UDP-Beta1,4-galactosyltransferase (Beta1,4-GT) is a trans-Golgi membrane- bound enzyme that participates in the coordinate biosynthesis of the carbohydrate moieties of glycoproteins and glycolipids. Beta1,4-GT has also been localized to the plasma membrane of a diverse variety of cells and tissues by immunohistochemical and biochemical procedures. This cell surface distribution has led to the postulate that, in addition to its biosynthetic role, the enzyme also has a functional role in intercellular recognition and adhesion. Analysis of the cell surface Beta1,4-GT, localized to the plasma membrane overlying the intact acrosome of mouse sperm, supports a functional role in sperm-egg recognition. Failure of early mouse embryos to form blastocysts when cultured in vitro in the presence of monospecific anti-Beta1,4-GT IgG, suggests that cell surface Beta1,4-GT is functionally important during late morula compaction. This proposal takes advantage of a full-length cDNA clone for murine Beta1,4-GT that we have characterized. We have demonstrated that this gene is unusual in that it specifies two related forms of the enzyme that differ only in their NH2-terminal domains. We have used this clone to determine the genomic organization of murine Beta1,4-GT and have demonstrated that both the levels, structure of the mRNA and the form of the protein expressed are modulated during murine spermatogenesis.
The specific aims are: (1) to determine if the different NH2-terminal domains of the two forms of Beta1,4-GT selectively target the protein to different subcellular compartments, (2) to clone/analyze the unique Beta1,4-GT transcripts in murine haploid germ cells, (3) to characterize the regulatory sequences directing expression of the two somatic forms and the germ cell specific form of Beta1,4-GT, and 4) to introduce into embryonic stem (ES) cells by homologous recombination, a modified Beta1,4-GT gene that codes for an enzymatically inactive form of the enzyme. Our long range objectives are to define the regulation, structure, function and organization of this enzyme in both its Golgi and plasma membrane compartments, to determine the mechanism(s) responsible for the compartmentalization of this membrane- bound enzyme and to understand the role of the cell surface Beta1,4-GT in early mouse development. The proposed experiments lay the foundation for our long range goals.
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