Folyl polyglutamate and folate-requiring enzymes as bacteriophage T4D baseplate structural components

Abstract

The hexagonal-shaped baseplate on the tail of Escherichia coli bacteriophage T4D has been shown to contain 5–6 molecules of an unusual form of folic acid, dihydropteroyl hexaglutamate. In addition to this viral-induced small molecule, 2 viral-induced folate-requiring enzymes, namely dihydrofolate reductase ( frd) and thymidylate synthetase ( td), have also been found as phage baseplate components. The pteridine portion of the folate has been located near the baseplate frd and td and all 3 components are partially covered by T4D gene product 11 (gp11) which lies largely on the distal surface of the baseplate. On the other hand, the polyglutamate portion of the baseplate folic acid lies under or near the attachment region for the long tail fibers. Recent results have shown that the baseplate frd is a component of the outer wedge-like structures of the baseplate and highly suggestive evidence has now been obtained that the td molecule is a component of the central plug of the baseplate. These findings support the hypothesis that the baseplate folate plays a unique role in linking together the baseplate wedges and central plug by binding simultaneously to the wedge frd and to the plug td. This structural role for the viral folate and folate-requiring enzymes indicates that these baseplate components are not acting primarily in their usual roles as a coenzyme or as enzymes but are participants in a unique biological assembly process. Some molecular properties of dihydropteroyl hexaglutamate are presented which bear on its binding energies as a structural linking element. Finally some speculation is offered on the abiogenic origin of pteridine compounds.