Glutactin is important in structuring the segmented divisions of the fly. Glutactin is a calcium binding protein of the basement membrane extracellular matrix, the ground substance providing an adhesive support for cells. It is found adjacent to sheets of epithelial cells in the basement membrane, lining segmentally spaced channels lying between segmental nerves in the abdominal segments. It is also found lining channels at the exact medial position between the central nervous system posterior commissures of one segment and the anterior commissures of the next segment. Glutactin is also found lining the boundaries of segmentally arranged muscle cells (Olson, 1990).

GENE STRUCTURE

Bases in 5' UTR -112; the first intron breaks up the 5'UTR

Exons - three

Bases in 3' UTR - 124

PROTEIN STRUCTURE

Amino Acids - 1023

Structural Domains

The N-terminal contains a signal peptide of 17 amino acids with a cleavage site. The signal sequence enables protein transport across the cell membrane during protein synthesis. Subsequently the sequence is cleaved. The N-terminal domain is homologous to serine esterases, but lacks the catalytically critical serine residue. The amino and carboxyl domains are separate by 13 contiguous threonine residues. Glutamine and glutamic acid make up 44% of Glutactin's extremely acidic carboxyl domain (Olson, 1990). Four of the 46 tyrosine sequences are sulfated.

Evolutionary Homologies

The amino terminal esterase domain is homologous to serine esterases, including acetylchorinesterase (Olsen, 1990)

REGULATION

Protein Interactions

A Drosophila UDP-glucose:glycoprotein glucosyltransferase was isolated, cloned and characterized. Its 1548 amino acid sequence begins with a signal peptide, lacks any putative transmembrane domains and terminates in a potential endoplasmic reticulum retrieval signal, HGEL. The soluble, 170 kDa glycoprotein occurs throughout Drosophila embryos, in microsomes of highly secretory Drosophila Kc cells and in small amounts in cell culture media. The isolated enzyme transfers [14C]glucose from UDP-[14C]Glc to several purified extracellular matrix glycoproteins (laminin, peroxidasin and glutactin) made by these cells, and to bovine thyroglobulin. These proteins must be denatured to accept glucose, which is bound at endoglycosidase H-sensitive sites. The unusual ability to discriminate between malfolded and native glycoproteins is shared by the rat liver homolog. The amino acid sequence presented differs from most glycosyltransferases. There is weak, though significant, similarity with a few bacterial lipopolysaccharide glycotransferases and a yeast protein Kre5p. In contrast, the 56%-68% amino acid identities with partial sequences from genome projects of Caenorhabditis elegans, rice and Arabidopsis suggest widespread homologs of the enzyme. This glucosyltransferase fits previously proposed hypotheses for an endoplasmic reticular sensor of the state of folding of newly made glycoproteins (Parker, 1995).

DEVELOPMENTAL BIOLOGY

Embryonic

Glutactin is a calcium binding protein of the extracellular matrix. Along with laminin and collagen IV, it is found lining the channels through the nerve cord between the posterior commissures of one segment and the anterior commissures of the next segment. Two dorsal medial cells above the nerve cord extend processes to the segmentally arranged muscles of abdominal segments. Glutactin lines the basement membrane channels established by these cells between segments (Olson, 1990).

Larval

Membranes that envelop imaginal discs also stain with antibodies against Glutactin and Laminin (Olson, 1990).

REFERENCES

Olson, P.F., Fessler, L.I., Nelson, R.E., Sterne, R.E., Campbell, A.G. and Fessler, J.H. (1990). Glutactin, a novel Drosophila basement membrane-related glycoprotein with sequence similarity to serine esterases. EMBO J. 9: 1219-1227

Parker, C. G., et al. (1995). Drosophila UDP-glucose:glycoprotein glucosyltransferase: sequence and characterization of an enzyme that distinguishes between denatured and native proteins. EMBO J. 14(7): 1294-303.

date revised: 20 feb 96 

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