TRAPP Complex - TRAPP As A Tether

TRAPP As A Tether

TRAPP also comes in two "flavors", TRAPP I & II. TRAPP I is a multisubunit complex that consists of seven subunits (Bet5, Bet3, Trs20, Trs23, Trs31, Trs33, Trs85). TRAPPII has three additional subunits (Trs65, Trs120 and Trs130) and functions as a tether at latter stages of the transport pathway. TRAPP I binds these ER derived vesicles and brings the vesicle closer to the acceptor membrane. This close juxtapositioning of the two membranes allows the interaction between SNARE's (soluble NSF (N-ethylmaleimide sensitive Factor) attachment protein receptor) on both compartments. The interacting SNARE's then pull the membranes close and allow for fusion.

A recent report has shown that the initial interaction between TRAPP and the ER derived vesicle is mediated via the interaction between the TRAPP subunit Bet3 and the COPII coat subunit, Sec23. The conventional view of tethering/fusion assumed that the vesicle coat is shed prior to tethering and fusion but this report argues for a model where the initial tethering step takes place on a coated or partially coated vesicle. The TRAPPI complex minus the subunit Trs85 was recently crystallized as two subcomplexes that were then modelled on EM etch data to reveal a possible crystal structure for the entire complex. A putative ypt/rab binding site was indicated on the crystal structure involving the subunits Trs23 and Bet5. Incidentally, all 5 essential small subunits (Trs31, Trs20, Bet3, Bet5 and Trs23) are required to reconstitute efficient exchange activity on ypt1 in vitro. Therefore, it remains to be seen whether the interface between ypt1 and TRAPPI is only confined to Trs23 and Bet5 and how the other subunit influence exchange activity.