The plasma protein fetuin: common structural features of the mammalian fetuin family

Abstract

The structure and tissue-specific expression of the bovine plasma glycoprotein fetuin and its homologues in other species has been examined. From the known partial amino acid sequence of bovine fetuin, degenerate oligonucleotide probes were designed. These probes were end-labelled using 14 polynucleotide kinase and were used to screen an adult bovine liver cDNA library by plaque hybridisation. A cDNA encoding the whole of the plasma protein was obtained and was fully sequenced on both strands. The deduced amino acid sequence was confirmed by protein sequence data in the literature [Christie et al., (1987), FEES Letts. 214, 45-49]. The bovine fetuin cDNA was labelled by the random-primer technique and w#s used to screen a fetal sheep liver (pooled 40-60 cDNA library. A cDNA encoding the whole of sheep fetuin was obtained and was fully sequenced. The sheep fetuin cDNA was similarly labelled and was used to screen an adult pig liver cDNA library. An almost full-length pig fetuin clone was obtained and was sequenced. The deduced amino acid sequences were confirmed by amino-terminal protein sequence analysis by Dr. D.L. Christie of the glycoproteins purified from plasma. During this project it was reported that the protein encoded kyf rat clone pp(# [Auberger et ai., (1989), Cell 58, 631-640] showed strong homology to bovine fetuin and human ag-HS glycoprotein. Further analysis, reported in this thesis, demonstrates that clone pp63 encodes rat fetuin.

The three fetuin sequences reported in this thesis are compared with the other known members of the mammalian fetuin family: human ag-HS glycoprotein, the rat protein encoded by clone pp63 and mouse fetuin. Sequence analysis reveals that fetuins comprise three domains: at the amino-terminus two cystatin-like domains and a unique carboxyl-terminal domain. A series of actual or potential sites for post-translational modification are apparent in the sequences.

By the techniques used (northern blot, RNAase protection assay and polymerase chain reaction) fetuin expression could only be detected in the liver. No fetuin mRNA could be detected in the fetal brain, although the protein can be immunocytochemically localised there. Data reported here show that in cattle, fetuin is a positive acute phase protein. It has been reported that human aj-HS glycoprotein and rat fetuin are negative acute phase proteins.

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