Golgi-dependent signaling: self-coordination of membrane trafficking.

Multiple studies have shown that endomembranes can act as signaling platforms for plasma-membrane-originated signaling. In particular, the Golgi complex operates as a relay station for signaling, which is initiated by classical ligand-receptor systems at the plasma membrane, acting as a positive or negative regulator of these plasma-membrane signals. Thus, the Golgi complex has emerged as a hub for intracellular signaling. Furthermore, recent evidence has indicated that the Golgi complex can also trigger its own signaling cascades, which involve some of the molecular players that are classically engaged in signal transduction at the plasma membrane. This aspect of the Golgi complex, namely, the ability to generate autonomous signaling, has been experimentally addressed only in the last few years. These studies have revealed that the transport vesicles that leave the ER for the Golgi complex also carry signal molecules that can then be sensed by a receptor in the Golgi complex to coordinate secretory organelles. The receptor involved in the sensing of incoming traffic at the Golgi complex has been shown to be the KDEL receptor (KDELR), a proposed new G-protein-coupled receptor. Upon binding to a KDEL-containing ligand (a chaperone), the KDELR can activate a signaling cascade that regulates anterograde intra-Golgi trafficking. However, this Golgi-based signaling response is only partially understood to date. Here we report on several approaches that are suitable for the study of traffic-initiated and KDELR-dependent signaling.