Characterization of histone modifying enzymes and their substrates in maize

Doctoral Thesis / Dissertation from the year 2007 in the subject Chemistry - Bio-chemistry, grade: Doctorate, University of Innsbruck (Institute of Molecular Biology), language: English, abstract: The maize ZmHda1 is a class II, HDA1-like histone deacetylase. ZmHda1 has been purified to homogeneity and found to be a 48 kDa monomeric protein (Brosch et al., 1996a). However, ZmHda1 specific antibodies can detect two other protein bands of molecular sizes of 84 and 65 kDa on immunoblots indicating that ZmHda1 exists natively in different molecular forms. These two protein forms of ZmHda1 are found in protein complexes of high molecular weights and they are enzymatically inactive (Pipal et al., 2003). Attempts to identify the protein components of the ZmHda1 protein complexes have been made. The second part of this study showed that histone methylation profile in maize embryos quantitatively and qualitatively changes as the germination progresses from 0 to 72 h. The total and chromatin unbound HMT activity decrease during the germination progress, while the chromatin bound HMT activity increases. This increase was most remarkable as an increase in methylation of H4K20 residue and to a lesser extent in mono- and tri- methylated H3K9 and dimethylated H3K36. With the progress of germination, the decrease in dimethylated H3K9 was remarkable. The decrease in H3K4 methylation and dimethylated H4R3 was less pronounced. Seven different histone methyltransferase activities could be separated from the 72h maize embryos; three of them were chromatin bound and four were found soluble in the cytoplasmic extract. Two of the chromatin bound activities, AI and AII2 methylate histone H3. AII2 showed H3K9 specificity and has a molecular size of 67 kDa. The third chromatin bound HMT activity, AII1, has a molecular size of 120 kDa and is H4 specific. Purification schemes could not yield purification of these activities to homogeneity; however, LC-MS/MS analysis of a chromatographic fraction containing the AII1 activity showed that it might have a homology to a putative SAM-dependent methyltransferase from Rhodobacter sphaeroides. Histone H3 is methylated by two activities from 72h maize embryos cytoplasmic extract; BI which has a molecular size of 30 kDa and another uncharacterized activity form. BII, from 72h embryos cytoplasmic extract, was purified to homogeneity using a series of FPLC chromatographic steps followed by native PAG. BII has been identified by nano-LC-MS/MS as a homologue of PRMT5, a type II PRMT and hence BII is referred to as ZmPRMT5.