Molecular Comparisons of the D-Type Retroviruses

Abstract

The D-type retroviruses represent the most recently discovered genus within the sub-family oncovirinae of the family Retroviridae. The assignment of viruses into this group has been primarily on the basis of similarities in virus structure and morphogenesis. Mason-Pfizer monkey virus (M-PMV), the prototype D-type retrovirus, was isolated from a breast carcinoma of a female rhesus monkey by Jensen et al. (12) in 1970. The virus has properties in common with both the mammalian B-type viruses (e.g. mouse mammary tumor virus, MMTV) and C-type viruses (e.g. baboon endogenous virus, BaEV) but is distinct from both (2, 3). Like MMTV, M-PMV preassembles a virus core structure or A-type particle within the cell, and thus virus particles bud from the plasma membrane with a complete nucleoid (Fig 1, panels a and b). However, the morphology of the mature D-type virion more closely resembles that of C-type viruses, in that in general the condensed, cylindrical nucleoid is centrally located (Fig.l, panel c) in contrast to the eccentric location of the B-type nucleoid. Furthermore, the virion lacks the dense fringe of glycoprotein seen on MMTV, possessing instead a halo of knobs or knobbed spikes (Fig. 1, panels d,e and f). A second feature in common with MMTV is the presence of a reverse transcriptase with a preference for magnesium over manganese (4). The latter divalent cation is preferred by most mammalian C-type retrovirus enzymes (5).