HTLV-1 p13, a small protein with a busy agenda

Abstract

Human T-cell leukemia virus type 1 (HTLV-1) infection is characterized by life-long persistence of the virus in the host. While most infected individuals remain asymptomatic, 3-5% will eventually develop adult T-cell leukemia/lymphoma (ATLL) or tropical spastic paraparesis/HTLV-associated myelopathy (TSP/HAM) after a clinical latency that can span years (TSP/HAM) to decades (ATLL). The major oncogenic determinant among HTLV-1 proteins is the Tax transactivator, which influences the expression and function of a great number of cellular proteins, drives cell proliferation, reduces cell death, and induces genetic instability. The present review is focused on the current knowledge of p13, an HTLV-1 accessory protein targeted to the inner mitochondrial membrane and, under certain conditions, to the nucleus. In mitochondria, p13 produces an inward K+current that results in an increased production of ROS by mitochondria. These effects are linked to the protein's effects on cell turnover which include activation of primary T-cells and reduced proliferation/sensitization to death of tumor cells. Recent findings suggest that in the presence of Tax, p13 is subjected to ubiquitylation and partly targeted to the nucleus. Nuclear p13 binds Tax and inhibits its transcriptional activity. These findings suggest that the protein might exert distinct functions depending on its intracellular localization and influence both the turnover of infected cells and the balance between viral latency and productive infection.

Figure 1

Indicated is the domain structure of p13 suggested by biochemical and in silico analyses. AA indicates the amphipathic alpha helix (residues 20-35) which includes the mitochondrial targeting signal (MTS) of p13 and is essential for the protein's function in mitochondria. TM indicates the transmembrane region (residues 30-40); H indicates a region with a high flexibility score (residues 42-48) which is likely to form a hinge in the structure of p13. B indicates a predicted βsheet hairpin structure (residues 65-75). The C-terminal portion of p13 also includes PXXP SH3 domain-binding motifs and a cryptic nuclear localization sequence (NLS) whose exact position remains to be determined.

Figure 2

Working model of p13 function based on the results of studies on isolated mitochondria. p13 induces inner mitochondrial membrane potential (Δψ)-driven influx of K⁺. The influx of K⁺ induced by p13 triggers an increase in the activity of the electron transport chain (ETC) that, by extruding H⁺ from the matrix, dampens the depolarization induced by p13. Increased ETC activity also leads to increased production of reactive oxygen species (ROS) by the ETC that lowers the opening threshold of the permeability transition pore (PTP), suggesting that p13 might trigger apoptosis. OM, outer membrane; IMS, intermembrane space; IM, inner membrane.

Figure 3

Mitochondrial and nuclear functions of p13. The observations indicating a dual localization of p13 suggest that the protein might exert distinct functions depending on its intracellular localization. Effects on mitochondria, mediated through increased ROS production, control cell turnover. The effects of p13 on inner membrane potential also inhibit the mitochondrial uptake of Ca²⁺, which in turn may modulate Ca²⁺ signalling that in T cells also plays a pivotal role in controlling cell activation and death. Nuclear localization, which follows p13 ubiquitylation (U) in the presence of Tax results in the formation of transcriptionally inactive p13-Tax dimers. This effect is likely to reduce the expression of both HTLV-1 and Tax-controlled cellular genes.

Figure 4

Effects of p13 on T-cell proliferation. Shown is a proliferation analysis of Jurkat T cells transduced with a lentiviral vector expressing p13 or with a control empty vector. Cells were labelled with the cell membrane fluorescent probe PKH26 (sigma) on day 0 and proliferation was assessed by measuring the dilution of the fluorescent signal resulting from cell division was monitored on days 2 and 6. A) Flow cytometry analysis of PKH26 staining demonstrating a higher proportion of cells of higher generation in control compared to p13-expressing cells. Colors indicate generations of cells exhibiting progressively lower intensities of PKH26 staining (Blue, generation 1= parental; orange, generation 2; green, generation 3; purple, generation 4; light blue, generation 5); arrow indicates peak fluorescence values of parental (time o) generation for the two cell lines. B) Mean proliferation indexes over time calculated from the PKH26 analyses in control vs. p13-expressing cells.

Figure 5

Functional interactions of Tax and p13 in HTLV-1 persistence and T-cell transformation. Studies carried out so far suggest that Tax and p13 might act in concert to maintain the proliferation and survival rates of infected cells at a level compatible with long term viral persistence and survival of the host. Solid lines indicate demonstrated effects, while dotted lines indicate hypothetical interactions. Tax drives T-cell proliferation and reduces cell death resulting in the accumulation of infected cell pool; however due to the effects of Tax on cell cycle checkpoints and genetic stability these cells are prone to accumulate genetic lesions resulting in the emergence of neoplastic clones giving rise to ATLL. p13 activation of resting T-cells might cooperate with Tax in expanding the pool of “normal” infected T-cells; however, the proapototic effect of p13 on tumor cells might contrast with the anti-apoptotic effect of Tax, reducing the probability of ATLL transformation.