I could have opted to close this question as an exact duplicate of at least 5 other questions (if you search for "cakephp another controller").
But the answers there are just terrible. They actually try to invoke new Dispatchers or requestAction().
So if your question is about another controller method:
The short answer is: You don't.
The long answer: You still dont. That's a typical beginners mistake.
You should put the functionality into a component if it is mainly business logic. The component then can be accessed from multiple controllers.
If it is more like model data (as in your example), put the functionality into the model layer (in an appropriate model). this way you can also access it from anywhere in your application.
Also: Accessing protected methods from other objects is never a good idea. Use public methods if you intend to use it from "outside" the object.
If your question is about a model method:
You need to include your model in your controller before you can use it.
Either by using public $uses
or by using loadModel('ModelName')
or even ClassRegistry::init('ModelName')
.