Final answer:
The protein that loses affinity for the glucocorticoid receptor upon hormone binding is SMRT, resulting in an unmasking of the Nuclear Localization Domain.
Step-by-step explanation:
The protein that loses affinity for the glucocorticoid receptor upon hormone binding is the SMRT (Silencing Mediator of Retinoid and Thyroid hormone receptors). When hormone binding occurs, SMRT dissociates from the glucocorticoid receptor, resulting in an unmasking of the Nuclear Localization Domain. This allows the receptor to translocate to the nucleus and bind to a DNA sequence called a hormone response element (HRE) to trigger gene transcription and translation.