The actin domain of Gardner-Rasheed feline sarcoma virus inhibits kinase and transforming activities

The role of v-Fgr myristoylation and the Gag domain in membrane binding and cellular transformation

The v-fgr oncogene encodes a chimeric oncoprotein composed of feline sarcoma virus (FeSV)-derived gag and cellular-derived actin and c-Fgr sequences. v-Fgr is myristoylated and membrane bound, two criteria which must be met for src kinases to induce cellular transformation. Although inhibition of myristoylation resulted in a decreased ability of v-Fgr to sediment with membranes from an NIH-3T3 P100 fraction, deletion of the gag domain caused nearly all of the protein to remain unbound and cytosolic. Systematic deletions within gag indicate that while amino acids 3 through 9 are critical determinants of myristoylation and/or define a domain which directs membrane localization, these residues cooperate with additional gag sequences when anchoring the protein to the plasma membrane. Furthermore, nonmyristoylated and/or cytoplasmic variants of v-Fgr failed to induce anchorage-independent growth of NIH-3T3 cells, indicating that proper subcellular localization of v-Fgr is a key factor in its ability to induce transformation.

Characterization of the transforming domain of a feline sarcoma virus encoding a fgr-related tyrosine kinase

We investigated the transforming domain of a recently isolated feline sarcoma virus (TP1-FeSV) which encodes a fgr-related tyrosine kinase expressed as a gag-fgr fusion protein. The gag portion was removed and replication-competent expression vectors (RCAS) with inserted v-fgr sequences were established. Chicken embryo fibroblasts (CEF) were transfected and monitored for replication, integration and transcription of the proviral constructs. We demonstrated that transfected cells display morphological changes and are able to form colonies in soft-agar. This suggests that the gag portion of the fusion protein from TP1-FeSV is not necessary for the transformation of fibroblasts.

Fusion between transcription factor CBF beta/PEBP2 beta and a myosin heavy chain in acute myeloid leukemia

The pericentric inversion of chromosome 16 [inv(16)(p13q22)] is a characteristic karyotypic abnormality associated with acute myeloid leukemia, most commonly of the M4Eo subtype. The 16p and 16q breakpoints were pinpointed by yeast artificial chromosome and cosmid cloning, and the two genes involved in this inversion were identified. On 16q the inversion occurred near the end of the coding region for CBF beta, also known as PEBP2 beta, a subunit of a heterodimeric transcription factor regulating genes expressed in T cells; on 16p a smooth muscle myosin heavy chain (SMMHC) gene (MYH11) was interrupted. In six of six inv(16) patient samples tested, an in-frame fusion messenger RNA was demonstrated that connected the first 165 amino acids of CBF beta with the tail region of SMMHC. The repeated coiled coil of SMMHC may result in dimerization of the CBF beta fusion protein, which in turn would lead to alterations in transcriptional regulation and contribute to leukemic transformation.

A novel c-fgr exon utilized in Epstein-Barr virus-infected B lymphocytes but not in normal monocytes

The fgr proto-oncogene encodes a nonreceptor protein-tyrosine kinase, designated p55c-fgr. In this study, we have isolated human fgr cDNA molecules from normal monocyte mRNA templates. Nucleotide sequence analysis of the longest fgr cDNA revealed a 5' untranslated region of 927 bp which included two Alu-like repeats as well as three translation stop codons immediately upstream of the initiator for p55c-fgr synthesis. Within genomic DNA, these sequences were distributed over 13 kbp as three distinct 5' untranslated exons. Previous studies have shown that Epstein-Barr virus (EBV) increases c-fgr mRNA levels in B lymphocytes. By comparing the nucleotide sequence reported for transcripts isolated from EBV-infected B lymphocytes with those of our monocyte cDNA as well as genomic DNA, we identified a novel untranslated exon utilized only in EBV-infected cells. The transcriptional initiation sites of fgr mRNA expressed in EBV-converted cells were mapped and shown to reside within a region identified as an intron for fgr mRNA that is expressed in normal myelomonocytic cells. Furthermore, the region of the fgr locus upstream of the novel exon displayed properties of a transcriptional promoter when transfected into heterologous cells. We conclude from all of these findings that activation of the fgr gene by EBV is achieved by mechanisms distinct from those normally regulating its programmed expression in myelomonocytic cells.

A novel c-fgr exon utilized in Epstein-Barr virus-infected B lymphocytes but not in normal monocytes

The fgr proto-oncogene encodes a nonreceptor protein-tyrosine kinase, designated p55c-fgr. In this study, we have isolated human fgr cDNA molecules from normal monocyte mRNA templates. Nucleotide sequence analysis of the longest fgr cDNA revealed a 5' untranslated region of 927 bp which included two Alu-like repeats as well as three translation stop codons immediately upstream of the initiator for p55c-fgr synthesis. Within genomic DNA, these sequences were distributed over 13 kbp as three distinct 5' untranslated exons. Previous studies have shown that Epstein-Barr virus (EBV) increases c-fgr mRNA levels in B lymphocytes. By comparing the nucleotide sequence reported for transcripts isolated from EBV-infected B lymphocytes with those of our monocyte cDNA as well as genomic DNA, we identified a novel untranslated exon utilized only in EBV-infected cells. The transcriptional initiation sites of fgr mRNA expressed in EBV-converted cells were mapped and shown to reside within a region identified as an intron for fgr mRNA that is expressed in normal myelomonocytic cells. Furthermore, the region of the fgr locus upstream of the novel exon displayed properties of a transcriptional promoter when transfected into heterologous cells. We conclude from all of these findings that activation of the fgr gene by EBV is achieved by mechanisms distinct from those normally regulating its programmed expression in myelomonocytic cells.

Tyrosine phosphorylation coupled to IgE receptor-mediated signal transduction and histamine release

Antigen-induced cross-linking of IgE bound to its receptors at the surface of basophils or mast cells initiates a number of biochemical events culminating in the release of histamine-containing granules. In the present study, we investigated the possible involvement of tyrosine phosphorylation in signaling by the high-affinity IgE receptor (Fc epsilon RI). Cross-linking of Fc epsilon RI in rat basophilic leukemia cells (RBL-2H3) led to the phosphorylation of several proteins on tyrosine, the most prominent having a mass of 72 kDa. Tyrosine phosphorylation was rapid, detectable 1 min after stimulation, and correlated with both the time course and antigen dose for histamine release. Reversal of Fc epsilon RI cross-linking prevented continuation of the degranulation process and resulted in rapid loss of tyrosine phosphorylation. The receptor-mediated tyrosine phosphorylation was still induced in the absence of calcium in the medium. Depletion of protein kinase C with phorbol 12-myristate 13-acetate did not dramatically affect the tyrosine phosphorylation signal or the release of histamine. In contrast, the calcium ionophore A23187 induced histamine release in the absence of a perceptible increase in protein tyrosine phosphorylation. Thus, tyrosine phosphorylation is an early signal following Fc epsilon RI aggregation, independent of the exocytotic process itself. Taken together, our findings functionally link protein phosphorylation on tyrosine residues to Fc epsilon RI-mediated signal transduction leading to histamine release.