Abstract
The accepted model for the interaction of α and β integrins in the transmembrane (TM) domain is based on the pair αIIbβ3. This involves the so-called outer and inner membrane association clasps (OMC and IMC, respectively). In the α chain, the OMC involves a GxxxG-like motif, whereas in the IMC a conserved juxtamembrane GFFKR motif experiences a backbone reversal that partially fills the void generated by TM separation towards the cytoplasmic half. However, the GFFKR motif of several α integrin cytoplasmic tails in non-bicelle environments has been shown to adopt an α-helical structure that is not membrane-embedded and which was shown to bind a variety of cytoplasmic proteins. Thus it is not known if a membrane-embedded backbone reversal is a conserved structural feature in α integrins. We have studied the system αLβ2 because of its importance in leukocytes, where integrin deactivation is particularly important. Herein we show that the backbone reversal feature is not only present in αIIb but also in αL-TM when reconstituted in bicelles. Additionally, titration with β2 TM showed eight residues clustering along one side of αL-TM, forming a plausible interacting face with β2. The latter orientation is consistent with a previously predicted reported polar interaction between αL Ser-1071 and β2 Thr-686.
Highlights
Integrins are heterodimeric type I transmembrane (TM) proteins formed by non-covalent association of an a and a b subunit
The TMs separate towards the cytoplasmic half, and the void is partially filled with an inner membrane association clasp (IMC), formed by a juxtamembrane backbone reversal in aIIb that participated by two phenylalanines, Phe-992 and Phe-993
The structure of integrin aL TM domain has been deposited on the Protein Data Bank (PDB) with ID 2m3e, and the assigned chemical shifts have been deposited on the Biological Magnetic Resonance Bank (BMRB) with ID 18958
Summary
Integrins are heterodimeric type I transmembrane (TM) proteins formed by non-covalent association of an a and a b subunit. The TMs separate towards the cytoplasmic half, and the void is partially filled with an inner membrane association clasp (IMC), formed by a juxtamembrane backbone reversal in aIIb that participated by two phenylalanines, Phe-992 and Phe-993 The latter are part of a conserved a chain GFFKR motif. In contrast to the above model for the juxtamembrane backbone reversal, the GFFKR motif in several a integrins e.g., in aIIb, a1, aM, aL, aX or a4 [18,19,20,21,22,23,24], formed an a-helical structure that was not membrane-embedded Most of these reports included the cytoplasmic tail without the TM domain, in some cases they were myristoylated and attached to detergent micelles. Titration experiments with non-labeled b2 TM show which residues in aL are most affected by this interaction
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