title: Gene 32 protein, the single-stranded DNA binding protein from bacteriophage T4, is a zinc metalloprotein.
abstract: Gene 32 protein (g32P) isolated from bacteriophage T4-infected Escherichia coli and from an overproduction vector derived from the plasmid pKC30 contains 1 mol of tightly incorporated Zn(II) per mol of protein. A linear incorporation of three molar equivalents of p-hydroxymercuriphenylsulfonate (PMPS) results in a linear release of 1.1 mol of Zn(II) from the protein. Reversal of formation of the g32P-PMPS complex with thiol in the presence of EDTA results in a zinc-free apo-g32P. Cd(II) and Co(II) can be exchanged with the intrinsic Zn(II) ion. The Cd(II) protein shows a charge-transfer band at approximately 250 nm. The Co(II) protein shows a set of absorption bands typical of a tetrahedral Co(II) complex (epsilon max = 660 M-1 X cm-1 at 645 nm), and two intense charge-transfer bands are present at 355 nm (epsilon = 2,250 M-1 X cm-1) and 320 nm (epsilon = 3,175 M-1 X cm-1). These observations are consistent with three cysteines as ligands to the Zn(II) ion in g32P. Zn(II) g32P undergoes precise limited proteolysis by trypsin to produce the small fragments A and B and the core, g32P-(A + B). Under identical conditions, apo-g32P is hydrolyzed rapidly beyond the g32P-(A + B) stage to produce many proteolyzed fragments. Fluorescence quenching experiments show that at low protein concentration apo-g32P has markedly altered binding affinity for poly(dT) relative to native g32P. Three of the four cysteines of g32P are found in a tyrosine-rich sequence corresponding to residues 72-116 and implicated in DNA binding by 1H NMR investigations. Zn(II) appears to provide a conformational element contributing to DNA binding by coordinating the cysteine and possibly histidine side chains in the sequence -Cys-X3-His-X5-Cys-X2-Cys-, residues 77-90, located in the DNA binding domain of g32P.
mesh_terms: D000595:Amino Acid Sequence; D002104:Cadmium / Q000032:analysis; D003035:Cobalt / Q000032:analysis; D004277:DNA, Single-Stranded / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000032:analysis*; D008667:Metalloproteins / Q000032:analysis*; D011071:Poly T / Q000378:metabolism; D013604:T-Phages / Q000032:analysis*; D014357:Trypsin / Q000494:pharmacology; D014764:Viral Proteins / Q000032:analysis* / Q000378:metabolism; D015032:Zinc / Q000032:analysis*
chemical_list: D004277:DNA, Single-Stranded; D004268:DNA-Binding Proteins; D008667:Metalloproteins; D014764:Viral Proteins; D002104:Cadmium; D011071:Poly T; D003035:Cobalt; D014357:Trypsin; D015032:Zinc