Immunofluorescence analysis was carried out as previously described (Sabbattini et al., 2007 (link)). Secondary antibodies used for fluorescence detection were Alexa 488–anti-rabbit and Alexa 488–anti-rat (both from Molecular Probes, Eugene, OR). Images were collected by confocal microscopy using a SP5 microscope (Leica Microsystems, Wetzlar, Germany) and LAS-AF software.