Final answer:
The likely destination of a protein detected in the endoplasmic reticulum is the Golgi apparatus. The endomembrane system processes and ships proteins specified by the nucleus, and after synthesis in the rough ER, the protein goes through the Golgi apparatus for further modification and transport to the plasma membrane.
Step-by-step explanation:
The likely destination of a protein detected in an extract of the endoplasmic reticulum is Golgi apparatus. The endomembrane system processes and ships proteins specified by the nucleus. After synthesis in the rough endoplasmic reticulum (RER), the protein is packaged into a transport vesicle which fuses with the cis face of the Golgi apparatus. The protein then travels along the Golgi's cisternae and gets further modified before being packaged into a secretory vesicle for transport to the plasma membrane.