A multifunctional protein: involvement of the alpha-1 serum protease inhibitor in SDS and high salt-stable DNA-protein complexes

Proteins tightly bound to DNA: new data and old problems

Proteins tightly bound to DNA (TBP) comprise a group of proteins that remain bound to DNA after usual deproteinization procedures such as salting out and treatment with phenol or chloroform. TBP bind to DNA by covalent phosphotriester and noncovalent ionic and hydrogen bonds. Some TBP are conservative, and they are usually covalently bound to DNA. However, the TBP composition is very diverse and significantly different in different tissues and in different organisms. TBP include transcription factors, enzymes of the ubiquitin-proteasome system, phosphatases, protein kinases, serpins, and proteins of retrotransposons. Their distribution within the genome is nonrandom. However, the DNA primary structure or DNA curvatures do not define the affinity of TBP to DNA. But there are repetitive DNA sequences with which TBP interact more often. The TBP distribution within genes and chromosomes depends on a cell's physiological state, differentiation type, and stage of organism development. TBP do not interact with DNA in the sites of its association with nuclear matrix and most likely they are not components of the latter.

Unusually stable and long-lived ligand-induced conformations of integrins

Integrins are a large family of cell surface receptors that are involved in a wide range of biological processes. The integrin alpha(IIb)beta3 (glycoprotein IIb-IIIa) is a major platelet glycoprotein heterodimeric receptor that mediates platelet aggregation and is currently a target for pharmaceutical intervention. Ligand binding to the receptor has been shown to induce conformational changes by physical methods and the exposure of neoepitopes (the ligand-induced binding sites). Here we show that the antagonist XP280 induces a conformation that is stable to treatment with SDS and that the protein retains this conformation for several days even after dissociation of the inhibitor. These ligand-induced conformational changes take place with purified protein and on intact platelets. They are competable with an RGDS peptide and are stable to reduction but not boiling or treatment with EDTA. The retention of an altered conformation in the absence of the ligand implies the possibility of ligand-induced alteration of biological function even in the absence of ligand. Finally, similar behavior is observed with the integrin alpha(v)beta3, suggesting that access to SDS stable conformations may be conserved throughout the integrin superfamily. The unusual stability, long-lived nature, and potential generality of these conformations could have profound implications for integrin biology.

Intracellular location and nuclear targeting of the Spi-1, Spi-2 and Spi-3 gene-derived serine protease inhibitors in non-secretory cells

Proteases and their inhibitors are indispensable for the regulated activation and/or degradation of structural and functional proteins involved in basic cellular processes, e.g. in cell cycle control, cell growth, differentiation and apoptosis. In this context the serine protease inhibitors derived from the murine Spi-1, Spi-2 and Spi-3 genes, and their human homologs, deserve reconsideration. Microsequencing data indicate that a fraction of the three serpins has the capability to constitute a well characterized proteinase K, high salt and SDS-stable complex which coisolates with DNA under salting out conditions from various cell and tissue types. This tight association with DNA isolated under conditions designed to deproteinize DNA efficiently points to an in situ preformed chromatin complex. Accordingly, in addition to their well known functions as 'serum protease inhibitors' the Spi-1 and Spi-2 gene-derived proteins appear to have intracellular functions as well. The involvement of the three serpins in chromatin complexes requires their nuclear translocation. Application of (enhanced) green fluorescent protein technology and optical section microscopy reveals that truncation of the N-terminal signal sequences of the Spi-1 and Spi-2 gene-encoded proteins is a prerequisite for their nuclear translocation while non-truncated fusion proteins are enriched at the nuclear indentation which is the site of the Golgi apparatus and the centrosome. The identification of new species of intracellular serpins is of potential interest with respect to accumulating evidence for serine protease inhibitor-dependent inhibition or prevention of apoptosis.