GLYCOSYLATED HUMAN GROWTH HORMONE VARIANT

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Abstract
The human growth hormone variant (hGH-V) gene is expressed by the syncytiotrophoblastic layer of the human placenta in two forms: hGH-V mRNA encoding a 22 kD protein, and hGH-V2 mRNA which retains intron 4 and is expected to encode a 26 kD protein. There is a predicted N-linked glycosylation site in hGH-V at amino acid 140 that is absent in both hGH-V2 and in the highly homologous normal pituitary GH (hGH-N). Cell lines transfected with the hGH-N gene secrete 22 kD GH and the 20 kD product of an alternatively spliced mRNA, while cell lines transfected with the hGH-V gene secrete three proteins of 22, 24, and 26 kD. To determine whether any of these hGH-V isoforms are glycosylated, the cell lines were grown in the absence and presence of tunicamycin. In addition, conditioned medium from metabolically labelled HGH-V transfected cells was separately digested with peptide:N-glycosidase F and endoglycosidase H. The 26 and 24 kD bands were both absent from the media after tunicamycin treatment and were both sensitive to peptide:N-glycosidase F treatment. Endoglycosidase H digestion resulted in the selective loss of the 24 kD band. These results indicate tht HGH-V is partially modified posttranslationally by N-linked glycosylation in a fibroblastic cell line.