Journal of Neuroscience, Vol 16, 1470-1478, Copyright © 1996 by Society for Neuroscience
Association of beta 1 integrin with phosphotyrosine in growth cone filopodia
DY Wu, LC Wang, CA Mason and DJ Goldberg
Department of Pharmacology, Columbia University, New York, New York 10032, USA.
Filopodia of growth cones are key elements in the transduction of
extracellular cues that guide axon growth during development. How they are
specialized to carry out this role is poorly understood. We previously had
found tyrosine phosphorylated protein to be heavily concentrated at the
tips of many filopodia of Aplysia growth cones in certain culturing
conditions, suggesting that tyrosine phosphorylation might be involved in
filopodial specialization. Immunocytochemistry was used to analyze the
protein composition of the tip aggregates to determine whether there was an
association of the tip phosphorylation with any important extracellular
cue. beta 1 integrin, a subunit of the receptor for laminin-type neurite
growth promoters, coconcentrated with phosphotyrosine at filopodial tips of
both Aplysia and mouse growth cones. Several observations indicated that
the association of beta 1 integrin with phosphotyrosine is close. beta 1
integrin and phosphotyrosine are known to colocalize at focal contacts,
sites of adhesion of cells to the extracellular matrix, but the composition
and behavior of the tip aggregates mark them as distinct structures. Also
found in the tip aggregates was a member of the ezrin-radixin-moesin family
of proteins, which are thought to link membrane proteins to submembranous
bundles of actin filaments. Use of an inhibitor of protein-tyrosine kinases
to deplete tip phosphotyrosine also caused disappearance of beta 1 integrin
from the tip, suggesting a role for tyrosine phosphorylation in
facilitating interaction of growth cones with certain environmental cues by
fostering the aggregation of receptors in filopodia.