Using BLASTP (Altschul et al., 1990), non-redundant database comparisons indicate a striking similarity of the MOF protein with Tip60, a recently identified human protein that appears to interact with the HIV-1 Tat transactivator (P = 4.1×10−104). Discovered by means of the yeast two-hybrid selection system, Tip60 has been demonstrated to greatly enhance Tat transactivation of the HIV-1 promoter in transient expression assays (Kamine et al., 1996). MOF also displays extended amino acid homology to MOZ, the human monocytic leukemia zinc finger protein (P = 8.6×10−90). The MOZ gene was recently identified as one of the two breakpoint-associated genes in the translocation found in the M4/M5 subtype of acute myeloid leukemia. The chromosome translocation fuses MOZ in-frame to CBP, the CREB transcriptional factor-binding protein (Borrow et al., 1996). Finally, a significant level of sequence similarity is found between MOF, the SAS2 (P = 2.8×10−45) and SAS3 (P = 3.3× 10−65) gene products of Saccharomyces cerevisiae, and other proteins of yet unidentified function (Figure 4A). In yeast, SAS2 is involved in silencing the telomeres; SAS2 and, to a lesser extent SAS3, are also involved in HMR locus silencing (Reifsnyder et al., 1996). Within this region of homology, which extends for ∼250 amino acids (Figure 4B), is present a domain common to many acetyl transferases and shown to be required for the binding of acetyl coenzyme A (Coon et al., 1995). Deduced from its homology to mammalian spermidine/spermine acetyl transferases and microbial antibiotic acetyl transferases (Lu et al., 1996), this domain is also found in enzymes known to acetylate histones, such as histone acetyl transferase 1 of yeast (Kleff et al., 1995), histone acetyl transferase A of Tetrahymena (Brownell et al., 1996) and its yeast homologue Gcn5p (Georgakopoulos and Thieros, 1992), and p300/CBP-associated factor P/CAF (Xiang-Jao et al., 1996) (Figure 4C). A second domain, identified as a C2HC/H zinc finger found in a variety of transcription factors and in oncogenes (Berg and Shi, 1996), is present in all of the MOF-related proteins with the exception of ScYOR244w. Some examples of proteins in which this type of zinc finger has been studied at the structural level are listed in Figure 4D. An additional region of homology is shared by MOF, Tip60 and the S.cerevisiae YOR244w (Figure 4A). This region is very similar to the chromo domain contained within a large number of proteins (Koonin et al., 1995). With respect to this domain, the highest level of similarity with MOF is exhibited by human retinoblastoma-binding proteins RBP-1 and RBP–2 (Fattaey et al., 1993) and the frog XNF7 (Reddy et al., 1991) and newt PwA33 nuclear factors (Figure 4E). The latter is a maternal protein associated with nascent transcripts on the lampbrush chromosome loops of the oocyte (Bellini et al., 1993). The chromo-like domain and the single zinc finger may represent sites of protein–protein interaction, although binding to DNA should not necessarily be ruled out (Pedone et al., 1996).
Figure 4. Similarity of MOF with other proteins. (A) The regions of strongest homology are represented as large open boxes. (B) Sequence alignment of MOF with Tip60 (accession No. U74667), MOZ (accession No. U47742), ScYOR244w (accession No. Z75152), SpAC17G8.13c (accession No. Z69795), CeRO7B5.8 (accession No. Z72512), SAS3 (accession No. P34218) and SAS2 (accession No. S48299). Identity values range from 53% and 50% with respect to Tip60 and MOZ to 35% with respect to SAS2. The zinc finger and the acetyl coenzyme A putative binding site are indicated by the open or hatched bar, respectively. (C) Alignment of the acetyl coenzyme A domain of MOF with ScHAT1, TtHAT A1, ScGCN5, HuP/CAF, HuSSAT, SaPAC (Saccharomyces alboniger puromycin N-acetyl transferase, accession No. P13249) and HiRIMI (Haemophilus influenzae ribosomal-protein-alanine acetyl transferase, accession No. P44305). The arrowhead indicates a conserved glycine residue that is substituted with a glutamate in the mof mutant gene product. (D) Alignment of the C2HC zinc finger of MOF with zinc fingers from XlTFIIIA (Xenopus laevis transcription factor IIIA, accession No. P17842), HuZFY (human zinc finger Y-chromosomal protein, accession No. P08048), ScADR1 (S.cerevisiae alcohol dehydrogenase II regulator protein 1, accession No. P07248), HuGLI (human glioma protein, accession No. P08151) and ScSWI5 (S.cerevisiae switch 5 gene product, accession No. P08153). The distance between the C and H marked by asterisks is constant in all of the examples listed. (E) The colored areas highlight the similarities that exist between the MOF chromo-like domain and those of TIP60, ScYOR244w, PwA33, XlXNF7, HuRBP1, DmHP1 (D.melanogaster heterochromatin protein 1) and DmPc (D.melanogaster polycomb protein; Paro and Hogness, 1991).
Download figure to PowerPoint