special relativity - Potential energy in $E_f^2=(mc^2)^2+(pc)^2$?

Let's consider

$$E_f^2=(mc^2)^2+(pc)^2$$

where the $mc^2$ is the rest energy due to the rest mass -- in Finnish "lepomassa".

$$ \sqrt{(mc^2)^2+(pc)^2} - mc^2~=~(\gamma-1)mc^2$$

is the kinetic energy due to the movement because of momentum $p=\gamma mv$.

Now where is potential energy if $E_f=\gamma mc^2$ is the total energy?