Intersectin goes nuclear: secret life of an endocytic protein

Intersectin 1-short (ITSN1-s) is a 1220 amino acids ubiquitously-expressed scaffold protein presenting a multi-domain structure that allows to spatiotemporally regulate the functional interaction of a plethora of proteins. Beside its well-established role in endocytosis, ITSN1-s is involved in regulation of cell signaling and is implicated in tumorigenesis processes, although the signaling pathways involved are still poorly understood. Here we identify ITSN1-s as a nucleocytoplasmic trafficking protein. We show that, by binding to importin (IMP)a, a small fraction of ITSN1-s localizes in the cell nucleus at the steady state, where it preferentially associates with the nuclear envelope (NE) and interacts with lamin A/C. However, upon pharmacological ablation of Chromosome region maintenance 1 (CRM-1) dependent nuclear export pathway, the protein accumulates into the nucleus, thus revealing its moonlighting nature. Analysis of deletion mutants revealed that the coiled coil (CC) and Src homology (SH3) regions play the major role in its nucleocytoplasmic shuttling. While no evident nuclear localization signal (NLS) was detected in the CC region, a functional bipartite NLS was identified within the SH3D region of ITSN1-s (RKK NPGGWWEGELQA R G KKR QIGW-1127), capable of conferring energy-dependent nuclear accumulation to reporter proteins and whose mutational ablation affects nuclear import of the whole SH3 region. Thus, ITSN1-s is an endocytic protein, which shuttles between the nucleus and the cytoplasm in a CRM-1 and IMPa dependent fashion.