The exocyst complex is an essential component of the mammalian constitutive secretory pathway

Secreted proteins fulfil a vast array of functions, including immunity, signalling and extracellular matrix remodelling. In the trans-Golgi network, proteins destined for constitutive secretion are sorted into post-Golgi carriers, which fuse with the plasma membrane. The molecular machinery involved is poorly understood. We have developed assays for studying this process. 

We have performed biochemical purification and mass spectrometry of the carriers to characterise their contents. We have subsequently performed a CRISPR-KO screen to identify novel components of the secretory pathway. Interestingly, we identify several subunits of the exocyst complex. Depletion of all canonical exocyst subunits causes cargo accumulation in post-Golgi carriers.

Exocyst subunits are recruited to and co-localise with carriers. Exocyst abrogation followed by kinetic trafficking assays of soluble cargoes results in intracellular cargo accumulation. Unbiased secretomics reveals impairment of soluble protein secretion after exocyst subunit knock-out.

Importantly, in specialised cell types, the loss of exocyst prevents constitutive secretion of antibodies in lymphocytes and of leptin in adipocytes. These data identify exocyst as the functional tether of secretory post-Golgi carriers at the plasma membrane and an essential component of the mammalian constitutive secretory pathway. 

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