FRAP reveals that mobility of oestrogen receptor-alpha is ligand- and proteasome-dependent

Here we report the use of fluorescence recovery after photobleaching (FRAP) to examine the intranuclear dynamics of fluorescent oestrogen receptor-alp ha (ER). After bleaching, unliganded ER exhibits high mobility (recovery t( 1/2) < 1 s). Agonist (oestradiol; E2) or partial antagonist (4-hydroxytamox ifen) slows ER recovery (t(1/2) <similar to>5-6 s), whereas the pure antago nist (ICI 182,780) and, surprisingly, proteasome inhibitors each immobilize ER to the nuclear matrix. Dual FRAP experiments show that fluorescent ER a nd SRC-1 exhibit similar dynamics only in the presence of E2. In contrast t o reports that several nuclear proteins show uniform dynamics, ER exhibits differential mobility depending upon several factors that ave linked to its transcription function.