Molecular cloning and characterization of phocein, a protein found from the Golgi complex to dendritic spines

Phocein is a widely expressed, highly conserved intracellular protein of 22 5 amino acids, the sequence of which has limited homology to the a subunits from clathrin adaptor complexes and contains an additional stretch bearing a putative SH3-binding domain. This sequence is evolutionarily very conser ved (80% identity between Drosophila melanogaster and human). Phocein was d iscovered by a yeast two-hybrid screen using striatin as a bait. Striatin, SG2NA, and zinedin, the three mammalian members of the striatin family, are multimodular, WD-repeat, and calmodulin-binding proteins. The interaction of phocein with striatin, SG2NA, and zinedin was validated in vitro by coim munoprecipitation and pull-down experiments. Fractionation of brain and HeL a cells showed that phocein is associated with membranes, as well as presen t in the cytosol where it behaves as a protein complex. The molecular inter action between SG2NA and phocein was confirmed by their in vivo colocalizat ion, as observed in HeLa cells where antibodies directed against either pho cein or SG2NA immunostained the Golgi complex. A 2-min brefeldin A treatmen t of HeLa cells induced the redistribution of both proteins. Immunocytochem ical studies of adult rat brain sections showed that phocein reactivity, pr esent in many types of neurons, is strictly somato-dendritic and extends do wn to spines, just as do striatin and SG2NA.