Role of cyclic GMP and calcineurin in homologous and heterologous desensitization of natriuretic peptide receptor-A

The natriuretic peptide receptor-A (NPR-A) mediates natriuretic, hypotensive, and antihypertrophic effects of natriuretic peptides through the production of cGMP. In pathological conditions such as heart failure, these effects are attenuated by homologous and heterologous desensitization mechanisms resulting in the dephosphorylation of the cytosolic portion of the receptor. In contrast with natriuretic peptide-induced desensitization, pressor hormone-induced desensitization is dependent on protein kinase C (PKC) stimulation and (or) cytosolic calcium elevation. Mechanisms by which PKC and Ca(2+) promote NPR-A desensitization are not known. The role of cGMP and of the cytosolic Ca(2+) pathways in NPR-A desensitization were therefore studied. In contrast with the activation of NPR-A by its agonist, activation of soluble guanylyl cyclases of LLC-PK1 cells by sodium nitroprusside also leads to a production of cGMP but without altering NPR-A activation. Consequently, cGMP elevation per se does not appear to mediate homologous desensitization of NPR-A. In addition, cytosolic calcium increase is required only for the heterologous desensitization pathway since the calcium chelator BAPTA-AM blocks only PMA or ionomycin-induced desensitization. Calcineurin inhibitors block the NPR-A guanylyl cyclase heterologous desensitization induced by ionomycin, suggesting an essential role for this Ca(2+)-stimulated phosphatase in NPR-A desensitization. In summary, the present report demonstrates that neither cGMP nor Ca(2+) cytosolic elevation cause NPR-A homologous desensitization. Our results also indicate for the first time a role for calcineurin in NPR-A heterologous desensitization.

ZRP-1, a zyxin-related protein, interacts with the second PDZ domain of the cytosolic protein tyrosine phosphatase hPTP1E

Protein-protein interactions play an important role in the specificity of cellular signaling cascades. By using the yeast two-hybrid system, a specific interaction was identified between the second PDZ domain of the cytosolic protein tyrosine phosphatase hPTP1E and a novel protein, which was termed ZRP-1 to indicate its sequence similarity to the Zyxin protein family. The mRNA encoding this protein is distributed widely in human tissues and contains an open reading frame of 1428 base pairs, predicting a polypeptide of 476 amino acid residues. The deduced protein displays a proline-rich amino-terminal region and three double zinc finger LIM domains at its carboxyl terminus. The specific interaction of this novel protein with the second PDZ domain of hPTP1E was demonstrated both in vitro, using bacterially expressed proteins, and in vivo, by co-immunoprecipitation studies. Deletion analysis indicated that an intact carboxyl terminus is required for its interaction with the second PDZ domain of hPTP1E in the yeast two-hybrid system and suggested that other sequences, including the LIM domains, also participate in the interaction. The genomic organization of the ZRP-1 coding sequence is identical to that of the lipoma preferred partner gene, another Zyxin-related protein, suggesting that the two genes have evolved from a recent gene duplication event.

Comparison of the thymidine kinase genes from three entomopoxviruses

The entomopoxviruses (insect poxviruses) of eastern spruce budworm (Choristoneura fumiferana), two year cycle spruce budworm (C. biennis) and the Indian red army worm (Amsacta moorei) are being studied in our laboratory for their potential as biological insecticides and expression vectors. These viruses characteristically replicate in the cytoplasm of insect cells and produce occlusion bodies that serve to protect the virion from the environment. By analogy to mammalian poxviruses, they should also contain a viral thymidine kinase (TK) that functions in viral DNA synthesis. The replication of the A. moorei entomopoxvirus was inhibited by bromodeoxyuridine whereas the baculovirus of Autographa californica was insensitive to this drug. This result was a biochemical indication that entomopoxviruses contained a kinase that phosphorylated this nucleoside analogue and thus viral DNA synthesis was inhibited. TK genes from the three different insect poxviruses were identified, cloned and sequenced. The sequences of the TK genes of the entomopoxviruses were closely related and exhibited 63.2% identity and 9.9% similarity at the protein level. However, there was only 36.7% identity and 13.6% similarity when these enzymes were compared to their mammalian poxvirus counterpart in vaccinia virus. Finally, one entomopoxvirus TK gene was expressed in Escherichia coli mutants lacking the enzyme. These bacteria were converted to a phenotype that could incorporate radioactive thymidine into their chromosomal DNA. The results presented in this paper provide impetus for the design of a recombinant entomopoxvirus expression system in which foreign genes could be introduced into the viral TK locus under selective pressure from bromodeoxyuridine.

Mutations conferring resistance to azide in Escherichia coli occur primarily in the secA gene

Mutant strains of Escherichia coli were screened for the ability to grow on L agar plates containing 3.4 or 4.6 mM sodium azide. Most mutants had mutations located in the leucine region, presumably at the azi locus. Two of these mutants were found to have a mutation in the secA gene, but expression of the resistance phenotype also required the presence of upstream gene X. While a plasmid carrying the X-secA mutant gene pair was able to confer azide resistance to a sensitive host, a similar plasmid harboring the wild-type secA allele rendered a resistant strain sensitive to azide, indicating codominance of the two alleles. That azide inhibits SecA is consistent with the fact that SecA has ATPase activity, an activity that is often prone to inhibition by azide.