STKE Changes in the morphology of neuronal dendritic spines are correlated with changes in synaptic plasticity. The cell surface proteoglycan syndecan-2 is clustered at the surface of mature hippocampal neurons and is thought to regulate structural changes of the spines. Now Ethell et al. have demonstrated that syndecan-2 is phosphorylated on at least two tyrosine residues by the receptor-type tyrosine kinase EphB2 in vitro and in vivo. The two transmembrane proteins colocalized in dendrites of hippocampal neurons and were isolated in a complex from mouse brain neurons. Phosphorylation was required for their interaction, for syndecan-2 to cluster, and for normal dendritic spine formation in transfected neurons. Phosphorylation could trigger clustering of syndecan-2, and the EphB2-syndecan-2 complex may subsequently initiate the recruitment of downstream signaling molecules that control dendritic spine formation.— LC Neuron 31 , 1001 (2001).