Tethering factors are a large group of proteins or multiple-protein complexes that link transport vesicles to their cognate target membranes. Tethering factors can be divided into two major groups: homodimeric long coiled-coil proteins and multisubunit tethering complexes (MTCs). Coiled-coil tethers are large hydrophilic dimeric proteins comprising two globular heads connected by long coiled-coil domains, which can interact with vesicles over distances of more than 200 nm. MTCs, which contain 3–10 subunits with an overall molecular weight of approximately 250-800 kDa, can interact with vesicles over much shorter distances (up to 30 nm).
SM proteins are evolutionarily conserved soluble, peripheral membrane proteins of 60-90 kDa. SM proteins are considered key universal components of the fusion machinery. They function in distinct intracellular transport steps, and physically interact with different SNARE proteins