2d sprite alwayes overlay 3d game object?

in unity 4.3 you can apply the concept of sorting layer to 2d sprite
but how i can sort 2d sprite with 3d object???

i tried to change Z position but i always get the sprite drawn above 3d objects .

i found the solution
you can set sorting layer name and sorting order for any 3d renderer using the following script

using UnityEngine;
using System.Collections;

public class SetSortingLayer : MonoBehaviour {

	public string sortingLayerName;		// The name of the sorting layer .
	public int sortingOrder;			//The sorting order
	void Start ()
		// Set the sorting layer and order.
		renderer.sortingLayerName = sortingLayerName;

this script already exists in unity 2d demo