👤 Santam Saha

The MLL gene is reciprocally translocated with one of a number of different partner genes in a proportion of human acute leukaemias. The precise mechanism of oncogenic transformation is unclear since most of the partner genes encode unrelated proteins. However, two partner genes, AF10 and AF17 are related through the presence of a cysteine rich region and a leucine zipper. The identification of other proteins with these structures will aid our understanding of their role in normal and leukaemic cells. We report the cloning of a novel human gene (BRL) which encodes a protein containing a cysteine rich region related to that of AF10 and AF17 and is overall most closely related to the previously known protein BR140. BRL maps to chromosome 22q13 and shows high levels of expression in testis and several cell lines. The deduced protein sequence also contains a bromodomain, four potential LXXLL motifs and four predicted nuclear localization signals. A monoclonal antibody raised to a BRL peptide sequence confirmed its widespread expression as a 120 Kd protein and demonstrated localization to the nucleus within spermatocytes. Show less

We have cloned Af10, the murine homologue of the MLL partner gene AF10. The predicted open reading frame of Af10 contains 1069 aa which are 90% identical to those of AF10. Af10 contains an N-terminal cysteine-rich region with a LAP/PHD finger, a leucine zipper domain and a glutamine-rich region at the C-terminus, features also found in the human proteins AF10 and AF17. A single 5. 5-kb transcript was detected in murine tissues with the highest level of expression in the testes. A polyclonal antibody raised to the cysteine-rich region of AF10 was able to identify a double band of 140 kDa on Western analysis in mouse testicular extracts. After subcellular separation Af10 was identified in both the nuclear and cytoplasmic extracts, again as a double band of 140 kDa in size. In situ hybridisation studies were performed with sense and antisense digoxigenin-labelled oligonucleotides. High levels of expression were noted in postmeiotic germ cells, especially in spermatids from around stage VI to stage VIII. High levels of expression were also seen in the white matter of the cerebellum, extending into the granular layer. The expression in differentiated rather than in proliferating cells suggests that the role of Af10 may lie in the suppression of proliferation rather than in differentiation. Since the LAP/PHD finger domains are lost in the MLL-AF10 fusion, arguably such a function could be carried out by this domain. Show less

The genes AF10 and AF17 have been identified as the basis of the t(10;11) and t(11;17) translocations, events that result in their fusion to the MLL/HRX gene in acute myeloid leukaemias. AF10 and AF17 bear significant homology to each other within their putative zinc finger and leucine zipper domains, although they are diverged outside these regions. The BR140 gene encodes a 140 kDa protein of unknown function that contains a putative zinc finger domain, a leucine zipper region, and, in addition, a bromo domain. The zinc finger and leucine zipper domains of BR140 have significant homology to those of AF10 and AF17, suggesting that it belongs to this newly described gene family and, therefore, could be a target for chromosome translocation. To assess the potential involvement of BR140 in chromosome translocations in leukaemia, the chromosomal location of the BR140 gene has been determined by using several independent methods. A combination of Southern analysis, polymerase chain reactions (PCR) on monochromosomal cell hybrids, and fluorescence in situ hybridisation (FISH) has been used to show that the BR140 gene maps to chromosome band 3p25. Show less

We have identified and further characterized a Caenorhabditis elegans gene, CEZF, that encodes a protein with substantial homology to the zinc finger and leucine zipper motifs of the human gene products AF10, MLLT6, and BR140. The first part of the zinc finger region of CEZF has strong similarity to the corresponding regions of AF10 (66%) and MLLT6 (64%) at the cDNA level. As this region is structurally different from previously described zinc finger motifs, sequence homology searches were done. Twenty-five other proteins with a similar motif were identified. Because the functional domain of this motif is potentially disrupted in leukemia-associated chromosomal translocations, we propose the name of leukemia-associated protein (LAP) finger. On the basis of these comparisons, the LAP domain consensus sequence is Cys1-Xaa1-2-Cys2-Xaa9-21-Cys3-Xaa2-4 -Cys4-Xaa4-5-His5-Xaa2-Cys6-Xaa12-46 - Cys7-Xaa2-Cys8, where subscripted numbers represent the number of amino acid residues. We review the evidence that this motif binds zinc, is the important DNA-binding domain in this group of regulatory proteins, and may be involved in leukemogenesis. Show less

The gene on chromosome 10 at band p12 (AF10), involved in the t(10;11) translocation in acute myeloid leukemia, has been identified and shown to contain conserved zinc finger and leucine zipper domains. These regions are highly homologous to the equivalent regions on AF17, the gene involved in the t(11;17) translocations. A series of adult, childhood, and infant leukemias with either simple or complex versions of the t(10;11) has been examined by Southern analysis and shown to involve rearrangement to the HRX locus. Reverse transcriptase-polymerase chain reaction from either bone marrow or peripheral blood cells showed that HRX sequence was fused to AF10 sequence in all 8 cases and subsequent sequence analysis showed an in-frame fusion between the HRX and AF10 sequence. A consistent feature of these fusions was the juxtaposition of the leucine dimerization motif of AF10 onto the NH2-terminal region of HRX. The published data suggest that a similar conclusion can be drawn about the t(11;17) translocation, implying a critical role for this motif in the chimaeric HRX protein. Show less

A novel class of conserved transcription factors has been identified from the molecular cloning of AF10, the gene involved in the t(10;11)(p12;q23) translocation of acute myeloid leukemias. AF10 encodes a 109-kD protein of 1,027 amino acids and contains an N-terminal zinc finger region and a C-terminal leucine zipper. These structures have been found to be conserved in sequence and position in three other proteins, AF17, BR140, and a previously unrecognized Caenorhabditis elegans gene, provisionally named CEZF. The overall structure, level of sequence conservation, and expression pattern suggest that these genes encode a new class of transcription factors, some of which are targets for chromosomal translocation in acute leukemia. Show less

The legionellae are facultative intracellular bacterial pathogens which multiply in host phagocytes. Legionella micdadei cells contain an acid phosphatase (ACP2) which blocks superoxide anion production by human neutrophils stimulated with formyl-Met-Leu-Phe (fMLP) [A. K. Saha, et al. (1985) Arch. Biochem. Biophys. 243, 150-160]. In the present study, we have purified the Legionella phosphatase to homogeneity as indicated by the finding of a single 68,000-Da band following sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We explored the possibility that ACP2 acts by interfering with polyphosphoinositide hydrolysis and the production of the intracellular second messengers, inositol trisphosphate (IP3) and diacylglycerol, following neutrophil stimulation. Phosphatidylinositol 4,5-bisphosphate (PIP2) was hydrolyzed rapidly by ACP2 in vitro. The rate of hydrolysis of PIP2 was higher at pH 7.0 (Km 2.0 microM; 4 X 10(3) units/mg protein; 1 unit equals 1 nmol of Pi released/h) than at lower pH. IP3 was also a good substrate for ACP2 in vitro. When human neutrophil phosphoinositides were prelabeled with 32Pi, subsequent incubation with ACP2 resulted in an 85% loss of the labeled PIP2 over 2 h. Following fMLP stimulation of [3H]inositol-labeled neutrophils, the quantity of IP3 produced by ACP2-treated cells was reduced by 44%. Prior treatment of neutrophils with ACP2 also reduced by 45% the amount of diacylglycerol they produced when stimulated by fMLP. These results indicate that the Legionella phosphatase may compromise the neutrophils' microbicidal response to the organism by hydrolyzing PIP2, the progenitor of IP3 and diacylglycerol, and by hydrolyzing IP3 itself. Show less

The high-speed supernatant (100,000 g, 1 h) obtained after centrifuging a suspension of Legionella micdadei that had been freeze-thawed and sonicated contained (i) considerable acid phosphatase activity when assayed using 4-methylumbelliferyl phosphate (MUP) as the substrate, and a factor that blocked superoxide anion production by human neutrophils stimulated with f-Met-Leu-Phe. Chromatography of the extract on a hydroxylapatite column resolved two acids phosphatases (designated ACP1 and ACP2). Subsequent chromatography of ACP2 on a Sephadex G-150 column revealed coincident elution of phosphatase activity and neutrophil blocking activity. When heated at 45 degrees C for various periods of time, the phosphatase activity of the acid phosphatase preparation was lost at the same rate as the ability of the preparation to block superoxide anion production by neutrophils. Furthermore, preincubation of neutrophils and acid phosphatase together in the presence of a heteropolymolybdate complex that inhibits the phosphatase eliminated the effect of the L. micdadei phosphatase on neutrophil superoxide anion production. ACP2 had the following properties: pH optimum, 6.0; Km for MUP, 3.8 mM; isoelectric point, 4.5; substrate specificity, MUP greater than ADP greater than phosphoenolpyruvate greater than phosphothreonine greater than phosphoserine greater than phosphotyrosine; molecular weight (estimated by sucrose density gradient centrifugation and gel filtration chromatography), 71,000-86,000. These results indicate that a cell-associated phosphatase may play a role in the virulence of L. micdadei. Show less