Abstract. We have detected and begun to characterize a 17-kD centromere-specific protein, CENP-A (Earnshaw, W. C., and N. Rothfield, 1985, Chromosoma., 91:313-321). Sera from several humans with CREST scleroderma autoimmune disease (CREST: calcinosis, Raynaud's phenomenon, esophageal dsymotility, sclerodactyly, and telangiectasia) bind this protein in immunoblot assays of HeLa whole cell or nuclear extracts. We have affinity purified the anti-17-kD centromere protein (anti-CENP-A) specific antibodies from immunoblots of HeLa nuclear protein. The antibodies react with epitopes present on CENP-A derived from humans but apparently do not recognize specific epitopes in either rat or chicken nuclei. Only human nuclear protein is CENP-A positive by immunoblot. Furthermore, human cells show localization of anti-CENP-A antibody to centromeres by immunofluorescence microscopy, whereas rat cells do not. On extraction from the nucleus, CENP-A copurifies with core histones and with nucleosome core particles. We conclude that this centromerespecific protein is a historic-like component of chromatin. The data suggest that CENP-A functions as a centromere-specific core histone.

CENP-A, a centromere-specific 17-kDa protein, has histone-like properties. However, in contrast to the common somatic histones, CENP-A is quantitatively retained in bull spermatozoa, and we have exploited this fact to purify CENP-A to apparent homogeneity. Partial sequence analysis of the purified protein indicates that CENP-A is a distinctive gene product. Some CENP-A sequences are highly similar to regions of histone H3. Other segments of CENP-A are not related to H3 or any other histone. These unrelated segments are presumably involved in loaliing CENP-A to centromeric DNA or in centromere-specific functions of CENP-A..The centromeric region of the chromosome is responsible for its integration into the mitotic spindle and for its proper segregation poleward during anaphase (1)(2)(3)(4). Several proteins have been identified that are associated with the centromere (5-10). For the most part, their functions are presently unknown. Recently, however, Bischoff et al. (11) reported that a 47-kDa centromere-specific autoantigen is highly homologous to the translation product ofRCCI, a gene involved in regulating mammalian chromosome condensation. This antigen may be identical to CENP-D, a 50-to 60-kDa centromeric protein reported to be present in a variety of species (7, 9, 10).CENP-A, a centromere-specific protein of 17 kDa, appears to be associated with kinetochore chromatin or with chromatin closely apposed to the outermost domain of the kinetochore, as judged by indirect immunofluorescence (9) and immunoelectron microscopy (12). This localization suggests it may play a direct role in kinetochore function during mitosis.We have extensively analyzed CENP-A and have shown that it appears to be a core histone by the following criteria: (i) it is extracted with histones from chromatin by dilute mineral acids, (ii) it elutes with histones H3 and H4 in apparent tetrameric complexes during ion exchange (13) or sizing-column chromatography (D.K.P., K.O., and R.L.M., unpublished observations), and, most importantly, (iii) it is a component of highly purified nucleosome core particles (13). CENP-A is exceptional among somatic histones not only because it is centromere specific but also because it is quantitatively retained in chromatin during spermatogenesis in mammals, even in species where other histones are quantitatively replaced by protamines (14).The selective retention of this protein in bull sperm, where other histones are absent, has now allowed us to extract and purify CENP-A to homogeneity. We have also subjected the purified CENP-A from bull sperm to partial sequence analysis and report here that it is a distinctive histone, with sequences similar to those of H3, as well as segments that are not related to histones or homologous to any other known mammalian centromeric protein sequences (11, 15). MATERIALS AND METHODSPurification and Acid Extraction of Nuclei ad ReversePhase Chromatography of Acid-Extracted Proteins. Calf thymus nuclei and bull sperm nuclei were purified and extracted with 1 M NaCl/0.25...

No abstract

We have developed a competitive enzyme-linked immunosorbent assay for solubilized kinetochore components, using human CREST (calcinosis, Raynaud's phenomenon, esophageal dysfunction, sclerodactyly, telangiectasia) scleroderma autoimmune antibodies specific for these kinetochore elements. Using this quantitative assay, we found interphase persistent or "pre-kinetochore" components in low-and moderately high-salt (375 mM salt) extracts of micrococcal nuclease-digested rat liver and chicken erythrocyte nuclei. The release of antigen activity from nuclei under these conditions has been correlated with loss of pre-kinetochore foci as determined by immunofluorescence microscopy. Combined biochemical and competition assay analysis of chicken erythrocyte nuclear extracts indicates that pre-kinetochore components are tightly bound to chromatin of mononucleosome size. The conclusions based on competition assay data are supported by a direct binding assay, which confirms that antigens recognized by CREST sera are present on chromatin. These results raise the possibility that the kinetochore-specific chromosomal antigen(s) we have detected substitutes for "standard" mononucleosome components, such as histone HI. Furthermore, they suggest approaches to the isolation of kinetochore-specific DNA sequences from higher eucaryotes.

Histones are major structural components of the basic repeating subunit of chromatin, the nucleosome (1). Core histone sequences are remarkably conserved during evolution. Nonetheless, a number of histone variants are known (2-4). Synthesis of core histone variants is stage-specific during sea urchin development (3, 4), tissue-specific in mammals (5), and correlated with erythroid differentiation in Friend leukemia cells (6, 7). These observations suggest that nucleosome heterogeneity is related to the functional specialization of specific elements of chromatin. In this context, it may be important that mononucleosomes isolated from Drosophila chromatin contain large amounts of a histone-like protein that is not a standard core histone. This protein was first identified by Alfageme et al. (8), and designated D2, or "Drosophila 2."In this communication, we demonstrate that D2 is nucleosomal and histone-like, yet is readily distinguished from each of the four "standard" core histones. It is also conserved during the evolution of Drosophila. The curious features of D2 raise questions about histone evolution and the organization of histone genes in Drosophila.MATERIALS AND METHODS Purification of Nuclei. D. melanogaster embryo nuclei and adult head nuclei were prepared with three modifications of published procedures (9, 10): (i) buffer A of Hewish and Burgoyne (11), containing 1 mM EDTA, 0.2 mM ethylene glycol bis(f3-aminoethyl ether)-N,N,N'N'-tetraacetic acid (EGTA), and 0.2 mM phenylmethylsulfonyl fluoride was used for the nuclear isolation; (ii) the crude nuclear pellet was washed in buffer containing 0.1% Triton X-100; (iii) the detergent-washed nuclei were then resuspended and recentrifuged three times in buffer lacking Triton X-100.Purification of Mononucleosomes. Nuclei were suspended in buffer A of Hewish and Burgoyne (11), made 4 mM in CaCl2, and digested at 250C to 25-29% acid solubility with staphylococcal nuclease (EC 3.1.31.1). Digested nuclei were lysed according to Noll and Kornberg (12). Digests were fractionated by sedimentation through linear (5-20%) sucrose gradients containing 0.5 M NaCl (13) or by electrophoresis in 5% acrylamide gels (14).Analysis of Nucleosomal DNA. DNA was extracted from pooled, sucrose-gradient-purified nucleosomes (15), omitting RNase treatment. DNA was released from electrophoretically purified nucleosomes with 1% NaDodSO4 and proteinase K at 100 ,ug/ml. DNA was electrophoresed on 5% acrylamide slab gels (16) and visualized by staining with ethidium bromide.Analysis of Nucleosomal Proteins. Gradient-purified nucleosomes were precipitated with 10 mM MgCl2 (17) and extracted with protamine/urea/acetic acid (18). The released chromosomal proteins were applied directly to acetic acid/ urea/Triton DF-16 gels (see below). Electrophoretically purified nucleosomes were electrophoresed into dialysis bags and extracted with 0.25 M HCO (8). The extracted proteins were analyzed by two-dimensional electrophoresis (see below). Purification of Histones. D. melanogaster embryo histones w...