Most of the adaptor proteins are heterotetramers. In the AP complexes, there are two large proteins (∼100 kD) and two smaller proteins. One of the large proteins is termed β (beta), with β1 in the AP-1 complex, β2 in the AP-2 complex, and so on. The other large protein has different designations in the different complexes. In AP-1 it is named γ (gamma), AP-2 has α (alpha), AP-3 has δ (delta), AP-4 has ε (epsilon) and AP-5 has ζ (zeta). The two smaller proteins are a medium subunit named μ (mu ∼50 kD) and a small subunit σ (sigma ∼20 kD), and named 1 through 5 corresponding to the 5 AP complexes. Components of COPI (cop one) a coatomer, and TSET (T-set) a membrane trafficking complex have similar heterotetramers of the AP complexes.