Evolution of proteins of the cystatin superfamily

Levels of salivary cystatins in periodontally healthy and diseased older adults

Cystatins are cysteine protease inhibitors present in a variety of tissues and body fluids, including saliva. One possible function of these molecules may be to modulate tissue destruction in periodontal diseases. To investigate the potential role of salivary cystatins in these events, the levels of cystatins in saliva from periodontally healthy and diseased individuals were measured by enzyme-linked immunosorbent assay. Flow rates and total protein content were determined in all the samples collected, while cysteine protease inhibitory activity was assessed in submandibular-sublingual secretions. Statistical analysis showed no significant differences in the levels and activity of salivary cystatins in periodontally healthy and diseased individuals. These findings suggest that comparing the levels of cystatins in glandular salivas may not be a suitable indicator of periodontal disease status.

The nucleotide and partial amino acid sequences of rat fetuin. Identity with the natural tyrosine kinase inhibitor of the rat insulin receptor

Fetuins are among the major plasma proteins, yet their biological role has remained elusive. Here we report the molecular cloning of rat fetuin and the sequence analysis of a full-length clone, RF619 of 1456 bp with an open reading frame of 1056 bp encoding 352 amino acid residues. The coding part of RF619 was identical with the cDNA sequence of the natural inhibitor of the insulin receptor tyrosine kinase from rat (pp63) except for four substitutions and a single base insertion causing divergence of the predicted protein sequences. Partial amino acid sequences of rat plasma fetuin were in agreement with the predictions based on the RF619 cDNA. Purified rat fetuin inhibited the insulin receptor tyrosine kinase in vitro. Therefore, we conclude that RF619 and pp63 cDNA encode the same protein, i.e. authentic rat fetuin which is a functional tyrosine kinase inhibitor.

The nucleotide and deduced amino acid structures of sheep and pig fetuin. Common structural features of the mammalian fetuin family

This study was initiated to gain further insight into the structural features of the mammalian fetuin family. The cDNA structures of sheep and pig fetuin were determined. The cDNA insert encoding sheep (pig) fetuin comprised 1550 (1470) nucleotides, including 54 (46) nucleotides encoding a signal peptide of 18 (15) residues and 1038 (1041) nucleotides encoding the 346 (347) amino acids of the mature plasma protein. The predicted amino-terminal sequence of the mature pig fetuin was confirmed by the amino-terminal sequence of the purified protein. However, two alternative sheep amino-terminal sequences were found in fetuin purified from the plasma of a single sheep fetus; the minor product was the one predicted by comparison with other fetuin sequences while the major product was two amino acids longer. Comparison of the deduced amino acid sequences of sheep and pig fetuin showed an extensive sequence identity between them (75%) and with other proteins of the mammalian fetuin family, i.e. human alpha 2-HS glycoprotein, and bovine and rat fetuins. Twelve cysteine residues were found at invariant positions in all fetuin sequences, suggesting strongly that the arrangement of disulphide bridges identified in human alpha 2-HS glycoprotein is common to the members of the family. Further sequence comparisons revealed that the structures of mammalian fetuins are organised in three domains: two cystatin-like domains (D1 and D2) and a complex carboxyl-terminal domain (D3). The proposed three-domain structure of the protein is reflected in the organisation of the rat fetuin structural gene which has recently been published.

Incorporation and degradation by kidney lysosomes of cystatin alpha injected intravenously

Cystatin alpha was purified from Escherichia coli transfected with a recombinant cystatin alpha gene and injected into the tail vein of rats. Its fate was then followed using a sensitive enzyme immunoassay for cystatin alpha. Results showed that it was rapidly removed from the blood and taken up by the kidney. Percoll density-gradient analysis showed that it was rapidly incorporated into lysosomes in the kidney. Its level in the kidney was maximal 30 min after its injection then rapidly decreased. The activity of cathepsin H in the kidney lysosomal fraction was markedly decreased 30 min after injection of cystatin alpha but recovered rapidly. Anti-(cystatin alpha) antibody precipitated cathepsin H and anti-(cathepsin H) antibody precipitated cystatin alpha in an extract of the lysosome-rich fraction of the kidney 30 min after injection of cystatin alpha. These results indicate that cystatin alpha was taken up by kidney lysosomes, formed a complex with cathepsin H and initially decreased the activity of cathepsin H, but that later the level of cystatin alpha in the kidney rapidly decreased.

The complete primary structure of bovine stefin B

A new stefin B-type low-Mr CPI was isolated from bovine thymus and subjected to structural analysis. The inhibitor consisted of 98 amino acids and its Mr was calculated to be 11,178. The NH2-terminal amino acid residue was blocked. The sequence was determined by automated sequencing of peptides derived by cleavage with cyanogen bromide and fragments of the inhibitor resulting from enzymatic digestion with beta-trypsin and Staphylococcus aureus V-8 proteinase. The NH2-terminal blocking group was established with mass spectrometry. The inhibitor exhibits considerable sequence homology with inhibitors from the stefin family. Furthermore, a highly conserved QVVAG region within the stefin family is for the first time replaced by the QLVAG sequence.

Human salivary cystatin S. Cloning, sequence analysis, hybridization in situ and immunocytochemistry

A human submandibular-gland (SMG) cDNA library was constructed in a lambda was constructed in a lambda gt11 Sfi-Not orientation-specific expression vector and then screened with antibody generated against human salivary cystatins. The clone C4-4 encoded an N-terminally truncated cystatin S, whereas the others encoded cystatin SN. The library was then rescreened with the C4-4, and the inserts of several positive clones were directly amplified from the eluted plaques by linear PCR and the PCR products analysed by Southern blotting and direct DNA sequencing. Two clones (C3 and C12) encoded a full-length secreted cystatin S and its leader peptide and included 5'- and 3'-untranslated regions. These clones showed a high degree of sequence similarity to cDNA clones encoding human salivary cystatin SN and genomic clones encoding cystatin SN and SA. Hybridization in situ of normal human SMG and parotid-gland (PG) tissue sections localized the cystatin-gene transcripts to the cytoplasm of serous acinar cells of both glands, with a much higher concentration of cystatin mRNA in the SMG. Immunocytochemistry localized the salivary cystatin gene products also to the serous cells, and the levels of cystatin protein correlated with the amount of cystatin mRNA, with a much stronger signal in the SMG than in the PG.

The cystatins: protein inhibitors of cysteine proteinases

The last decade has witnessed enormous progress of protein inhibitors of cysteine proteinases concerning their structures, functions and evolutionary relationships. Although they differ in their molecular properties and biological distribution, they are structurally related proteins. All three inhibitory families, the stefins, the cystatins and the kininogens, are members of the same superfamily. Recently determined crystal structures of chicken cystatin and human stefin B established a new mechanism of interaction between cysteine proteinases and their inhibitors which is fundamentally different from the standard mechanism for serine proteinases and their inhibitors.

The human kininogen gene (KNG) mapped to chromosome 3q26-qter by analysis of somatic cell hybrids using the polymerase chain reaction

Kinins, peptide products of kininogens, may be involved in hypertensive and diabetic diseases, and inflammatory disorders. The human kininogen gene (KNG) has been mapped to chromosome 3, using a panel of human-hamster somatic cell hybrids by polymerase chain reaction of hybrid DNA with gene-specific primers. KNG was further assigned to 3q26-3qter, using DNA from a second panel of chromosome 3 deletion mapping cell hybrids.

Rat tyrosine kinase inhibitor shows sequence similarity to human alpha 2-HS glycoprotein and bovine fetuin

Human alpha 2-HS glycoprotein and bovine fetuin, abundant proteins of fetal plasma, are structural members of the fetuin family within the cystatin superfamily. They are characterized by the presence of two N-terminally located cystatin-like units and a unique C-terminal sequence segment not present in the other members of the cystatin superfamily. Search for related sequences revealed that the natural inhibitor of the insulin receptor tyrosine kinase [Auberger, Falquerho, Contreres, Pages, Le Cam, Rossi & Le Cam (1989) Cell (Cambridge, Mass.) 58, 631-640] shows sequence similarity to the mammalian fetuins. The sequence identity between rat tyrosine kinase inhibitor, human alpha 2-HS glycoprotein and bovine fetuin is 56 and 60% respectively (percentage of residues in identical positions). The sequence similarity extends over the entire protein structures, except the extreme C-terminal portions. In particular, the number and relative positions of the cysteine residues are invariant among the proteins, suggesting that the characteristic array of linearly arranged and tandemly repeated disulphide loops of the cystatin superfamily is also present in rat tyrosine kinase inhibitor. We conclude that rat tyrosine kinase inhibitor may be classified as a novel member of the mammalian fetuin family.

Transforming growth factor beta regulates cystatin C in serum-free mouse embryo (SFME) cells

Differential screening of a cDNA library derived from mRNA of TGF beta-treated serum-free mouse embryo (astrocyte precursor) cells isolated a strongly TGF beta-regulated mRNA that codes for cystatin C, a cysteine protease inhibitor. Increase in cystatin C mRNA level was observed within four hours after treatment with picomolar concentrations of TGF beta. The increase was reversible upon removal of TGF beta and was not prevented by cycloheximide. These results suggest that cystatin C expression may represent a developmentally regulated differentiated function of astrocytes, and also suggest that cystatin C expression may be involved in the response of brain cells to platelet release of TGF beta after trauma or injury.