both NRL-and linker histone-dependent. Only the 197-bp NRL array can form 30-nm higher-order chromatin structure. Importantly for understanding the regulation of compaction, this array shows a cooperative linker histone-dependent compaction. The 167-bp NRL array displays a limited linker histone-dependent compaction, resulting in a thinner and topologically different fiber. These observations provide an explanation for the distribution of NRLs found in nature.30-nm fiber ͉ electron microscopy ͉ heterochromatin ͉ nucleosome array reconstitution ͉ sedimentation velocity analysis D uring the past decade it has emerged that the packaging of eukaryotic DNA by histones into chromatin is a key regulator of nuclear processes involving DNA, such as transcription and replication. Although the structure of the first level of DNA folding, the nucleosome core, is known at atomic resolution (1, 2), the structure of the second level of folding, whereby a string of nucleosomes folds into a fiber with an approximate diameter of 30 nm (the 30-nm chromatin fiber) remains undetermined (3). Early evidence for the presence of a 30-nm chromatin fiber in vivo came from EM analysis of Balbiani ring genes in Chironomus tentans (4) and x-ray diffraction studies of nuclei that show spacings of 30-40 nm (5).The structure of the 30-nm chromatin fiber is controversial (reviewed in ref.3). Recent structural analyses using in vitroreconstituted model nucleosome arrays based on the strong 601 DNA nucleosome positioning sequence (6) have led to the proposal of two models for the 30-nm chromatin fiber that differ in topology, dimension, and nucleosome packing density. The first model was constructed by using the crystal structure at 9 Å of a tetra-nucleosome core array and is of a two-start helix type (7). It is based on a zigzag arrangement of nucleosome cores that stack on top of each other and has a 24-to 25-nm diameter with a packing density of five to six nucleosomes per 11 nm. The second model was derived from tight constraints obtained from measurements of the physical dimensions of long nucleosome arrays visualized by EM (8). It is of the one-start helix type in which nucleosomes from adjacent gyres are interdigitated. It has a diameter of 34 nm with a packing density of 11 nucleosomes per 11 nm.The key difference between the two 30-nm chromatin fiber models is that the interdigitated model was derived from nucleosome arrays saturated with linker histone, whereas the two-start helix model was derived from a tetra-nucleosome core array in the absence of linker histone...

Retroviral Gag polyproteins drive virion assembly by polymerizing to form a spherical shell that lines the inner membrane of nascent virions. Deletion of the nucleocapsid (NC) domain of the Gag polyprotein disrupts assembly, presumably because NC is required for polymerization. Human immunodeficiency virus type 1 NC possesses two zinc finger motifs that are required for specific recognition and packaging of viral genomic RNA. Though essential, zinc fingers and genomic RNA are not required for virion assembly. NC promiscuously associates with cellular RNAs, many of which are incorporated into virions. It has been hypothesized that Gag polymerization and virion assembly are promoted by nonspecific interaction of NC with RNA. Consistent with this model, we found an inverse relationship between the number of NC basic residues replaced with alanine and NC's nonspecific RNA-binding activity, Gag's ability to polymerize in vitro and in vivo, and Gag's capacity to assemble virions. In contrast, mutation of NC's zinc fingers had only minor effects on these properties.

A molecular model that provides a framework for interpreting the wealth of functional information obtained on the E. coli F-ATP synthase has been generated using cryo-electron microscopy. Three different states that relate to rotation of the enzyme were observed, with the central stalk’s ε subunit in an extended autoinhibitory conformation in all three states. The Fo motor comprises of seven transmembrane helices and a decameric c-ring and invaginations on either side of the membrane indicate the entry and exit channels for protons. The proton translocating subunit contains near parallel helices inclined by ~30° to the membrane, a feature now synonymous with rotary ATPases. For the first time in this rotary ATPase subtype, the peripheral stalk is resolved over its entire length of the complex, revealing the F1 attachment points and a coiled-coil that bifurcates toward the membrane with its helices separating to embrace subunit a from two sides.DOI: http://dx.doi.org/10.7554/eLife.21598.001

The single protein caveolar coat complex comprises only cavins and caveolins, coats the caveolar bulb, and is probably responsible for creating caveolae.