Title : Cell-type-specific and site-specific N-glycosylation of type I and type II human
tissue plasminogen activator
Abstract :
- Tissue plasminogen activator ( t-PA ) is an important initiator of fibrinolysis
- The t-PA polypeptide has four potential N-glycosylation sites of which three are occupied in type I ( Asn-117, -184, and -448 ) and two in type II ( Asn-117 and -448 )
- In an effort to elucidate the factors controlling the expression of N-linked oligosaccharides on this polypeptide , we have used a combination of sequential exoglycosidase digestion, methylation analysis, and controlled acetolysis to determine the oligosaccharide structures at each of the N-glycosylation sites of type I and type II t-PA when isolated from a human colon fibroblast cell strain and from a Bowes melanoma cell line
- Our results suggest the following: (i) type I and type II t-PA are N-glycosylated in an identical way at Asn-117 and Asn-448, when isolated from the same cell line; (ii) Asn-117 is predominantly associated with oligomannose-type structures in all cases; (iii) Asn-184 and Asn-448 are predominantly associated with complex-type structures when t-PA is isolated from fibroblast cells, but with both complex- and oligomannose-type structures when isolated from melanoma cells; (iv) fibroblast cell derived t-PA is associated with both neutral and sialylated oligosaccharides , while melanoma cell derived t-PA is also associated with sulfated oligosaccharides , which are located exclusively at Asn-448 of type II t-PA ; (v) no complex-type structures occur in common between t-PA from the two cell lines
- These results indicate that the t-PA glycoprotein is secreted by each cell line as a set of glycoforms, each glycoform being unique with respect to the nature and dis position of oligosaccharides on a common polypeptide
- Further, the two cell lines express no glycoform in common, despite expressing the same t-PA polypeptide
- The implications of these results for both the control of oligosaccharide processing in different cell lines and the genetic engineering of mammalian glycoproteins are discussed