Heterotrimer

A heterotrimer is a protein trimer containing more than one type of protein subunit. It may have composition $A_{2}B$ or $ABC$, for example; the total number of subunits is three in either case. This distinguishes it from a homotrimer, whose three subunits have the same type.

A heterotrimeric G protein illustrates the $ABC$ arrangement, with distinct $\alpha$, $\beta$ and $\gamma$ subunits. A structural inhibitor study crystallized an engineered $\alpha \beta \gamma$ complex and separately measured nucleotide binding and release. Such experiments distinguish the composition of the assembled complex from the chemical activity of one of its components.^[ref]

For an original assembly bound, start with $3\mu \mathrm{M}$ of $A$, $4\mu \mathrm{M}$ of $B$ and $1\mu \mathrm{M}$ of $C$, allowing only $ABC$ complexes. At most $1\mu \mathrm{M}$ complexes can form, leaving at least $2\mu \mathrm{M}$ of $A$ and $3\mu \mathrm{M}$ of $B$ outside them. Adding more $A$ alone cannot overcome the shortage of $C$. Actual assembly may be lower because of incomplete binding or competing complexes.