Human cells are divided into compartments such as the nucleus, endoplasmic reticulum, and Golgi apparatus. To keep order, newly made proteins must be shipped to the proper compartment by a system called vesicle trafficking. In this process a membrane-bound vesicle buds off, travels, and fuses with a target membrane to unload its cargo. Randy Schekman used temperature-sensitive yeast mutants to identify SEC genes that control each step of the route. James Rothman reproduced vesicle fusion in vitro and proved that SNARE proteins pair with high specificity, acting like matching keys and locks. Thomas Südhof discovered calcium-sensing proteins like synaptotagmin that trigger fusion within milliseconds, explaining rapid neurotransmitter release. Their work clarifies diseases such as diabetes and neurodegeneration and provides new drug targets. It also explains how botulinum toxin causes paralysis by cutting SNAREs.

In eukaryotic cells, proteins synthesized in the endoplasmic reticulum (ER) reach the Golgi, lysosomes, or plasma membrane through four conceptual stages: vesicle budding, transport, tethering, and fusion. Schekman’s genetic screen in yeast identified 23 SEC genes and led to the functional categorization of COPII coat components, motor adaptors, and fusion modules. Rothman established an in-vitro assay using VSV-G to purify NSF, SNAPs, and the v- and t-SNAREs that form a four-helix bundle, pulling membranes within nanometers to overcome the fusion energy barrier. Structural and kinetic studies confirm that assembly of the trans-SNARE complex converts chemical energy to mechanical work to open a fusion pore. Südhof demonstrated that synaptotagmin, a synaptic vesicle protein with tandem C2 domains, binds Ca2+ and displaces the clamp protein complexin, thereby synchronizing fusion with an action potential. These principles extend to hormone secretion, immune-cell granule exocytosis, and autophagosome formation. The discoveries also illuminate how SNARE-cleaving toxins act and why mutations in fusion factors lead to diabetes, epilepsy, and immune disorders, providing multiple avenues for therapeutic intervention.

Schekman’s systematic analysis of temperature-sensitive sec mutants in S. cerevisiae delineated COPII-mediated ER exit (sec23/sec24/sec13/sec31), early Golgi targeting (sec21, etc.), and SNARE-dependent fusion (sec17/sec18). Rothman, using NEM-sensitive biochemistry, elucidated the ATPase cycle of NSF within 20S particles and proposed α-SNAP-based recycling of cis-SNARE complexes. The canonical SNARE bundle consists of Qa, Qb, Qc, and R motifs whose highly conserved 0-layer glutamine/arginine pattern underpins pairing specificity. Südhof showed that Munc13-1 converts the closed Munc18-1/Syntaxin conformation into an open state and that synaptotagmin-1 inserts into PIP2-rich target membranes upon Ca2+ binding, inducing membrane curvature and catalyzing fusion. Ultrafast optical assays now visualize SNARE zippering dynamics on the millisecond scale, and post-translational modifications such as multisite phosphorylation and ubiquitination modulate complex stability. These advances refine organelle identity concepts and frame current debates on phase separation and membrane nano-domain functions within the secretory pathway.