Leukocyte integrin α _L β ₂ headpiece structures: The αI domain, the pocket for the internal ligand, and concerted movements of its loops

Abstract

High-resolution crystal structures of the headpiece of lymphocyte function-associated antigen-1 (integrin αLβ2) reveal how the αI domain interacts with its platform formed by the α-subunit β-propeller and β-subunit βI domains. The αLβ2 structures compared with αXβ2 structures show that the αI domain, tethered through its N-linker and a disulfide to a stable β-ribbon pillar near the center of the platform, can undergo remarkable pivoting and tilting motions that appear buffered by N-glycan decorations that differ between αL and αX subunits. Rerefined β2 integrin structures reveal details including pyroglutamic acid at the β2 N terminus and bending within the EGF1 domain. Allostery is relayed to the αI domain by an internal ligand that binds to a pocket at the interface between the β-propeller and βI domains. Marked differences between the αL and αX subunit β-propeller domains concentrate near the binding pocket and αI domain interfaces. Remarkably, movement in allostery in the βI domain of specificity determining loop 1 (SDL1) causes concerted movement of SDL2 and thereby tightens the binding pocket for the internal ligand.

Salt bridge interactions within the β2 integrin α7 helix mediate force‐induced binding and shear resistance ability

“…Integrins are composed of heterodimeric α and β subunits connected by noncovalent interactions, and they can be divided into two types based on the presence or absence of the αI domain in the α subunit . The αI domain is folded into a spherical conformation with six β sheets (β 1 –β 6 ) and seven α helixes (α 1 –α 7 ), and it protrudes on the head of the integrin with both the N and C termini inserted into a β propeller domain . It serves as the key domain for integrin–ligand interactions by directly binding to external ligands.…”

“…The aI domain is folded into a spherical conformation with six b sheets (b 1 -b 6 ) and seven a helixes (a 1 -a 7 ), and it protrudes on the head of the integrin with both the N and C termini inserted into a b propeller domain [3,4]. It serves as the key domain for integrinligand interactions by directly binding to external ligands.…”

“…Allosteric pathways have been extensively investigated with regard to key amino acids. Several static structures of b 2 integrin aI domains in different states have been crystallized with partial or full extracellular domains [3,4,9,[15][16][17][18][19]. The extent that the a 7 helix is pulled down corresponds with changes in metal coordination in the MIDAS of the external ligand binding interface and the lateral tilt of both the a 1 helix and the b 6 sheet neighboring the two sides of the a 7 helix.…”

Direction of actin flow dictates integrin LFA-1 orientation during leukocyte migration

“…In this work, we make use of previous structural studies on integrins 2,4,21,22 , and orient these structures in a reference frame that corresponds to the plasma membrane of a migrating lymphocyte. In addition to general structural knowledge on many integrin families 2,4 , we make specific use of crystal structures for the αI domain of LFA-1 bound to ICAMs, the LFA-1 headpiece, αL-T αL-F αL-L GFP Fig.…”

“…N (number of cells) from left was 18, 22, 17, 37. *p < 0.05; **p < 0.01; ****p < 0.0001 and two states of the bent ectodomain of the LFA-1 (αLβ2) relative, αXβ2 2,21,22 . We also use negative stain EM class averages showing the bent-closed, extended-closed, and extended-open conformations of the αLβ2 and αXβ2 ectodomains 2 .…”