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The Lung Biology Center, Department of Medicine, University of California, San Francisco, California 94143
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 
4
7, L2, and M2 were heterologously expressed in K562 cells. 47 initiated
adhesion under flow and localized to microvilli,
whereas 2 integrins did not initiate adhesion and localized to the cell body. Chimeric integrins were produced by replacing the 47 cytoplasmic and/or transmembrane domains with the homologous domains of
L2 or M2. Unexpectedly, these chimeras efficiently mediated adhesion to the 47 ligand mucosal
addressin cell adhesion molecule-1 under flow and localized to microvilli. Therefore, differences between the transmembrane and cytoplasmic domains of 4 and
2 integrins do not account for differences in ability to
support attachment under flow or in membrane localization. Integrins 41, 51, 6A1, v3, and E7
also localized to microvilli. Transmembrane proteins
known or 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.
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