Esperanto for histones: CENP-A, not CenH3, is the centromeric histone H3 variant

The first centromeric protein identified in any species was CENP-A, a divergent member of the histone H3 family that was recognised by autoantibodies from patients with scleroderma-spectrum disease. It has recently been suggested to rename this protein CenH3. Here, we argue that the original name should be maintained both because it is the basis of a long established nomenclature for centromere proteins and because it avoids confusion due to the presence of canonical histone H3 at centromeres.

Epigenetic engineering shows H3K4me2 is required for HJURP targeting and CENP-A assembly on a synthetic human kinetochore

Kinetochores assemble on distinct 'centrochromatin' containing the histone H3 variant CENP-A and interspersed nucleosomes dimethylated on H3K4 (H3K4me2). Little is known about how the chromatin environment at active centromeres governs centromeric structure and function. Here, we report that centrochromatin resembles K4-K36 domains found in the body of some actively transcribed housekeeping genes. By tethering the lysine-specific demethylase 1 (LSD1), we specifically depleted H3K4me2, a modification thought to have a role in transcriptional memory, from the kinetochore of a synthetic human artificial chromosome (HAC). H3K4me2 depletion caused kinetochores to suffer a rapid loss of transcription of the underlying α-satellite DNA and to no longer efficiently recruit HJURP, the CENP-A chaperone. Kinetochores depleted of H3K4me2 remained functional in the short term, but were defective in incorporation of CENP-A, and were gradually inactivated. Our data provide a functional link between the centromeric chromatin, α-satellite transcription, maintenance of CENP-A levels and kinetochore stability.

Sowing the Seeds of Centromeres

The centromere is a chromatin-based platform that accumulates microtubule-binding proteins that drive chromosome segregation during cell division. Despite their size (on the order of megabases of DNA in mammals) and conserved role, centromeres have the remarkable capacity to leave their usual comfort zone and to reform at a new chromosomal site (1). Although found rarely, these so-called neocentromeres are by most measures bona fide and segregate chromosomes with high fidelity. What accounts for this nomadic behavior?

Temporal control of epigenetic centromere specification

All living organisms require accurate mechanisms to faithfully inherit their genetic material during cell division. The centromere is a unique locus on each chromosome that supports a multiprotein structure called the kinetochore. During mitosis, the kinetochore is responsible for connecting chromosomes to spindle microtubules, allowing faithful segregation of the duplicated genome. In most organisms, centromere position and function is not defined by the local DNA sequence context but rather by an epigenetic chromatin-based mechanism. Centromere protein A (CENP-A) is central to this process, as chromatin assembled from this histone H3 variant is essential for assembly of the centromere complex, as well as for its epigenetic maintenance. As a major determinant of centromere function, CENP-A assembly requires tight control, both in its specificity for the centromere and in timing of assembly. In the last few years, there have been several new insights into the molecular mechanism that allow this process to occur. We will review these here and discuss the general implications of the mechanism of cell cycle coupling of centromere inheritance.

A two-step mechanism for epigenetic specification of centromere identity and function

The basic determinant of chromosome inheritance, the centromere, is specified in many eukaryotes by an epigenetic mark. Using gene targeting in human cells and fission yeast, chromatin containing the centromere-specific histone H3 variant CENP-A is demonstrated to be the epigenetic mark that acts through a two-step mechanism to identify, maintain and propagate centromere function indefinitely. Initially, centromere position is replicated and maintained by chromatin assembled with the centromere-targeting domain (CATD) of CENP-A substituted into H3. Subsequently, nucleation of kinetochore assembly onto CATD-containing chromatin is shown to require either the amino- or carboxy-terminal tail of CENP-A for recruitment of inner kinetochore proteins, including stabilizing CENP-B binding to human centromeres or direct recruitment of CENP-C, respectively.

Basic properties of epigenetic systems: lessons from the centromere

Chromatin-based epigenetic inheritance cooperates with cis-acting DNA sequence information to propagate gene expression states and chromosome architecture across cell division cycles. Histone proteins and their modifications are central components of epigenetic systems but how, and to what extent, they are propagated is a matter of continued debate. Centromeric nucleosomes, marked by the histone H3 variant CENP-A, are stable across mitotic divisions and are assembled in a locus specific and cell cycle controlled manner. The mechanism of inheritance of this unique chromatin domain has important implications for how general nucleosome transmission is controlled in space and time.