Regulated exocytosis is a hallmark in secretory cells. Cargo proteins to be secreted are involved in sequential steps of sorting, budding and forming vesicles in the Golgi complexes. Among these processes, adaptor proteins participate in vesicle formation during retrograde or anterograde trafficking to or from the Golgi, and small GTPase ADP-ribosylation factor 1 (Arf1) plays a crucial role in recruiting them to Golgi. Due to the properties of being activated and inactivated, Arf1 acts as a switch determining where and when those vesicles are to be formed....[ Read more ]

Regulated exocytosis is a hallmark in secretory cells. Cargo proteins to be secreted are involved in sequential steps of sorting, budding and forming vesicles in the Golgi complexes. Among these processes, adaptor proteins participate in vesicle formation during retrograde or anterograde trafficking to or from the Golgi, and small GTPase ADP-ribosylation factor 1 (Arf1) plays a crucial role in recruiting them to Golgi. Due to the properties of being activated and inactivated, Arf1 acts as a switch determining where and when those vesicles are to be formed.

Protein-interacting with C-kinase 1 (PICK1) has a BAR (B̲in1/A̲mphiphysin/R̲vs167) domain highly similar to arfaptin, an effector of Arf1. Current studies of PICK1 have been focused on its role in recycling GluR2-containing AMPA receptors during neuronal plasticity. In hippocampal neurons, PICK1 predominantly forms heterodimer with another BAR domain-containing protein Islet autoantigen 69-kD (ICA69). ICA69 homolog ric-19 has been implicated to play an important role in secretion in C. elegans. In mammal, the high expression level of PICK1 and ICA69 in organs actively participate in secretion such as brain and pancreas suggests the association of PICK1-ICA69 heteromeric complex with vesicular trafficking. Nonetheless, the details are remaining elusive.

In this study, we will follow two lines of direction regarding to the functions of PICK1-ICA69 complex: its general roles and regulatory mechanism in vesicular trafficking. First, we showed that PICK1/ICA69 immuno-positive vesicles were Golgi-related and their subcellular localization was conserved among different secretory cell lines. We also found that PICK1-ICA69 complex interacted with small GTPase Arf1 in vitro with different binding preference. Active Arf1 could also recruit PICK1-ICA69 complex to Golgi. Using fluorescence resonance energy transfer (FRET) approach, PICK1-ICA69 heteromer was found to be potentially regulated by inactive Arf1 through disrupting the complex. Under stimulated secretion, PICK1 colocalized with Chromogranin A containing vesicles in AtT-20 cells. Given the sign that Arf1 is involving in vesicle formation in Golgi, these observations suggest the significant roles of PICK1-ICA69 complex in Golgi that could possibly function cooperatively with Arf1 during vesicular trafficking.