Natural biology of polyomavirus middle T antigen

Abstract

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

FIG. 1

Schematic of MT. MT is 421 aa long and is encoded by nucleotides 175 to 748 (aa 1 to 191) spliced to 811 to 1499 (aa 192 to 421) based upon the sequence numbering of the A2 wild-type strain. The membrane-spanning domain consists of hydrophobic aa 394 to 415. On interaction with the plasma membrane, MT associates with (although does not directly bind to) src, a tyrosine kinase, resulting in the phosphorylation of tyrosines at aa 250, 315, and 322 and in the binding of Shc, PI 3-kinase, and PLCγ-1. In addition, MT contains serine (aa 257 and 283) and threonine (aa 160 and 291) phosphorylation sites. The phosphorylation of serine 257 results in the interaction with the 14-3-3 proteins. Finally, MT associates with both the catalytic subunit C and the regulatory domain A of PP2A at aa 90 to 120.

FIG. 2

MT and apoptosis. A schematic diagram of the potential antiapoptotic effects of the PI 3-kinase activation of Akt/PKB is shown. See the text for details. Akt can prevent apoptosis by inhibiting Ced3/ICE-like activity and phosphorylating and deactivating bad and glycogen synthase kinase-3 (GSK-3). In addition, Akt induces the expression of bcl2 and c-myc, which also helps protect against apoptosis. This pathway resembles those induced by many cellular growth factors, including EGF and PDGF.

FIG. 3

Proposed role of MT in polyomavirus replication. Following infection and establishment of persistence, polyomavirus is reactivated following a signal (external) to differentiate in the infected cell (I). This signal triggers the expression of transcription factors (II), which induce early-gene expression, and the production of LT, MT, and ST (III). In an autocatalytic (autoregulatory) loop, MT induces the expression of transcription factors (IV) through its activation of various signal transduction pathways and further induces the expression of the early proteins (V). During this time, the early proteins (LT, MT, and ST) reprogram the infected cell to allow viral DNA replication (IV). There is then a switch from early gene-expression to viral DNA replication (V) and late-region expression (V) at some time during the differentiation of the infected cell. MT and the other early proteins are proposed to establish all the factors required for efficient virus production in a cell during its final stages of differentiation (VI).

FIG. 4

In vivo life cycle of polyomavirus in the mouse lung. A schematic of polyomavirus acute replication in the epithelial cells of the mouse bronchiole is shown. Ciliated cells and Clara cells (nonciliated epithelial cells) which make up a majority of the epithelium in mouse bronchioles, are indicated. In the more basal cells, T antigens (early-gene proteins) are expressed, often at very low levels. The cells then transition from a basal location to more apical position near the bronchiolar lumen. During this transition, capsid proteins (late-gene proteins) are expressed and the nuclei of the cells are found apically as well. The cytoplasm of the cells is squeezed from the basement membrane and then dislodged as the cells exfoliate into the bronchiolar lumen (unattached cell). The cells expressing viral proteins (early or late) do not express markers for either ciliated cells or Clara cells, indicating a new differential pathway.