Several of the endothelial cell polypeptide mitogens that have been described probably play a role in blood vessel homeostasis. Two overlapping complementary DNA clones encoding human endothelial cell growth factor (ECGF) were isolated from a human brain stem complementary DNA library. Southern blot analysis suggested that there is a single copy of the ECGF gene and that it maps to human chromosome 5 at bands 5q31.3 to 33.2 A 4.8-kilobase messenger RNA was present in human brain stem messenger RNA. The complete amino acid sequence of human ECGF was deduced from the nucleic acid sequence of these clones; it encompasses all the well-characterized acidic endothelial cell polypeptide mitogens described by several laboratories. The ECGF-encoding open reading frame is flanked by translation stop codons and provides no signal peptide or internal hydrophobic domain for the secretion of ECGF. This property is shared by human interleukin-1, which is approximately 30 percent homologous to ECGF.

The epidermal growth factor (EGF)‐receptor is composed of an extracellular ligand‐binding region connected by a single transmembrane region to the cytoplasmic kinase domain. In spite of its importance for understanding signal transduction, the ligand‐binding domain of the EGF‐receptor is not yet defined. We describe the identification of a major ligand‐binding domain of the EGF‐receptor by utilizing chimeras between the human EGF‐receptor and the chicken EGF‐receptor. This approach is based on the fact that murine EGF binds to the chicken EGF‐receptor with 100‐fold lower affinity as compared to the human EGF‐receptor. Hence, the substitution of various domains of the chicken EGF‐receptor by domains of the human EGF‐receptor may restore the higher binding affinity towards EGF, characteristic of the human receptor. We show that chimeric chicken/human EGF‐receptor, which contains domain III of the extracellular region of the human receptor, behaves like the human EGF‐receptor with respect to EGF binding affinity and biological responsiveness. However, a chimeric chicken/human EGF‐receptor containing domains I and II of the human receptor behaves like the chicken rather than the human EGF‐receptor. Moreover, two different monoclonal antibodies which compete for the binding of EGF to EGF‐receptor recognize specifically domain III of the human EGF‐receptor. It is concluded that domain III which is flanked by the two cysteine‐rich domains is a major ligand‐binding domain of the EGF‐receptor.

A human brainstem cDNA library in bacteriophage Agtll was screened under conditions of reduced hybridization stringency with a leukocyte common antigen (LCA) probe that spanned both conserved cytoplasmic domains. cDNA encoding a receptor-linked protein-tyrosinephosphatase (protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48), RPTPase a, has been cloned and sequenced. Human RPTPase a consists of 802 amino acids. The extracellular domain of 150 residues includes a hydrophobic signal peptide and eight potential N-glycosylation sites. This is followed by a transmembrane region and two tandemly repeated conserved domains characteristic of all RPTPases identified thus far. The gene for RPTPase a has been localized to human chromosome region 20pter-20q12 by analysis of its segregation pattern in rodent-human somatic cell hybrids. Northern blot analysis revealed the presence of two major transcripts of 4.3 and 6.3 kilobases. In addition to RPTPase a, two other RPTPases (, and y), identified in the same screen, have been partially cloned and sequenced. Analysis of sequence comparisons among LCA, the LCA-related protein LAR, and RPTPases a, A, and y reveal4 the existence of a multigene family encoding different RPTPases, each containing a distinct extracellular domain, a single hydrophobic transmembrane region, and two tandemly repeated conserved cytoplasmic domains.The degree and pattern of phosphorylation of tyrosine residues on cellular proteins are regulated by the opposing activities of protein-tyrosine kinases (PTKases; ATP:protein-tyrosine O-phosphotransferase, EC 2.7.1.112) and protein-tyrosine-phosphatases (PTPases; protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48). The structural characteristics and evolution of PTKases as well as their role in the regulation of cell growth have been considered elsewhere (1, 2). More recently, attention has also been focused on the growing family of PTPases. This family includes two types of molecules: (i) low molecular weight proteins such as PTPase 1B (3, 4), T-cell PTPase (5), and rat brain PTPase (6), which contain a single conserved phosphatase domain, and (ii) high molecular weight, receptor-linked PTPases (RPTPases) containing two tandemly repeated conserved domains separated by 56-57 amino acids. Examples of these include the leukocyte common antigen (LCA), also known as CD45 (7,8)

The spleen focus-forming virus (SFFV), a replication-defective murine leukemia virus that causes the rapid transformation of certain hematopoietic target cells, has acquired specific xenotropic viral genetic information not contained in Friend helper virus. In the current studies, it is shown that a cDNA that represents a xenotropic virus portion of SFFV detects genetic sequences derived from the env gene region of murine xenotropic virus. The significance of the acquisition of these xenotropic viral sequences by SFFV is discussed with regard to their possible role in the rapid leukemogenicity of SFFV, and an analogy is drawn between the formation of SFFV and the formation of the Kirsten and Harvey sarcoma viruses.

The primary structure of the chicken epidermal growth factor (EGF) receptor was deduced from the sequence of a cDNA clone containing the complete coding sequence and shown to be highly homologous to the human EGF receptor. NIH-3T3 ceOls devoid of endogeneous EGF receptor were transfected with the appropriate cDNA constructs and shown to express either chicken or human EGF receptors. Like the human EGF receptor, the chicken EGF receptor is a glycoprotein with an apparent molecular weight of 170,000. Murine EGF bound to the chicken receptor with approximately 100-fold lower affinity than to the human receptor molecule. Surprisingly, human transforming growth factor a (TGF-a) bound equally well or even better to the chicken EGF receptor than to the human EGF receptor. Moreover, TGF-a stimulated DNA synthesis 100-fold better than did EGF in NIH 3T3 cells that expressed the chicken EGF receptor. The differential binding and potency of mammalian EGF and TGF-ac by the avian EGF receptor contrasts with the similar affinities of the mammalian receptor for the two growth factors.Control of cell growth is regulated by interaction of soluble growth factors and cell membrane receptors. The study of the specificity of growth factor-receptor interaction and the following steps which lead to the mitogenic responses of the cell is essential for our understanding of normal cell growth control and oncogenesis. Several growth factor receptors possess intrinsic protein tyrosine kinase activity in their cytoplasmic domain, which is activated by ligand binding leading to autophosphorylation, phosphorylation of various cellular proteins, and initiation of pleiotropic responses resulting in DNA synthesis and cell proliferation (reviewed in references 4 and 20 and J. Schlessinger, Biochemistry, in press, and Y.

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The control of vascular endothelial and smooth muscle cell proliferation is important in such processes as tumor angiogenesis, wound healing, and the pathogenesis of atherosclerosis. Class I heparin-binding growth factor (HBGF-

A novel species of 30S RNA has been detected in a variety of mouse cell lines. The 30S RNA is specifically packaged by helper-independent type C viruses propagated in such cells. Nucleic acid hybridization detects no homology between the 30S RNA and the genomic RNA of helper-independent mouse type C viruses. The properties of the 30S RNA suggest that it is a defective endogenous mouse type C virus and that it is analogous to a previously described class of defective endogenous rat type C virus, which has been shown previously to be the progenitor of Kirsten and Harvey murine sarcoma viruses.