Laravel - limit FormRequest to certain parameters

In a form request class I use a method like this to validate input data.

class SignupRequest extends FormRequest
{
...
    public function rules()
    {
        return [
            'user.email' => 'required_with:user|email',
            'user.domain_name' => 'required_with:user|string',
            'user.password' => 'required_with:user|string|min:8',
            'user.username' => 'required_with:user',
        ];
    }
...
}

Later in a controller I use something like this

 $data = $request->get('user', []);
 return $this->response($this->userService->create($data, false), 201);

I want somehow to write to my SignupRequest which fields it should allow to be passed. So when later I get $data = $request->get('user', []); I'm sure there are only allowed fields in it.

Is this possible inside the FormRequest?

via Gruz