Pokémon with Protean as its Ability will change its type to be the same type as the move it is using (excluding Struggle). Protean always causes the user to become a single-type Pokémon. Since Protean activates prior to executing a move, the user will receive same-type attack bonus on every move it uses regardless of the move's type (except Struggle) and will always hit with the move Toxic.
If the move is prevented from triggering, be it by weather, status condition or Powder, the Pokémon will not change type. However, missing the move or the move being ineffective will still result in a type change.
The new type will remain in effect until it changes again or the user is switched out. Changing or negating the ability will not turn the Pokémon back to its original typing.