Glycosylation of the pituitary-specific transcription factor: PIT-1

A wide variety of nuclear and cytoplasmic proteins are modified with Nacetylglucosamines glycosidically linked to Ser/Thr residues (O-GlcNAc). OGlcNAc is an abundant and transient modification, being modulated during the cell cycle and in response to specific physiological stimuli. O-GlcNAc therefore has been postulated to act as a modulator of protein function much like protein phosphorylation. Pit-1 is a pituitary-specific transcription factor that regulates GH, Prl, and TSH-b expression. Pit-1 also has been implicated in maintaining the survival of these cell types during development. Galactosyltransferase and UDP-[3H]galactose were used to demonstrate that Pit-1 is O-GlcNAcylated. [3H]Galactose residues were transfered to Pit-1 indicating the presence of terminM GlcNAc residues. The labeled saccharide was resistant to PNGaseF but sensitive to b-elimination indicating that it was O-linked. Dionex HPLC showed the released saccharide was [3H]Galbl,4GlcNAcitol. Thus, the original acceptor was indeed O-GlcNAc. A Pit-1 chimera containing the HA epitope tag at its amino terminus was overexpressed using the baculovirus expression system. The expressed Pit-1 also was O-GlcNAcylated as demonstrated by its labeling with galactosyltransferase. Trypsinization, followed by lectin affinity chromatography and RP-HPLC yielded one radiolabeled glycopeptide from Pit-1. The location of the O-GlcNAc residue suggests that it is probably not involved in the stimulation of transcription from eL minimal promoter, but rather may be important for the interaction with other factors giving to rise to tissue specific gene expression. (Funded by NIH grants GM16229 to B.K.H. and HD13563 to G.W.H.).