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L A Henricksen

Publications and source records attributed to L A Henricksen.

8 recordsLinked to original sources
PubMed1998

Junction ribonuclease: an activity in Okazaki fragment processing.

The initiator RNAs of mammalian Okazaki fragments are thought to be removed by RNase HI and the 5'-3' flap endonuclease (FEN1). Earlier evidence indicated that the cleavage site of RNase HI is 5' of the last ribonucleotide at the RNA-DNA junction on an Okazaki substrate. In current work, highly purified calf RNase HI makes this exact cleavage in Okazaki fragments containing mismatches that distort the hybrid structure of the heteroduplex. Furthermore, even fully unannealed Okazaki fragments were cleaved. Clearly, the enzyme recognizes the transition from RNA to DNA on a single-stranded substrate and not the RNA/DNA heteroduplex structure. We have named this junction RNase activity. This activity exactly comigrates with RNase HI activity during purification strongly suggesting that both activities reside in the same enzyme. After junction cleavage, FEN1 removes the remaining ribonucleotide. Because FEN1 prefers a substrate with a single-stranded 5'-flap structure, the single-stranded activity of junction RNase suggests that Okazaki fragments are displaced to form a 5'-tail prior to cleavage by both nucleases.

Animals↗
PubMed1996

Phosphorylation of human replication protein A by the DNA-dependent protein kinase is involved in the modulation of DNA replication.

The single-stranded DNA-binding protein, Replication Protein A (RPA), is a heterotrimeric complex with subunits of 70, 32 and 14 kDa involved in DNA metabolism. RPA may be a target for cellular regulation; the 32 kDa subunit (RPA32) is phosphorylated by several cellular kinases including the DNA-dependent protein kinase (DNA-PK). We have purified a mutant hRPA complex lacking amino acids 1-33 of RPA32 (rhRPA x 32delta1-33). This mutant bound ssDNA and supported DNA replication; however, rhRPA x 32delta1-33 was not phosphorylated under replication conditions or directly by DNA-PK. Proteolytic mapping revealed that all the sites phosphorylated by DNA-PK are contained on residues 1-33 of RPA32. When wild-type RPA was treated with DNA-PK and the mixture added to SV40 replication assays, DNA replication was supported. In contrast, when rhRPA x 32delta1-33 was treated with DNA-PK, DNA replication was strongly inhibited. Because untreated rhRPA x 32delta1-33 is fully functional, this suggests that the N-terminus of RPA is needed to overcome inhibitory effects of DNA-PK on other components of the DNA replication system. Thus, phosphorylation of RPA may modulate DNA replication indirectly, through interactions with other proteins whose activity is modulated by phosphorylation.

DNA Replication↗
PubMed1996

Proteolytic mapping of human replication protein A: evidence for multiple structural domains and a conformational change upon interaction with single-stranded DNA.

Replication protein A (RPA) is multisubunit single-stranded DNA-binding protein required for multiple processes in DNA metabolism including DNA replication, DNA repair, and recombination. Human RPA is a stable complex of three subunits of 70, 32, and 14 kDa (RPA70, RPA32, and RPA14, respectively). We examined the structure of both wild-type and mutant forms of human RPA by mapping sites sensitive to proteolytic cleavage. For all three subunits, only a subset of the possible protease cleavage sites was sensitive to digestion. RPA70 was cleaved into multiple fragments of defined lengths. RPA32 was cleaved rapidly to a approximately 28-kDa polypeptide containing the C-terminus that was partially resistant to further digestion. RPA14 was refractory to digestion under the conditions used in these studies. The digestion properties of a complex of RPA32 and RPA14 were similar to those of the native heterotrimeric RPA complex, indicating that the structure of these subunits is similar in both complexes. Epitopes recognized by monoclonal antibodies to RPA70 were mapped, and this information was used to determine the position of individual cleavage events. These studies suggest that RPA70 is composed of at least two structural domains: an approximately 18-kDa N-terminal domain and a approximately 52-kDa C-terminal domain. The N-terminus of RPA70 was not required for single-stranded DNA-binding activity. Multiple changes in the digestion pattern were observed when RPA bound single-stranded DNA: degradation of the approximately 52-kDa domain of RPA70 was inhibited while proteolysis of RPA32 was stimulated. These data indicate that RPA undergoes a conformational change upon binding to single-stranded DNA.

Antibodies, Monoclonal↗
PubMed1995

RPA involvement in the damage-recognition and incision steps of nucleotide excision repair.

Human replication protein (RPA) functions in DNA replication, homologous recombination and nucleotide excision repair. This multisubunit single-stranded DNA-binding protein may be required to make unique protein-protein contacts because heterologous single-stranded binding proteins cannot substitute for RPA in these diverse DNA transactions. We report here that, by using affinity chromatography and immunoprecipitation, we found that human RPA bound specifically and directly to two excision repair proteins, the xeroderma pigmentosum damage-recognition protein XPA (refs 8, 9) and the endonuclease XPG (refs 10-13). Although it had been suggested that RPA might function before the DNA synthesis repair stage, our finding that a complex of RPA and XPA showed a striking cooperativity in binding to DNA lesions indicates that RPA may function at the very earliest stage of excision repair. In addition, by binding XPG, RPA may target this endonuclease to damaged DNA.

Chromatography, Affinity↗
PubMed1994

Replication protein A mutants lacking phosphorylation sites for p34cdc2 kinase support DNA replication.

Replication Protein A (RPA) is a multisubunit, single-stranded DNA-binding protein essential for DNA metabolism in eukaryotic cells. The 32-kDa subunit of RPA is phosphorylated in a cell cycle-dependent manner becoming phosphorylated during S phase. It has been postulated that this phosphorylation may regulate the activities of RPA and that the family of p34cdc2 kinases directly catalyzes the phosphorylation of RPA in the cell. We have mutated the two consensus p34cdc2 sites in the 32-kDa subunit of RPA individually and in combination and purified the mutant protein complexes. Mutant RPA with both consensus p34cdc2 sites converted to alanine was not phosphorylated by purified p34cdc2 kinase. Nevertheless, we found that the properties of these RPA mutants were identical to those of the wild-type protein. The mutated RPA proteins had normal single-stranded DNA binding activity and were completely functional for DNA replication. In addition, the mutants became hyperphosphorylated when incubated under DNA replication conditions. These results demonstrate that phosphorylation by p34cdc2 kinase is not essential for RPA function in DNA replication in vitro. Possible roles of RPA phosphorylation on DNA metabolism are discussed.

CDC2 Protein Kinase↗
PubMed1994

Recombinant replication protein A: expression, complex formation, and functional characterization.

Replication protein A (RPA) is a multisubunit, single-stranded DNA-binding protein that is absolutely required for replication of SV40 DNA. The three cDNAs encoding the subunits of human replication protein A (70, 32, and 14 kDa) have been expressed individually and in combination in Escherichia coli. When subunits were expressed individually, appropriately sized polypeptides were synthesized, but were found to be either insoluble or aggregated with other proteins. We examined the interactions between individual RPA subunits by expressing pairs of subunits and determining if they formed stable complexes. Only the 32- and 14-kDa subunits formed a soluble complex when coexpressed. This complex was purified and characterized. The 32-14 kDa subcomplex did not have any effect on DNA replication and was not phosphorylated efficiently in vitro. We believe that the 32.14-kDa subcomplex may be a precursor in the assembly of the complete RPA complex. Coexpression of all three subunits of RPA resulted in a significant portion of each polypeptide forming a soluble complex. We have purified recombinant RPA complex from E. coli and demonstrated that it has properties similar to those of human RPA. Recombinant human RPA has the same subunit composition and the same activities as the authentic complex from human cells. Recombinant human RPA binds single-stranded DNA and is capable of supporting SV40 DNA replication in vitro. In addition, recombinant RPA became phosphorylated when incubated under replication conditions.

Base Sequence↗