Biology subjects

Ali-Ahmad, A.

Publications and source records attributed to Ali-Ahmad, A..

3 recordsLinked to original sources

bioRxiv2025

Non-nucleosomal (CENP-A/H4)2 - DNA complexes as a possible platform for centromere organization

The centromere is a part of the chromosome that is essential for the even segregation of duplicated chromosomes during cell division. It is epigenetically defined by the presence of the histone H3 variant CENP-A. CENP-A associates specifically with a group of 16 proteins that form the centromere-associated network of proteins (CCAN). In mitosis, the kinetochore forms on the CCAN to connect the duplicated chromosomes to the microtubules protruding from the cell poles. Previous studies have shown that CENP-A replaces H3 in nucleosomes, and recently the structures of CENP-A-containing nucleosomes in complex with CCANs have been revealed, but they show only a limited interaction between CCANs and CENP-A. Here, we report the cryoEM structure of 2x(CENP-A/H4)2-di-tetramers assembled on DNA in the absence of H2A/H2B histone dimer and speculate how (CENP-A/H4)2-tetramers and -di-tetramers might serve as a platform for CCAN organization. Main pointsO_LICENP-A/H4 in the absence of H2A/H2B dimers associate on DNA to form di-tetrasomes C_LIO_LICENP-A/H4 di-tetrasomes are stable but highly flexible structures resembling nucleosomes C_LIO_LICENP-A/H4 tetrasomes bind CENP-N/L and can serve as platform for centromere organisation C_LI

biochemistry↗

bioRxiv2024

Insects evolved a monomeric histone-fold domain in the CENP-T protein family

The histone fold domain (HFD) is a conserved protein interaction module that requires stabilization through a handshake interaction with an HFD partner. All HFD proteins known to date form obligate dimers to shield the extensive hydrophobic residues along the HFD. Here, we find that the lepidopteran kinetochore protein CENP-T is soluble as a monomer. We attribute this stability to a structural rearrangement, which leads to the repositioning of the HFD helix 3. This brings a conserved two-helical extension closer to the histone fold, where it takes over the position and function of the CENP-T partner CENP-W. This change has no effect on the DNA binding ability of the lepidopteran CENP-T. Our analysis suggests that the monomeric HFD originated in the last common ancestor of insects, with a possible second independent origin in acariformes, both of which lack CENP-W. Our study highlights an unexpected structural variation in a protein module as conserved and optimized as the HFD providing a unique perspective on the evolution of protein structure and the forces driving it.

cell biology↗

bioRxiv2019

CryoEM structure of human CENP-A nucleosome alone and in complex with central domain of human CENP-C

Centromeres are defined epigenetically by nucleosomes containing the histone H3 variant CENP-A, upon which the constitutive centromere-associated network of proteins (CCAN) is built. CENP-C, is considered to be a central organizer of the CCAN. We provide new molecular insights into the structure of CENP-A nucleosomes, in isolation and in complex with the CENP-C central region (CENP-CCR), the main CENP-A binding module of CENP-C. We establish that the short N-helix of CENP-A promotes DNA flexibility at the nucleosome ends, independently of the sequence it wraps.\n\nFurthermore, we show that, in vitro, two regions of CENP-C (CENP-CCR and CENP-Cmotif) both bind exclusively to the CENP-A nucleosome. We find CENP-CCR to bind with high affinity due to an extended hydrophobic area made up of CENP-AV532 and CENP-AV533. Importantly, we identify two key conformational changes within the CENP-A nucleosome upon CENP-C binding. First, the loose DNA wrapping of CENP-A nucleosomes is further exacerbated, through destabilization of the H2A N-terminal tail. Second, CENP-CCR rigidifies the N-terminal tail of H4 in the conformation favoring H4K20 monomethylation, essential for a functional centromere.\n\nSynopsisCENP-A nucleosomes have a short N helix incompatible with complete DNA wrapping, independently of DNA sequence. CENP-C binds exclusively to CENP-A nucleosomes and this binding induces conformational changes that further differentiate CENP-A-containing from canonical nucleosomes.\n\nO_LICENP-C binds CENP-A nucleosomes specifically\nC_LIO_LIDNA ends of the CENP-A nucleosome are further unwrapped in the CENP-A/CENP-C complex, due to flexible H2A C-terminal tails\nC_LIO_LIThe N-terminal tail of H4 adopts a conformation favored for centromere specific H4K20 monomethylation when CENP-C is bound\nC_LI

biochemistry↗