Presentation of integrins on leukocyte microvilli: A role for the extracellular domain in determining membrane localization

Adhesion of blood leukocytes to the endothelium involves multiple steps including initial attachment (tethering), rolling, and firm arrest. Presentation of adhesion molecules on leukocyte microvilli can substantially enhance tethering, Localization of L-selectin to microvilli and of CD44 to the planar cell body have been shown to depend upon their transmembrane and cytoplasmic domains. We investigated the role of leukocyte integrin transmembrane and cytoplasmic domains in initiating adhesion under flow and in microvillous localization. Integrins alpha 4 beta 7, alpha L beta 2, and alpha M beta 2 were heterologously expressed in K562 cells. alpha 4 beta 7 initiated adhesion under flow and localized to microvilli, whereas beta 2 integrins did not initiate adhesion and localized to the cell body. Chimeric integrins were produced by replacing the alpha 4 beta 7 cytoplasmic and/or transmembrane domains with the homologous domains of alpha L beta 2 or alpha M beta 2. Unexpectedly, these chimeras efficiently mediated adhesion to the alpha 4 beta 7 ligand mucosal addressin cell adhesion molecule-1 under flow and localized to microvilli. Therefore, differences between the transmembrane and cytoplasmic domains of alpha 4 and beta 2 integrins do not account for differences in ability to support attachment under flow or in membrane localization, Integrins alpha 4 beta 1, alpha 5 beta 1, alpha 6A beta 1, alpha v beta 3, and alpha E beta 7 also localized to microvilli. Transmembrane proteins known dr suspected to associate with extracellular domains of microvillous integrins, including tetraspans and CD47, were concentrated on microvilli as well. These findings suggest that interactions between the extracellular domains of integrins and associated proteins could direct the assembly of multimolecular complexes on leukocyte microvilli.