Figure 4.

Structural highlights of the cytoplasmic domain of α_IIb. (A) Structure of the m-α_IIb-wt showing the side chains of K994, R997, E1001, D1003, D1004, and E1005 for the salt-bridge network. A proposed Ca²⁺-binding site is also shown in the figure involving R997, E1001, D1003, and D1004, which were identified by comparing the two-dimensional ¹H total correlation spectroscopy spectra of Ca²⁺-free and Ca²⁺-bound forms (Ca²⁺ was added up to 5-fold). The binding of Ca²⁺ to the α_IIb-tail has been shown by biochemical studies (12, 27). (B) Backbone overlay of m-α_IIb-wt (pink) and m-α_IIb-mut (yellow), showing the structural difference. The C-terminal loop of m-α_IIb-wt folds back to interact with the N-terminal helix, whereas in the mutant, the C-terminal part is highly flexible and makes no interactions with the helix. The C-terminal part (D1003–E1008) of the m-α_IIb-mut is disordered.