Aminoacyl-tRNA synthetases, which catalyze the aminoacylation of tRNA molecules, are ancient proteins that are essential for decoding genetic information during the process of translation. In higher eukaryotes, nine aminoacyl-tRNA synthetases associate with at least three other polypeptides to form a supramolecular multienzyme complex (Mirande et al., 1985, Eur. J. Biochem. 147:281-89). Each of the eukaryotic tRNA synthetases consists of a core enzyme, which is closely related to the prokaryotic counterpart of the tRNA synthetase, and an additional domain that is appended to the amino-terminal or carboxyl-terminal end of the core enzyme (Mirande, 1991, Prog. Nucleic Acid Res. Mol. Biol. 40:95-142). Human tyrosyl-tRNA synthetase (TyrRS), for example, has a carboxyl-terminal domain that is not part of prokaryotic and lower eukaryotic TyrRS molecules (FIG. 1) (Rho et al., 1998, J. Biol. Chem. 273:11267-73). It has also been suggested that both the bovine and rabbit TyrRS molecules possess an extra domain (Kleeman et al., 1997, J. Biol. Chem. 272:14420-25).
In most cases, the appended domains appear to contribute to the assembly of the multienzyme complex (Mirande, supra). However, the presence of an extra domain is not strictly correlated with the association of a synthetase into the multienzyme complex. Higher eukaryotic TyrRS, for example, is not a component of the multienzyme complex (Mirande et al., supra).
The carboxyl-terminal domain of human TyrRS shares a 51% sequence identity with the mature form of human endothelial monocyte-activating polypeptide II (EMAP II) (Rho et al., supra). TyrRS is the only higher eukaryotic aminoacyl-tRNA synthetase known to contain an EMAP II-like domain. The EMAP-like domain of TyrRS has been shown to be dispensable for aminoacylation in vitro and in yeast (Wakasugi et al., 1998, EMBO J. 17:297-305).
EMAP II is a proinflammatory cytokine that was initially identified as a product of murine methylcholanthrene A-induced fibrosarcoma cells. Pro-EMAP II is cleaved and is secreted from apoptotic cells to produce a biologically active 22-kD mature cytokine (Kao, et al., 1994, J. Biol. Chem. 269:25106-19). The mature EMAP II can induce migration of mononuclear phagocytes (MPs) and polymorphonuclear leukocytes (PMNs); it also stimulates the production of tumor necrosis factor-α (TNFα) and tissue factor by MPs and the release of myeloperoxidase from PMNs.
The catalytic core domain of tryptophanyl-tRNA synthetase (TrpRS) is a close homolog of the catalytic domain of TyrRS (Brown et al., 1997, J. Mol. Evol. 45:9-12). As shown in FIG. 15, mammalian TrpRS molecules have an amino-terminal appended domain. In normal human cells, two forms of TrpRS can be detected: a major form consisting of the full-length molecule and a minor truncated form (“mini TrpRS”). The minor form is generated by the deletion of the amino-terminal domain through alternative splicing of the pre-mRNA (Tolstrup et al., 1995, J. Biol. Chem. 270:397-403). The amino-terminus of mini TrpRS has been determined to be the met residue at position 48 of the full-length TrpRS molecule (id.). Alternatively, truncated TrpRS may be generated by proteolysis (Lemaire et al., 1975, Eur. J. Biochem. 51:237-52). For example, bovine TrpRS is highly expressed in the pancreas and is secreted into the pancreatic juice (Kisselev, 1993, Biochimie 75:1027-39), thus resulting in the production of a truncated TrpRS molecule. These results suggest that truncated TrpRS has a function other than the aminoacylation of tRNA (Kisselev, supra).
Angiogenesis, or the proliferation of new capillaries from pre-existing blood vessels, is a fundamental process necessary for embryonic development, subsequent growth, and tissue repair. Angiogenesis is a prerequisite for the development and differentiation of the vascular-tree, as well as for a wide variety of fundamental physiological processes including embryogenesis, somatic growth, tissue and organ repair and regeneration, cyclical growth of the corpus luteum and endometrium, and development and differentiation of the nervous system. In the female reproductive system, angiogenesis occurs in the follicle during its development, in the corpus luteum following ovulation and in the placenta to establish and maintain pregnancy. Angiogenesis additionally occurs as part of the body's repair processes e.g., in the healing of wounds and fractures. Angiogenesis is also a factor in tumor growth, since a tumor must continuously stimulate growth of new capillary blood vessels in order to grow. Angiogenesis is an essential part of the growth of human solid cancer, and abnormal angiogenesis is associated with other diseases such as rheumatoid arthritis, psoriasis, and diabetic retinopathy (Folkman, J. and Klagsbrun, M., Science 235:442-447, (1987)).
Several factors are involved in angiogenesis. Both acidic and basic fibroblast growth factor molecules that are mitogens for endothelial cells and other cell types. Angiotropin and angiogenin can induce angiogenesis, although their functions are unclear (Folkman, J., 1993, Cancer Medicine pp. 153-170, Lea and Febiger Press). A highly selective mitogen for vascular endothelial cells is vascular endothelial growth factor or VEGF (Ferrara, N., et al., Endocr. Rev. 13:19-32, (1992)).