Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 199

Bacteriophage T4, which has been studied for more than 50 years, is the classic example of an architecturally elaborate virus that contains distinct parts that exhibit icosahedral or helical symmetry. The T4 particle, which is built from ~50 of the proteins encoded in the ~170-kbp double-stranded DNA genome, is a structurally elegant machine tailored for active delivery of the genome to host cells. The most striking feature is the presence of morphologically distinct and functionally specialized structures, notably the head containing the genome and a long tail that terminates in a baseplate from which six long tail fibers protrude (Fig. 4.26A).

The head of the mature T4 particle, an elongated icosahedron, is built from hexamers of a single viral protein (gp23*). In contrast to the other capsids considered so far, two T numbers are needed to describe the organization of gp23* in the two end structures (T = 13) and in the elongated midsection (T = 20). As in adenoviral capsids, the pentamers that occupy the vertices contain a different viral protein, and additional proteins reside on the outer or inner surfaces of the icosahedral shell (Fig. 4.26B). One of the 12 vertices is occupied by a unique structure termed the connector, which joins the head to the tail. Such structures are derived from a nanomachine termed the portal, which pulls DNA into immature heads. Portals are a characteristic feature of the capsids of other families of DNA-containing bacteriophages, as well as of herpesviruses.