GLUTACTIN, A NOVEL DROSOPHILA BASEMENT MEMBRANE-RELATED GLYCOPROTEIN WITH SEQUENCE SIMILARITY TO SERINE ESTERASES

Glutactin, a new acidic sulfated glycoprotein, was isolated from Drosophila Kc cell culture media. Immunofluorescence microscopy located it to embryonic basement membranes, particularly to the sequentially invaginated envelope of the central nervous system, muscle apodemes and dorsal median cell processes. Its chromosome locus is 29D. The nucleic acid sequence coding for the 1023 residue long polypeptide contains one intron and was confirmed by partial amino acid sequencing. Glutactin has a signal peptide and an amino domain of > 500 residues that strongly resembles acetylcholine esterases and other serine esterases, but lacks the catalytically critical serine residue. The amino and carboxyl domains of glutactin are separated by 13 contiguous threonine residues. Glutamine and glutamic acid make up 44% of glutactin's very acidic carboxyl domain. Glutactin preferentially binds Ca2+ in the presence of excess Mg2+ and four of its tyrosines are O-sulfated. Several similarities with mammalian entactin caused our previous, preliminary mention of glutactin as a putative Drosophila entactin, but sequence comparison now shows them to be different proteins.