Structural stabilization of protein 4.1R FERM domain upon binding to apo-calmodulin: novel insights into the biological significance of the calcium-independent binding of calmodulin to protein 4.1R.

In erythrocytes, 4.1R(80) (80 kDa isoform of protein 4.1R) binds to the cytoplasmic tail of the transmembrane proteins band 3 and GPC (glycophorin C), and to the membrane-associated protein p55 through the N- (N-terminal), alpha- (alpha-helix-rich) and C- (C-terminal) lobes of R30 [N-terminal 30 kDa FERM (4.1/ezrin/radixin/moesin) domain of protein 4.1R] respectively. We have shown previously that R30 binds to CaM (calmodulin) in a Ca2+-independent manner, the equilibrium dissociation constant (K-d) for R30 CaM binding being very similar (in the submicromolar range) in the presence or absence of Ca2+. In the present study, we investigated the consequences of CaM binding on R30's structural stability using resonant mirror detection and FUR (Fourier-transform IR) spectroscopy. After a 30 min incubation above 40 degrees C, R30 could no longer bind to band 3 or to GPC. In contrast, R30 binding to p55, which could be detected at a temperature as low as 34 degrees C, was maintained up to 44 degrees C in the presence of apo-CaM. Dynamic light scattering measurements indicated that R30, either alone or complexed with apo-CaM, did not aggregate up to 40 degrees C. FTIR spectroscopy revealed that the dramatic variations in the structure of the beta-sheet structure of R30 observed at various temperatures were minimized in the presence of apo-CaM. On the basis of K-d values calculated at various temperatures, Delta C-p and Delta G degrees for R30 binding to apo-CaM were determined as -10 kJ.K-1.mol(-1) and similar to -38 kJ.mol(-1) at 37 degrees C (310.15 K) respectively. These data support the notion that apo-CaM stabilizes R30 through interaction with its beta-strand-rich C-lobe and provide a novel function for CaM, i.e. structural stabilization of 4.1R(80).