Jp. Yang et al., Mapping the functional domains of HAP95, a protein that binds RNA helicaseA and activates the constitutive transport element of type D retroviruses, J BIOL CHEM, 276(33), 2001, pp. 30694-30700
The complex retroviruses such as human immunodeficiency virus, type 1, empl
oy a virally encoded protein, Rev, to mediate the nuclear export of unsplic
ed and partially spliced mRNA. In contrast, the simian type D retroviruses
act through a cis-acting constitutive transport element (CTE) that presumab
ly interacts directly with cellular export proteins. We first reported that
RNA helicase A (RHA) is a shuttle protein that binds to functional CTE in
vitro and in vivo. Recently, we isolated a novel protein, HAP95, that speci
fically binds to the nuclear transport domain of RHA and up-regulates CTE-m
ediated gene expression. Here, using truncation and deletion mutations, we
mapped the domains of HAP95 that are important for RHA binding, transactiva
tion of CTE, and nuclear cytoplasmic shuttling. We report evidence for a no
vel nuclear export signal in HAP95 and showed that the domains involved in
RHA binding and nuclear localization are required for CTE activation. Final
ly, we showed that HAP95 synergizes significantly with RHA on CTE-mediated
reporter gene expression and promotes nuclear export of unspliced mRNA in t
ransfected cells. Taken together, these data support the proposal that HAP9
5 specifically facilitates CTE-mediated gene expression by directly binding
to RHA.