The secretion of human T cell leukemia virus type 1 Tax protein is a regulated process involving components of cellular secretory pathway and critical amino acid signals within the viral protein: Implications for HTLV-1 neuropathogenesis

HTLV-1 associated myelopathy/tropical spastic para-paresis (HAM/TSP) is a progressive neurological disorder. Many pathologic abnormalities associated with HAM/TSP have been attributed to the extracellular activity of the viral transactivator protein Tax. Cell-free Tax has been detected in the cerebrospinal fluid of HAM/TSP patients indicating its physiologic relevance in the neuropathogenesis. We have previously demonstrated the presence of a nuclear export signal within Tax and its active secretion in vitro. Studies reported herein elucidate the process by which Tax is secreted and identify domains of Tax that contribute to its subcellular localization and secretion. Tax was shown to co-localize with endoplasmic reticulum (ER) and Golgi, the cytoplasmic organelle relevant to protein secretion. An extensive Proteomics analyses followed by GST pull-down assay revealed the specific interaction of Tax with several proteins of the cellular secretory pathway including COPII, SCAMP1, SCAMP2 and SNAP23 in both the model cell line BHK (baby hamster kidney)-21 and a T cell line C8166 (relevant to HTLV-1 infection). The silencing of these proteins inhibited Tax secretion further confirming their involvement in this process. Comparison of the Tax protein sequence with, amino acid signals known to target proteins to the secretory pathway yielded the identification of a number of putative secretory signals. The mutations in two signals DHE and YTNI resulted in aberrant subcellular localization of Tax with a large amount of Tax localized to areas immediately surrounding the nucleus. In addition, Tax constructs containing mutations in both signals significantly altered protein secretion. Together, these studies suggest that secretion of Tax is a regulated event facilitated by the interaction of Tax with cellular secretory pathway proteins and the presence of critical secretory signals within the carboxy-terminal domain of protein.