No abstract

The centromere is the chromosomal locus that ensures fidelity in genome transmission at cell division. Centromere protein A (CENP-A) is a histone H3 variant that specifies centromere location independently of DNA sequence. Conflicting evidence has emerged regarding the histone composition and stoichiometry of CENP-A nucleosomes. Here we show that the predominant form of the CENP-A particle at human centromeres is an octameric nucleosome. CENP-A nucleosomes are very highly phased on α-satellite 171 bp monomers at normal centromeres, and also display strong positioning at neocentromeres. At either type of functional centromere, CENP-A nucleosomes exhibit similar DNA wrapping behavior as octameric CENP-A nucleosomes reconstituted with recombinant components, having looser DNA termini than those on their conventional counterparts containing canonical H3. Thus, the fundamental unit of the chromatin that epigenetically specifies centromere location in mammals is an octameric nucleosome with loose termini.

Data availability. All of the sequencing data is available via Gene Expression Omnibus (GEO) under the accession number GSE117826.

This paper reviews our present knowledge of sphingomyelinases as enzymes, and as enzymes acting on a membrane constituent lipid, sphingomyelin. Six types of sphingomyelinases are considered, namely acidic, secretory, Mg 2+ -dependent neutral, Mg 2+ -independent neutral, alkaline, and bacterial enzymes with both phospholipase C and sphingomyelinase activity. Sphingomyelinase assay methods and speci¢c inhibitors are reviewed. Kinetic and mechanistic studies are summarized, a kinetic model and a general-base catalytic mechanism are proposed. Sphingomyelinase^membrane interactions are considered from the point of view of the in£uence of lipids on the enzyme activity. Moreover, e¡ects of sphingomyelinase activity on membrane architecture (increased membrane permeability, membrane aggregation and fusion) are described. Finally, a number of open questions on the above topics are enunciated.