defineProperty on nested class object

Let's say we have this class:

var MyClass = function() {
    this.oneProp = 'one prop';
    this.myProps = {
        prop1: 'my prop1',
        prop2: 'my prop2'
    };
});

How can I configure a getter / setter on myProps.prop1 using Object.defineProperty so that when I change myProps.prop1, the value of myProps.prop2 is set to, for example, '_'+prop1?

I found an easy way to define a getter / setter on oneProp using:

Object.defineProperty(MyClass.prototype, 'oneProp', {
    ...
});

But I can't figure out how to do the same on myProps.prop1.

Thanks