The PY motif in the HECT domain of human E3 ubiquitin-protein ligase NEDD4L/KIAA0439 binds to the WW3 domain of human E3 ubiquitin-protein ligase NEDD4L/KIAA0439, suggesting a possible auto-ubquitination mechanism using two molecules, a partially unfolded one and a fully functional NEDD4L counterpart