Recombinant P32 binds to recombinant RNase H1, enhances its turnover rate, and reduces the binding affinity of the enzyme for the heteroduplex substrate.

<p>(<b>A</b>) Coomassie blue staining of the purified human His-H1, GST protein, and GST-P32 proteins separated by SDS-PAGE. The sizes for the standard protein markers are indicated. (<b>B</b>) RNase H1 but not P32 appears to bind the heteroduplex substrate. Gel shift assay was performed using 0.4 ug purified RNase H1, GST-P32, or GST proteins incubated at 4°C for 30 min with a non-cleavable heteroduplex containing ³²P labeled uniformly modified 2′-fluoro RNA annealed to DNA and subjected to native gel electrophoresis. (<b>C</b>) The interaction between RNase H1 and P32 appears to be equal molar. A fixed amount of GST-P32 was bound to GST affinity beads and then incubated with increasing amounts of RNase H1. Glutathione (GSH) eluted RNase H1 and P32 were quantified by Western blot as described in the Material and Methods. The amounts of bead-bound P32 and P32-associated RNase H1 were determined by loading known amounts of the respective proteins (left panel). The molecular ratio of bound RNase H1 relative to P32 was calculated and plotted in the right panel. (<b>D</b>) The effects of ionic strength on RNase H1/P32 interaction. Left panel: RNase H1 binds GST-P32 but not GST protein. GST or GST-P32 bound to anti-GST beads was incubated with RNase H1 in NaCl concentrations ranging from 0-950 mM as described in the Material and Methods. Middle panel: increasing NaCl concentration inhibits binding of RNase H1 to P32. Both unbound (flow through) and bound (affinity eluted) fractions were collected and the levels of RNase H1 and P32 evaluated by western blot. Right panel: Increasing pH reduced binding of RNase H1 to P32. (<b>E</b>) Michaelis-Menten kinetics and binding constants for RNase H1 cleavage of an RNA/DNA duplex in the presence or absence of P32. The K_m, V_max, and K_d were determined by incubating the Apo B RNA/DNA duplex with RNase H1 plus GST (as control) or RNase H1 plus different amounts of P32 resulting in an H1:P32 ratio = 1∶1 or 1∶5. An uncleavable competitive inhibitor (2′-fluororibonucleotide/DNA) was used to determine the binding to the RNA/DNA duplex, as described in the Material and Methods. The calculated constants are indicated in the right panel. The error bars indicate the standard error from three parallel experiments.