Sagia, G. M.; Georgiou, X.; Chamilos, G.; Diallinas, G.; Dimou, S.

Membrane proteins are thought to be sorted to the plasma membrane (PM) via Golgi-dependent trafficking. However, our recent studies in the fungus Aspergillus nidulans challenged the essentiality of Golgi in the biogenesis of non-polarly localized transporters and receptors. Here, we investigate the mechanism of trafficking of membrane proteins, by following the localization of a polar cargo (R-SNARE SynA) versus a non-polar cargo (UapA transporter), synchronously co-expressed in wild-type or isogenic genetic backgrounds repressible for conventional cargo secretion. In wild-type, the two cargoes dynamically label distinct secretory compartments, highlighted by the observation that, unlike SynA, UapA does not colocalize with the late-Golgi. In line with partitioning into distinct early secretory carriers, UapA and SynA translocation to the PM is differentially dependent on Sec13, and importantly the two cargoes collapse in distinct early secretory compartments in a sec31ts mutant or upon CopA repression. Trafficking via distinct cargo-specific carriers is further supported by the observation that repression or inactivation of key proteins essential for late-Golgi /TGN maturation and post-Golgi vesicular secretion did not affect proper trafficking of UapA, but totally blocked SynA secretion. Surprisingly, several specific SNARE proteins that are absolutely essential for conventional cargo vesicular secretion, as well as the exocyst effector RabDSec4, proved dispensable for UapA translocation to the PM. Our findings point to a model where UapA proper trafficking and insertion into the PM might involve non-canonical SNARE combinations. Overall, the present work establishes unequivocally the existence of distinct, cargo-dependent, trafficking mechanisms, initiating at early secretory compartments.