A serine/threonine kinase which causes apoptosis-like cell death interacts with a calcineurin B-like protein capable of binding Na(+)/H(+) exchanger.

We surveyed proteins capable of binding to the cytoplasmic domain of Na(+)/H(+) exchanger (NHE)1 in a rat brain cDNA library with the yeast two-hybrid system. One clone obtained coded for a protein reported previously as a human calcineurin homologous protein (CHP). Since CHP is homologous to the regulatory subunit B of calcineurin, we expected a possible interacting partner of CHP like the catalytic subunit of calcineurin (calcineurin A), and surveyed this putative partner again with the yeast two-hybrid system. A clone thus obtained coded for a kinase, which is basically the same as that reported for human DRAK2. Overexpression of the rat homologue of DRAK2 caused apoptosis-like cell death of NIH3T3 cells, which was dependent on the kinase activity, confirming the previous result for DRAK2. The purified CHP and rat DRAK2 proteins synthesized in Escherichia coli could bind in vitro. CHP and rat DRAK2 expressed in COS-7 cells were found to be localized in the Golgi apparatus and nucleus, respectively. Some of them was also found in the membrane peripheral region. When they were co-expressed in the same cells, most of CHP moved to the nucleus where rat DRAK2 is located, suggesting in vivo interaction of these proteins. However, minor but significant fractions of both proteins were also found in the membrane peripheral region. Rat DRAK2 is expressed highly in thymus, spleen, and testis, where the apoptosis plays an important role in physiology.

Cloning of poly(3-hydroxybutyrate) depolymerase from a marine bacterium, Alcaligenes faecalis AE122, and characterization of its gene product.

A DNA fragment that carries the gene coding for poly(3-hydroxybutyrate) (PHB) depolymerase was cloned from the chromosomal DNA of Alcaligenes faecalis AE122 isolated from seawater. The open reading frame encoding the precursor of the PHB depolymerase was 1905 base pairs (bp) long, corresponding to a protein of 635 amino acid residues (M(r) = 65,208). The promoter site, which could be recognized by Escherichia coli RNA polymerase, was upstream from the gene, and the sequence adhering to the ribosome-binding sequence was found in front of the gene. The deduced amino acid sequence agreed with the N-terminal amino acid sequence of the purified PHB depolymerase from amino acid 28 onwards. Analysis of the deduced amino acid sequence revealed the domain structure of the protein; a signal peptide of 27 amino acids long was followed by a catalytic domain of about 400 amino acids, a fibronectin type III module sequence, and a putative substrate binding domain. The molecular mass (62,526) of the mature protein deduced from the nucleotide sequence was significantly lower than the value (95 kDa) estimated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but coincided well with the value (62,426) estimated from matrix-assisted laser desorption ionization mass spectra. By comparison of the primary structure with those of other PHB depolymerases, the substrate binding domain was found to consist of two domains, PHB-specific and poly(3-hydroxyvalerate)-specific ones, connected by a linker region. The PHB depolymerase gene was expressed in Escherichia coli under the control of the tac promoter. The enzyme expressed in E. coli was purified from culture broth and showed the same catalytic properties as the enzyme from A. faecalis.