Prof. Jaakko Saraste, Cellular dynamics and protein transport.

The Saraste group investigates the functional organization of biosynthetic-secretory pathway in mammalian cells with special focus on the intermediate compartment (IC), a complex membrane system that operates in protein sorting and transport at the ER-Golgi boundary. According to the currently prevailing model the IC represents a transient organelle consisting of pleomorphic structures that form de novo at ER exit sites and then move along microtubules to the entry side of the Golgi apparatus where they fuse with or transform into cis-Golgi cisternae. However, contrasting with this view, our recent results revealed that the compositional, structural, spatial and dynamic properties of the IC are largely unaffected when the Golgi apparatus is experimentally disassembled using e.g. the drug Brefeldin A. Moreover, our recent studies revealed that the IC maintains many of its properties during mitosis when ER exit is blocked and the Golgi breaks down in a reversible manner.

These findings suggest that besides its well-established functions in protein sorting and transport the IC has other important cellular roles. Regarding the operation of the IC in the early biosynthetic pathway, of particular interest was the finding that part of the tubules extending from peripheral IC elements – instead of approaching the Golgi region – move to the cell cortex and accumulate beneath the plasma membrane (PM). Live cell imaging also showed that the widespread tubular IC network, like the endosomal system, is stably anchored next to the centrosome. These results opened the possibility that the transport process from the ER to the cell surface divides at the level of the IC into Golgi-dependent and -independent pathways. In accordance with this idea, we obtained evidence that the novel IC-endosome connection is related to polarized exocytic events that allow newly made molecules, such as cholesterol and the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel, to bypass the Golgi stacks en route to the PM.

Currently, we continue to employ various imaging techniques – including confocal microscopy, live cell imaging (time-lapse and spinning disk confocal microscopy) and electron microscopy (including immuno-EM) –  to study the structure and function of the IC during the different stages of the cell cycle, with special interest in the functional connection(s) between the IC and the endosomal system.