No abstract

Highly purified p53 protein from different sources was able to degrade DNA with a 3'-to-5' polarity, yielding deoxynucleoside monophosphates as reaction products. This exonuclease activity was dependent on Mg2+ and inhibited by addition of 5 mM nucleoside monophosphates. This exonuclease activity is intrinsic to the wild-type p53 protein: it copurified with p53 during p53 preparation; only purified wild-type p53, but not identically purified mutant p53 proteins displayed exonuclease activity; the exonuclease activity could be reconstituted from SDS gel-purified and urea-renatured p53 protein and mapped to the core domain of the p53 molecule; and finally, purified p53 protein could be UV-cross-linked to GMP. A p53-intrinsic exonuclease activity should substantially extend our view on the role of p53 as a "guardian of the genome."

Integrin activation, ligand binding, and integrin clustering were analyzed using alphaIIb beta3 reconstituted into phospholipid vesicles and into supported planar lipid bilayers. Strong and specific binding of fibrinogen and the gamma-chain dodecapeptide of fibrinogen to alphaIIb beta3 indicated that the integrin is in an activated state after membrane reconstitution. Cryoelectron and fluorescence microscopy suggested a nonclustered state of the protein in the vesicle membrane. Supported planar lipid membranes were generated by fusion of vesicles in which approximately equal fractions of integrins were pointing inside-out and outside-in. This distribution led to an immobilization of about 40% of the integrin in supported bilayers due to attachment of the large extracellular domains to the quartz support. Fluorescence recovery after photobleaching indicated a diffusion coefficient of D = (0.70 +/- 0.06) x 10(-8) cm2/s, consistent with a nonclustered state of the mobile integrin. Upon fibrinogen binding, the integrins became immobile, and fluorescence micrographs showed a patchy distribution of fibrinogen-integrin complexes consisting of approximately 250 molecules. In addition to the expected dimer formation by bivalent fibrinogen, additionally induced fibrinogen clustering may account for the large size of the complexes. In contrast, binding of monovalent GRGDS pentapeptide or the gamma-chain dodecapeptide of fibrinogen altered neither the mobile fraction nor the association state of alphaIIb beta3. Our data indicate that integrin alphaIIbb3 is activated while monodisperse, and became clustered upon fibrinogen binding, leading to an irreversibly bound state.

A full-length reverse-transcribed, infectious cDNA copy of coxsackievirus B3 (CVB3) was used to determine the nucleotide sequence of this cardiotropic enterovirus. Comparison of the nucleotide sequence and the deduced amino acid sequence of the viral precursor polyprotein with the sequences of other group B coxsackieviruses (CVB1 and CVB4) demonstrates a high degree of genetic identity. They share about 80% homology at the nucleotide level and about 90% when the amino acid sequences of the polyproteins are compared. The potential processing sites of the coxsackievirus polyproteins, as deduced from alignment with the poliovirus sequence, are conserved among these enteroviruses with the exception of the cleavage sites between VP1 and 2APro and between polypeptides 2B and 2C. Comparison of the 5' termini of the enteroviral genomes reveals a high degree of identity, including the initial 5' consensus UUAAAACAGC, suggesting essential functions in virus replication. An important finding concerning the molecular basis of infectivity was that both recombinant CVB3 cDNA and in vitro-synthesized CVB3 RNA transcripts are infectious, although two initial 5' uridine residues found on the authentic CVB3 RNA were missing. Here, we report that cDNA-generated CVB3, as well as CVB3 generated by in vitro-synthesized RNA transcripts, regains the authentic initial 5' uridine residues during replication in transfected cells, indicating that the picornaviral primer molecule VPg-pUpU may be uridylylated in a template-independent fashion. The generation of virus or virus mutants with infectious recombinant CVB3 cDNA and in vitro-synthesized infectious CVB3 transcripts should provide a valuable means for studying the molecular basis of the pathogenicity of this cardiotropic enterovirus.