An orphan receptor tyrosine kinase family whose members serve as nonintegrin collagen receptors

Mammalian cells constantly monitor and respond to a myriad of extracellular signals, often by using cell surface receptors. Two important classes of cell surface receptors include the receptor tyrosine kinases, which recognize peptide growth factors such as insulin, and the integrins, which most often mediate binding to components of the extracellular matrix. We report that the collagens serve as ligands for the previously orphan family of discoidin domain-containing receptor-like tyrosine kinases. The unexpected realization that an extracellular matrix molecule can directly serve as a ligand for receptor tyrosine kinases provides an example of ligands shared by integrins and receptor tyrosine kinases, and this finding seems likely to change prevailing views about the mechanisms by which cells perceive and respond to the extracellular matrix.

Probing the mechanism of CRP activation by site-directed mutagenesis: the role of serine 128 in the allosteric pathway of cAMP receptor protein activation

Upon activation by cAMP, Escherichia coli cAMP receptor protein (CRP) controls the expression of a network of catabolite sensitive genes. The activation of CRP by cAMP involves conformational changes such as realignments between subunits and domains within the protein. To understand the molecular events that lead to the activation of CRP, point mutations at position 128 were introduced via site-directed mutagenesis in an attempt to specifically affect the subunit interfacial interactions, as well as the ligand-binding reaction. The biochemical and biophysical properties of these mutants were rigorously tested with the goal of identifying the partial reactions in the activation pathway that are perturbed by this specific mutation. Results from this study suggest that mutation of Ser 128 to Ala or Pro does not significantly disturb the overall secondary structure as monitored by circular dichroism. The energetics of subunit-subunit interaction and protein stability were monitored by sedimentation and spectroscopic techniques. Although these mutants were designed to interrupt intersubunit interactions, the energetics of subunit association and protein stability remain quantitatively the same as those of the wild-type CRP. Nevertheless, the ability of the subunit to be realigned to the DNA-binding form is significantly affected as reflected by the pronounced decrease in the susceptibility of mutant CRP to proteolytic digestion in the presence of cAMP. In addition, the binding affinity of cAMP to the first ligand site in mutants S128A and S128P is the same as that of the wild type, but the affinity to the second ligand site is reduced. This observation indicates that mutation at position 128 affects ligand binding by amplifying the magnitude of negative cooperativity. Mutation at residue 128 does not impair the ability of interdomain interactions as indicated by the quantitative response of a spectroscopic probe in the DNA-binding domain to the binding of cAMP to the ligand-binding domain. The S128A mutant binds to a specific DNA sequence about 50-fold weaker than the wild-type CRP, while the mutant S128P has no measurable DNA affinity under the same conditions. This observation is consistent with the in vivo result that both mutants display an inactive CRP phenotype (CRP-). In summary, these results suggest that communication between domains induced by cAMP binding can be dissociated from the proper subunit realignment of the CRP dimer that is crucial for the activation of CRP.(ABSTRACT TRUNCATED AT 400 WORDS)

Linear mitochondrial DNAs from yeasts: telomeres with large tandem repetitions

The terminal structure of the linear mitochondrial DNA (mtDNA) from the yeast Candida parapsilosis was investigated. This mtDNA, 30 kb long, has symmetrical ends forming inverted terminal repeats. These repeats are made up of a variable number of tandemly repeating units of 738 bp each; the terminal nucleotide corresponds to a precise position within the last repeat unit sequence. The ends had an open structure accessible to enzymes, with a 5' single-stranded extension of about 110 nucleotides. No circular forms were detected in the DNA preparations. Two other unrelated species, Pichia philodendra and Candida salmanticensis also appear to have a linear mtDNA of similar organization. These linear DNAs (which we name Type 2 linear mtDNAs) are distinct from the previously described linear mtDNAs of yeasts whose termini are formed by a closed hairpin loop (Type 1 linear mtDNA). The terminal structure of C. parapsilosis mtDNA is reminiscent of the linear mitochondrial genomes of the ciliate Tetrahymena although, in the latter, the telomeric tandem repeat unit is considerably shorter.

Respiratory chain defect of myocardial mitochondria in idiopathic dilated cardiomyopathy of Doberman pinscher dogs

Idiopathic dilated cardiomyopathy (IDCM) is a primary myocardial disease of unknown cause. We tested the hypothesis that IDCM was associated with a myocardial metabolic defect by determining a comprehensive biochemical profile of metabolite concentrations and enzyme activities for the major metabolic pathways of the myocardium. We used the Doberman pinscher breed as a naturally occurring canine model of IDCM and compared its myocardial profile with that of healthy adult mongrels. Compared with controls, myocardium in IDCM had markedly reduced mitochondrial electron transport activity and myoglobin concentration, in association with acidosis and energy depletion following anoxic challenge: 60% decreased NADH dehydrogenase and 50% decreased ATP synthetase activities; 90% decreased myoglobin concentration; and 30% reduced ATP and 50% increased lactate and proton concentrations. Sarcoplasmic reticulum Ca(2+)-transport ATPase was decreased by 42%. There was a 15% compensatory increase in fatty acid oxidation and Krebs cycle activity. Other biochemical changes were mild by comparison with the mitochondrial defects. We conclude that IDCM is associated with a marked impairment of mitochondrial production of ATP, arising from decreased activity of the mitochondrial electron transport system, including myoglobin. These changes may be secondary to an underlying genetic defect or may indicate a deficiency of the mitochondrial respiratory chain that predisposes this breed to heart failure.

[The incorporation of macromolecules into the germ cells of male mice via electroporation and dimethyl sulfoxide]

Electroporation (incorporation of macromolecules into the living cells by means of electric pulses) provides inclusion of plasmid 14C-DNA into immature cells of spermatogenic epithelium. The highest level of foreign DNA incorporation into spermatocytes and spermatids has been induced by 8kV electric pulses applied 3 times with 20 sec intervals. Meanwhile, mature sperms are found to be exclusively resistant to exogenous DNA irrespective of the voltage level, the number of pulses and Ca++ uptake (contents). Incubation of mature sperms for two hours in the medium with Ca++ (10 mM) and dimethylsulfoxide--(DMSO, 33%) provides highly reliable incorporation of plasmid 14C-DNA into sperm heads. The sperm cells with foreign DNA incorporated by means of Ca++ and DMSO treatment still remain alive and mobile. The possibilities of mature sperms loaded with foreign DNA for the creation of transgenic mammals are discussed.